ATOLL RESEARCH BULLETIN NO. 187 THE ALGAL RIDGES AND CORAL REEFS OF ST. CROIX their structure and Holocene development by Walter H. Adey Issued by THE SMITHSONIAN INSTITUTION Washington, D.C., U.S.A. August 6,1975 THE ALGAL RIDGES AND CORAL REEFS OF ST. CROIX their structure and Holocene development by Walter H. Adey - 2/ ABSTRACT The shallow c o r a l r ee f and a l g a l r i dge systems on t h e e a s t e r n s h e l f o f S t . Croix a r e descr ibed and mapped i n some d e t a i l . Based on p r e s e n t reef morphology, a s e c t i o n through the b a r r i e r r e e f i n a s h i p channel , numerous sand probes and c14 da t ing , Holocene growth p a t t e r n s of t h e r e e f s a r e determined and a model o f Holocene evolu t ion developed. Based on many d r i l l cores through t h e a l g a l r i d g e s , ~ 1 4 da t ing and paleoecolo- gy r e l a t i v e t o modern r idge and r ee f su r f aces on S t . Croix, growth p a t t e r n s during the l a t e Holocene a r e a l s o developed f o r t h e a l g a l r i dges . Lithophyllum congestum, Poro l i thon pachydemum, and s e v e r a l s- gonio l i t hon spec i e s a r e t he primary a l g a l r i dge b u i l d e r s on S t . Croix. L. congestum requ i r e s t u r b u l e n t water and high l i g h t i n t e n s i t y t o achieve - t h e branching form which c h a r a c t e r i z e s i t s occurrence i n t h e a l g a l r i d g e s . Also, c o r a l l i n e acc re t ion r a t e s of 3-6 rnm/year necessary f o r r i dge con- s t r u c t i o n a r e achieved only i f i n t e n s i v e p a r r o t f i s h and Diadema graz ing a r e prevented by c o n s i s t e n t and i n t e n s i v e wave a c t i o n . A dead c o r a l su r f ace o r pavement a t a depth of 0 t o 2 m , w i l l be colonized by c rus tose c o r a l l i n e s and i n t u r b u l e n t water , can develop p rog res s ive ly by c o r a l l i n e a l g a l a c c r e t i o n i n t o an i n c i p i e n t mound, a high b o i l e r and even tua l ly by b o i l e r f u s i o n , i n t o a l i n e a r a l g a l r i dge . Off open, e a s t e r l y shores i n S t . Croix, c o r a l r e e f s , on bu i ld ing t o t he s u r f a c e , develop a l g a l r i dges . The p r e s e n t morphology of t he r idge- r ee f complex has developed p r imar i ly a s a r e s u l t of t h e c o n t r o l exe r t ed by p r e - e x i s t i n g s h e l f and bench l e v e l s and changing r a t e s of Holocene sea l e v e l r i s e on c o r a l - c o r a l l i n e and g raze r ecology. Spec ia l emphasis i s p laced he re on the impo.rtance of p re -ex i s t i ng s h e l f l e v e l i n d e t e m i n i n g t h e form and developmental s t a g e o f r idge- reef systems. L / ~ o n t r i b u t i o n no. 26 from t h e West I n d i e s Laboratory o f F a i r l e i g h Dickinson Univers i ty , S t . Croix, U.S. Virgin I s l ands . This s tudy was supported by t h e Research Awards Program of the Smithsonian I n s t i t u t i o n . %/~mi thson ian I n s t i t u t i o n , Washington, D. C. 20560 INTRODUCTION The massive, wave-beaten, i n t e r t i d a l a l g a l r i dges of t he P a c i f i c a t o l l s have been b r i e f l y descr ibed by numerous au thors , l a r g e l y beginning i n t he 1950 ' s ( s ee e . g . , Emery, Tracey and Ladd, 1954; Munk and Sargent , 1954; Tracey e t a l . , 1964; Wiens, 1962). More r e c e n t l y , Cheval ier e t a l . (1968) and L i t t l e r and Doty (1974) have descr ibed the morphology of t h e s e r idges and t h e i r c rus tose c o r a l l i n e components i n f u r t h e r d e t a i l . E lse- where i n t h e Indo-Pac i f ic , t he r i dges a r e g e n e r a l l y more weakly developed and o f t e n c a l l e d a l g a l r i m s (Maxwell, 1968, Great B a r r i e r Reef; S todda r t and Yonge, 1971, Indian Ocean) . S e t c h e l l (1926) emphasized t h e importance o f c rus tose c o r a l l i n e s i n genera l on P a c i f i c r e e f s . Espec ia l ly during the l a s t 25 yea r s , many bor ings have been made through the l imes tone caps o f P a c i f i c a t o l l s , and some of t h e s e have a l s o repor ted c o r a l l i n e a lgae a s t h e dominant s t r u c t u r a l elements ( s ee e .g . , Gross e t a l . , 1969-Midway). However, none o f t h e s e have s u r f i c i a l l y pene t r a t ed an a l g a l r idge . Easton and Olson (1968, 1973, i n m s . ) have d r i l l e d through an a l g a l r i m and dominantly c o r a l l i n e a l g a l r e e f of Holocene age i n Hanauma Bay, Oahu, Hawaii. However, t h i s r i m i s appar- e n t l y n o t i n t e r t i d a l and the c o r a l l i n e a lgae themselves were n o t s tud ied . Some authors (Fa i rb r idge , 1968) have considered t h e P a c i f i c a l g a l r i dges a s only t h i n a l g a l veneers over o l d e r dominantly c o r a l r ee f s t r u c t u r e s . This i n t e r p r e t a t i o n is probably n o t gene ra l ly c o r r e c t , b u t on ly ex tens ive cor ing w i l l s e t t l e t h e ques t ion . Although r e l a t i v e l y small c o r a l l i n e and cora l l ine-vermet id frame- works, c a l l e d b o i l e r s , cup r e e f s o r m i c r o a t o l l s have a l s o been desc r ibed f o r t h e Caribbean and t r o p i c a l A t l a n t i c (Boyd e t a l . , 1963; Rempf and Laborel , 1968; Gessner, 1970; Ginsburg and Schroeder, 19731, " t rue" a l g a l r i dges have gene ra l ly been considered a s l ack ing (though see Ottmann, 1963; ~ i g b y and McIntyre, 1966 and Glynn, 1973) . More r e c e n t l y , Adey and Burke (1975) have descr ibed the d i s t r i b u t i o n and morphology of a s e r i e s of a l g a l r i d g e s and t h e a s soc i a t ed r e e f s i n t h e e a s t e r n Caribbean from t h e Virgin Is., Anguil la and Barbuda i n t h e no r th t o Grenada i n t he south. The a l g a l r i d g e s of S t . Croix a r e Holocene i n development. They a r e t r u l y a l g a l r i d g e s i n t h a t they a r e b u i l t above mean low water and n o t only i s t h e i r upper carbonate framework domimantly c rus tose c o r a l l i n e , b u t t h e i r upper s u r f a c e s o f t e n support a r i c h f l e s h y a l g a l f l o r a o f high bio- mass and p r o d u c t i v i t y . Connor and Adey (1975) have descr ibed t h e non- c rus tose c o r a l l i n e a l g a l f l o r a of t h e r idges and i t s d i v e r s i t y and ecology i n terms of s t and ing crop. Adey and Vassar (1975) have examined t h e c rus tose c o r a l l i n e succession p a t t e r n s and t h e i r growth and a c c r e t i o n r a t e s , and Steneck and Adey (1975) have s t u d i e d i n d e t a i l Lithophyllum congestum, t h e ch i e f b u i l d e r of t h e r idges . I n t h i s paper , I desc r ibe t h e d i s t r i b u t i o n , n~orphology and geo log ica l s t r u c t u r e of t he a l g a l r i dges and the r e l a t i o n s h i p of t hese t o the co ra l r e e f s and t o bedrock geology and she l f o r bench l e v e l s a s we l l a s t i d e l e v e l s and meteorological cond i t i ons . The c rus tose c o r a l l i n e s a r e t h e ch i e f b u i l d e r s of the Caribbean a l g a l r i dges . On S t . Croix, Lithophyllum i s t h e primary element , although i n s p e c i a l i z e d s i t u a t i o n s Poro l i thon pachydermum and a complex of Neoyoniolithon spec i e s a r e a l s o important . On the gene r i c l e v e l , most of t he r idge components can be i d e n t i f i e d us ing t h e keys of Adey and Macintyre (1973). However, a t t h e s p e c i e s l e v e l , of t h e dozen t h a t a r e important i n r i dge cons t ruc t ion , nea r ly h a l f a r e new spec i e s . In t h e o lde r l i t e r a t u r e , a l a r g e percentage of t h e descr ibed spec i e s a r e syno- nyms of Lithophyllum congestum and Neogoniolithon s tr ic turn. I a m pres- en t ly prepar ing a b iosys temat ic s tudy of t h e c rus tose c o r a l l i n e s of t h e Caribbean Sea. The new taxa used here a r e b r i e f l y descr ibed by Adey and Vassar (1975) and w i l l be descr ibed i n d e t a i l i n t he b iosys temat ic t reatment . ACKNOWLEDGMENTS Many persons a s s i s t e d i n var ious a s p e c t s of t h i s p r o j e c t . Most important were my own col leagues and a s s i s t a n t s P . Adey, R. Burke, J . Connor, L. Gordon, R. Steneck and J. M. Vassar , a l l o f whom deserve s p e c i a l thanks f o r wi ths tanding t h e r i g o r s of d r i l l i n g and working on t h e a l g a l r i d g e s and l i v i n g an icably i n t h e c l o s e conf ines of "Cora l l ina" . In a d d i t i o n t o t h e s e co l leagues , L. Gerhard, I. t4acintyre, C. bioore and J. Ogden o f t e n d iscussed a spec t s of t h e s tudy wi th me and a l l o f t he above have read the manuscript and o f f e r e d va luab le sugges t ions f o r i t s improve- ment. L . Bingham and h i s a s s o c i a t e s a t t h e West I n d i e s Lab helped us i n many ways wi th t h e cons iderable t e c h n i c a l problems of t he p r o j e c t . Through the loan of t he rock d r i l l , which we o therwise could n o t have f i - nanc ia l ly a f forded , Clyde Moore made a major p a r t o f t he geo log ica l i n t e r p r e t a t i o n p o s s i b l e . P ro fe s s iona l d i v e r R. Malpass and Captain Carlson of Hess O i l Company made p o s s i b l e t h e h ighly va luable Hess channel examination. The c14 d a t i n g was accomplished by R. Stuckenrath of t h e Smithsonian Radiat ion Biology Laboratory. Nost of t h e i l l u s t r a t i o n s a s wel l a s t h e l a y up f o r pub l i ca t ion were done by C. Lmerick. METHODS The maps appearing i n t h i s paper were a l l cons t ruc ted from a e r i a l photographs taken from a s ingle-engined, high-winged p lane f l y i n g a t a l t i - tudes o f 20 t o 200 meters. Best r e s u l t s were obta ined by removing t h e photographer ' s door f o r b e t t e r v i s i b i l i t y and ease of camera handl ing, and the most u s e f u l s h o t s were high angle ob l iques (60-80'). R o l l i e and Easselblad cameras were used f o r t h e l a r g e negat ive s i z e . General ly , s h o t s were taken a t 1/500th of a second and wi th ektachrome-x c o l o r o r sometimes p lus x , b lack and white . The co lo r s l i d e s were most u se fu l and i n i t i a l mapping was accomplished on Mylar ( p l a s t i c drawing paper) by p r o j e c t i n g the c o l o r s l i d e s on a proper ly-or ien ted mapping board. Sca le and angle were c o n t r o l l e d us ing the 1/210,000 s c a l e h igh a l t i t u d e black and whi te v e r t i c a l s o f Mark Hurd Aer i a l Surveys. Bes t r e s u l t s were obta ined by photographing dur ing t h e s h o r t and more o r l e s s r a r e pe r iods o f n o r t h e r calms. Some o f t h e h igh a l t i t u d e commercial photographs were a l s o taken during no r the r calms and were e s p e c i a l l y u s e f u l i n determi'ning t h e l i m i t s of t he deeper r e e f s . The base maps were drawn wi th i n d i a i nk i n Mylar which could then be taken underwater on a drawing board f o r i n t e r p r e t a t i o n and d e t a i l . Most underwater mapping work was accomplished by us ing snorke l gea r a l though SCUBA was occasionalLy employed. The B o i l e r Bay maps (Figs. 29-37) were completed f i r s t , and wi th approximately 150 man hours i n t h e water a r e t h e most accu ra t e . Not a s much time p e r r i dge was a v a i l a b l e f o r t he south shore a l g a l r i dges (F igs . 16, 20, 22, 2 5 ) , and e s p e c i a l l y a s some o f them a r e cons iderably l a r g e r than t h e Bo i l e r Bay r i d g e , t h e accuracy i s l e s s . Areas n o t a c t u a l l y v i s i t e d underwater a r e l e f t blank. The Beach a l g a l r idge was t h e l a s t r i dge mapped. With t h e l e a s t time a v a i l a b l e f o r work on t h a t r i d g e , it is t h e l e a s t accura te i n d e t a i l . I n t e r t i d a l and upper s u b t i d a l r i dge e l e v a t i o n s were es t imated us ing a s tandard eye-level and s t a d i a rod. The base of t h e rod was placed i n a p l a s t i c tube wi th only a 2 mm hole f o r water flow t o dampen wave a c t i o n , t h e rod be ing s e t i n t h e q u i e t e r back r ee f a r ea . Tide l e v e l s were de t e r - mined a t t h e West l n d i e s Lab dock i n Tague Bay. Two, four t o f i v e hour p a r a l l e l t i d a l measurements were made a t B o i l e r Bay and a t t he Fancy-Robin a r e a on t h e south shore t o s e e i f a d i f f e r e n c e i n t i d e time e x i s t e d . No d i f f e rence was found. The t i d e s a r e d iscussed i n d e t a i l below. D r i l l i n g on t h e a l g a l r i dges was accomplished us ing a gaso l ine- dr iven hand he ld Acker D r i l l Company "pack-sack" and both diamond and carb ide b i t s . Most o f t h e cores taken on t h e south shore a l g a l r i d g e s were 2 9 mm i n d i m . ; most o f those taken i n B o i l e r Bay were 20 mm i n d i m . Flushing water was ob ta ined wi th a 3 hp, gaso l ine-dr iven pump and ho le s were n o t cased. Two people could o p e r a t e t h e d r i l l on a r i dge with some d i f f i c u l t y , b u t t h r e e o r p r e f e r a b l y fou r were r equ i r ed f o r an e f f i c i e n t opera t ion . Usually a p l a t fo rm about two meters square and 1-1/2 meters high was s e t up on t h e r idge t o hold t h e core boxes, rods , pump, gaso l ine , t o o l s e t c . , and d r i l l i n g was accomplished s tanding on t h e r idge i t s e l f . Quiet days were usua l ly chosen f o r d r i l l i n g , though on t h e l a r g e r r i dges some hazard t o equipment and personnel remained on a l l b u t excep t iona l ly calm days. Once the p la t form was s e t up and the hole s t a r t e d , t h e d r i l l e r s had some- th ing t o hold onto and t h e r e was usua l ly n o t a major problem wi th t h e waves. The most d i f f i c u l t t imes were s e t t i n g up and breaking down. On s e v e r a l occasions a l l of t he gea r was dumped i n t h e water and t h e ope ra t ion had t o be suspended u n t i l t h e engines could be thoroughly serv iced . Also occas iona l ly a wave would ca t ch one o f us o f f balance and t h e r e s u l t i n g bounce on t h e r idge would r e s u l t i n a few s c r a t c h e s and Echinometra sp ines . I n t h e B o i l e r Bay a l g a l r i d g e , we d r i l l e d 32 h o l e s , 13 through t o t h e underlying basement a t 2 t o 4 meters. On t h e more d i f f i c u l t south shore r idges , we d r i l l e d 7 h o l e s , two of which went through t o basement a t 8 t o 9 meters. Once t h e p l a t fo rm was s e t up t h e r e was usua l ly l i t t l e d i f f i c u l t y i n d r i l l i n g i n t h e upper 3 meters . Core recovery va r i ed widely from 80 t o 90% i n s o l i d c o r a l l i n e t o nothing f o r a s much a s 3 meters i n sand- f i l l e d c a v i t i e s . On t h e o t h e r hand below 6 meters i nc reas ing d i f f i c u l t y was enccuntered, and we o f t e n spen t a s much a s a whole day r e t r i e v i n g o u r b i t from one meter o f d r i l l i n g i n t h e 6-9 meter range. A more powerful water pump and an A-frame o r tower wi th a block and t a c k l e would probably a l l e v i a t e t h i s problem. i n t e r p r e t a t i o n o f t h e cores was achieved wi th 2 0 p t h i c k ground sec t ions . I n c o r a l l i n e cores t h e fol lowing four elements were determined i n s l i d e s Lithophyllum congestum, Poro l i thon pachydermum, iqeogoniolithon spp. and Tenarea sp . More d e t a i l could have been ob ta ined i n i d e n t i f i c a - t i o n , b u t t h i s was n o t considered necessary a t t h i s p o i n t i n the s tudy . Coral spec i e s were usua l ly determined from t h e hand specimens of co re s . Both cores and s e c t i o n s a r e housed i n t h e c o r a l l i n e c o l l e c t i o n a t t h e National Museum of Natura l History i n Washington, D.C . S u r f i c i a l b locks of r i dge ma te r i a l 30-50 cm on a s i d e were o f t e n obta ined , sometimes wi th considerable d i f f i c u l t y , us ing a crowbar and l a r g e c h i s e l s . In Shark Reef i n Bo i l e r Bay we c r e a t e d a s l o t i n t h e f r o n t of t h e r idge about 0.6 meters wide, 1 .6 meters i n t o the r idye and one meter i n depth using a combination o f chopping and d r i l l i n g . The s labbing of t h e s e blocks on a rock saw helped cons iderably i n our i n t e r p r e t a t i o n of t h e cores . Twenty-eight c14 d a t e s were ob ta ined , 17 on c o r a l and 11 on c o r a l l i n e . P.lthough a l l b u t one o f t h e c o r a l d a t e s a r e c o n s i s t e n t wi th each o t h e r and the i n t e r p r e t a t i o n presented he re , t h e o l d e s t being 4360 years B .P . - + 90, the c o r a l l i n e d a t e s a r e i r r e g u l a r l y young. However, t hese ages a l l occur below t h e envelope f o r the Bermuda s e a l e v e l curve o f Neumann, a s d i scus- sed below. A s i n g l e Acropora palmata d.ate from B o i l e r Bay a t 4900 yea r s B.P . p laced s e v e r a l f e e t above t h e sea l e v e l s tandard . However, a c a r e f u l - check of t h i s specimen showed t h a t the gray-darkening c h a r a c t e r i s t i c o f subaerial ly-weathered c o r a l was p re sen t . We have i n t e r p r e t e d t h i s spec i - men a s p a r t o f a s u p r a - t i d a l storm berm such a s a r e q u i t e common on eas t e rn S t . Croix today. The p o s i t i o n of s e a l e v e l i n t h e r idge co res was determined by o u r s e a l e v e l i n d i c a t o r Lithophyllum congestum ( see Steneck and Adey, 1975) . Through t h e cour tesy o f t he Hess O i l Ref inery i n S t . ~ r o i x ; we were ab le t o d i v e and take many subsurface samples of Long Reef, seaward of t h e r e f i n e r y , where t h e 19-meter deep sh ip channel s e c t i o n s t h e r ee f and ex- tends 6 meters i n t o t h e basement. ~ 1 4 da t ing o f l a r g e c o r a l samples from t h i s s e c t i o n forms t h e b a s i s f o r ou r time i n t e r p r e t a t i o n of t h e c o r a l r ee f s . Our 41- foot , ketch-r igged, motor -sa i lor t r imaran "Cora l l i na" , with a r e l a t i v e l y l a r g e c e n t r a l l a b and bunks f o r s i x served a s ou r home, f i e l d t r a n s p o r t a t i o n and base of opera t ions dur ing t h e course of t h i s s tudy. PHYSICAL ENVIRONMENT A l l o f t h e known a l g a l r i dges on S t . Croix occur e a s t of Canegarden Bay (Fig. l> , where t h e bed rock i s mostly weakly metamorphosed, upper Cretaceous, deep water sand and mud s tone o f t h e Caledonia Formation. Whetten (1966) d e s c r i b e s t h i s formation i n some d e t a i l . The westernmost r i dge , a t Vagthus P o i n t , on t h e south shore we have n o t s t u d i e d i n any d e t a i l . Apparently it l i e s on a l imestone member o f t h e J u d i t h Fancy Formation. We have a l s o seen a small r i dge o f f Spr ing Bay from the a i r , / \\~m$ 'caneearden Bay Reef Caribb,ean Sea o I e 3 4 5 1 I Nautical M~ les Fig. 1. Major reef - r idge environments on eas te rn S t . Croix and pos i t ions of " she l f " sec t ions . reef f l a t o r a l g a l r idge. -0 .5 m t o -1 m; dominantly Acropora palmata u2per fore r e e f ; $-J domi- nant ly Montastrea annular is and Diplor ia spp.. deeper f o r e r e e f ; Meandrina hard ground; a sediments, 6.ominantly sand s i z e and b i o c l a s t i c . b u t have no t v i s i t e d t h i s r i dge . The remaining r idges l i e e a s t o f Great Pond Bay. The sma l l e s t o f t h e s e , one a t E a s t End Bay (F ig . 4 ) , and another no r th of Buck I s l a n d , l i e c l o s e t o exposures o f t he Caledonia Formation, b u t t h e na tu re and depth of t h e i r immediate basement i s unknown. Three of t he l a r g e south shore r idges , Fancy, Robin and I s a a c and t h e i n c i p i e n t r idge a t Hughes P o i n t (Fig. 4 ) , l i e o f f exposures of t h e po in t - forming Eas t End Member of t h e Caledonia Formation (Whetten, 1966) . A l l o f t hese r idges a r e apparent ly developed on hard she lves c u t i n t h e bed- rock a t t h e minus 8-12 meter l e v e l . The f o u r t h major south shore r idge , Beach r i d g e , apparent ly l i e s on a s i m i l a r bench c u t on a f a u l t scarp . Adey and Burke (1975) d i s c u s s i n d e t a i l t h e c o n t r o l exe r t ed by bedrock geology on s h e l f l e v e l and consequently a l g a l r i dge d i s t r i b u t i o n i n t h e e a s t e r n Caribbean. This i s d iscussed f u r t h e r below i n terms o f r i dge placement on S t . Croix. The a l g a l r i dge i n B o i l e r Bay i s unusual i n s e v e r a l r e s p e c t s . A small p a r t o f it i s apparent ly developed on r i d g e s of t h e Caledonia Formation. However, t h e major lobes of t he r idge a r e l e s s than 3 meters t h i ck and they a r e formed on a bench of Caledonia boulders . These boulders a r e t h e l a g d e p o s i t o r remains o f a rapid-flow o r colluvium der ived from t h e Caledonia probably during t h e l a s t 100,000 yea r s of t h e l a t e P l e i s tocene . Formal meteoro logica l d a t a were n o t taken a t any l o c a l i t y on the e a s t end of S t . Croix during 1973-74. However, "Cora l l ina" has both an anemometer and a wind d i r e c t i o n i n d i c a t o r . During our s t a y on S t . Croix from l a t e 1972 t o e a r l y 1974, a t l e a s t some members of ou r group had occasion t o be i n the f i e l d v i r t u a l l y every day o f t h e pe r iod , and usu- a l l y we were concerned wi th both wind d i r e c t i o n and s t r e n g t h . The following wind and s e a d e s c r i p t i o n i s based t o some e x t e n t on t h i s sub jec t ive f a m i l i a r i t y . The e a s t e r l y t r a d e wind i s markedly cons t an t on e a s t e r n S t . Croix and during our s t a y it blew a t 10-20 knots from ENE t o ESE b e t t e r than 95% of t h e time. According t o t he U . S . Navy Marine Cl imat ic At las o f t h e World (1955), t h i s p a r t o f t h e Caribbean experiences two peaks of wind v e l o c i t y , one i n June and J u l y and the o t h e r i n December and January. This was roughly t r u e dur ing ou r s t a y , though t h e autumn of 1973 was e x t r a o r d i n a r i l y windy. During t h e sp r ing and autumn, con t inen ta l f r o n t s o r no r the r s sometimes reach S t . Croix. Usually, t h i s r e s u l t s i n a s h i f t o f t he wind i n t o t h e no r th o r n o r t h e a s t , where it blows a t 10-15 knots , r a r e l y 20 knots , f o r about a day. Af te r t h i s t h e wind o f t e n f a l l s t o l i g h t and v a r i a b l e f o r a day o r two before p ick ing up t o a s t rong e a s t e r again. During t h e autumn o f 1972 and then aga in i n t he s p r i n g of 1973, 3-4 n o r t h e r s followed t h i s p a t t e r n . On the o t h e r hand, i n t h e autumn of 1973 and then aga in i n t h e e a r l y spr ing of 1974, t h e r e were few nor the r s and f o r those t h a t d i d pas s through, the wind merely s h i f t e d from E t o NE and back wi thout markedly changing i n i n t e n s i t y . During t h i s l a t t e r per iod the wind was a l s o occas iona l ly wel l i n t o t h e SE. However, i n l a t e Apr i l 1974, a s t rong f r o n t passed which caused t h e wind t o blow from due nor th f o r one day a t about 15-20 knots , and a second a t 10-15 knots . This two-day no r the r was then followed by fou r days no r th t o e a s t winds a t about 5-10 knots . Perhaps t h e r e i s a l s o a d i u r n a l c y c l e , r e l a t i v e l y l i g h t dur ing t h e n i g h t , and s t r o n g e r during t h e day, b u t t h i s i s n o t marked on e a s t e r n S t . Croix. I t was o f t e n my impression t h a t t he wind on t h e exposed south shore was l i g h t e s t from about 1600-1800, p ick ing up again t o mid-day s t r e n g t h by 2100-2200. The cons t an t o f f s h o r e s e a r e s u l t i n g from t h i s wind p a t t e r n i s s h o r t and s t e e p and mostly from 1-2 meters i n he igh t . P o l l e r s , t he l a r g e swel l s r e s u l t i n g from i n t e n s e n o r t h e r s i n t h e sou theas t A t l a n t i c and o f t e n ex- per ienced on t h e nor thern c o a s t s o f t h e nor thern Vi rg in I s l a n d s d i d n o t occur dur ing ou r s t a y i n S t . Croix. They a r e appa ren t ly l a r g e l y blocked and d i s s i p a t e d by t h e no r the rn Vir9i.n I s l ands . Even dur ing t h e calm pe r iods with low mid-day t i d e s , t h e r i dges a t h e i g h t s o f 30-50 cm above mean sea l e v e l would t a k e a t l e a s t a we t t i ng s e a every few minutes. I t is p o s s i b l e t h a t an extended calm, l i k e t h a t experienced i n Apr i l 1974, i n conjunct ion wi th a p e r i o d o f daytime low water s p r i n g s might r e s u l t i n s e r i o u s d e s i c c a t i o n and k i l l - o f f of a lgae and c o r a l on t h e r i d g e s and r e e f s . However, t h i s d i d n o t happen on t h e a l g a l r i d g e s dur ing t h e autumn of 1972 t o t h e s p r i n g of 1974 per iod . Water c l a r i t y around t h e r i d g e s and r e e f s i s usua l ly l a r g e l y depend- e n t on t h e i n t e n s i t y o f wave a c t i o n and t h e r e s u l t i n g suspension of f i n e carbonate sediment. During rough pe r iods , v i s i b i l i t y i s only 4-6 meters , while a f t e r a few q u i e t days it can be 15-20 meters . The q u a n t i t y of p lankton i n t h e water appears t o be h i g h e s t i n t h e summer and, a s t h i s i s o f t e n a r e l a t i v e l y rough t ime , t h e v i s i b i l i t y tends t o be p a r t i c u l a r l y poor . During t h e pe r iod t h a t w e were mapping and l e v e l i n g t h e r i d g e s ( e a r l y Apr i l t o l a t e June, 1973) , we k e p t a continuous record of t i d e a t t h e West I n d i e s Lab dock by simply read ing sea l e v e l on a meter s t i c k a t t ached t o t h e dock. This was r ead every one t o two hours dur ing t h e day and every 3-4 hours a t n i g h t throughout Apr i l and May. The read ing was more desu l - t o r y du r ing June, b u t by l a t e June , John Adams and Jay Kunin of t h e Laboratory had i n s t a l l e d a computerized guage a t t h e dock. This record was no t cont inuous, l a r g e l y because of o t h e r computer use and t echn ica l problems, b u t i n conjunc t ion wi th our continuous read ing i n May and June, it provides a f a i r l y complete p i c t u r e of t i d a l p a t t e r n s du r ing J u l y and l a t e r i n December and e a r l y January. During sp r ing t i d e s , which roughly correspond wi th t h e f u l l and new moons, t h e t i d a l p a t t e r n i s markedly d i u r n a l and u s u a l l y has a range of 30-35 cm. The neap t i d e s , w i th a range o f about 10-15 cm, a r e sometimes c l e a r l y semidiurnal . Xore o f t e n , however , t h e neaps a r e r a t h e r i r r e g u l a r w i th one h igh and one low s t r o n g l y dominating t h e o t h e r . The pe r iod of neaps i s o f t e n r a t h e r sha rp ly begun a s a "no t i d e day", with t h e low p a r t o f t h e d i u r n a l cyc le be ing c l ipped-of f a t o r s l i g h t l y below mean t i d e l e v e l and then remaining w i t h i n a range of 2-3 cm f o r 10 t o 12 hours . Our r i dge l e v e l i n g was based on a p o s i t i o n f o r mean low water s p r i n g s es tab- l i s h e d from ou r d a t a f o r A p r i l and May. A long term record might r e q u i r e some adjustment o f t h i s v a l u e , b u t i t i s probably c o r r e c t t o w i th in 2 2 cm, which i s l i k e l y a s good a s ou r accuracy of r i d g e l e v e l i n g . Figure 2A shows a t y p i c a l sp r ings t o neaps cyc le from 1973, along wi th t h e r e l a t i v e p o s i t i o n s of t h e h igher p a r t s o f s e v e r a l a l g a l r i dges . A p r l 2 3 2 4 2 5 26 27 28 2 9 30 may 1 2 3 4 5 b 7 8 9 OCt Nov Dec June J J I ~ Auy! 5ept May 2400 ?*n Apr I I I 2 0 0 0 I - - - - - - I 0 4 0 0 Fig . 2 . Typical t i d e p a t t e r n s on e a s t e r n S t . Croix. (A) Bi-monthly cyc l e a t West I n d i e s Laboratory dock f o r l a t e Apr i l and e a r l y May 1973. Level of mean low water sp r ings based on t i d e s f o r Apr i l t o June, 1973. (B) Times of occurrence of t i d e l e v e l s a t o r below mean low water sp r ings . P p r i l - December, 1973. The degenerat ing B o i l e r Bay r idge c r e s t s range from about 17 cm above mean low water s p r i n g s t o mean t i d e l e v e l s . The l a r g e r , south shore r idges have c r e s t s t h a t range from about t he p o s i t i o n o f mean s e a l e v e l t o l e v e l s of mean h igh water sp r ings . The very a c t i v e I s a a c and Robin r idges have many o f t h e i r c r e s t s a t o r above mean high water s p r i n g s and t h e i r h ighes t c r e s t s a r e about 20 cm above t h a t l e v e l o r n e a r l y 10 cm above the h ighes t t i d e l e v e l recorded i n t h i s s tudy. ( ~ d e y and Burke (1975) desc r ibe r i d g e s i n t h e Lesser A n t i l l e s ranging up t o about one meter above mean low water . ) I n Apr i l 1973 t h e pe r iod o f low water s p r i n g s was cen te red about mid-afternoon, with t h e e a r l y p a r t o f t h e 7-day phase being cen te red about e a r l y af ternoon and t h e l a t e r days being cen te red about 1800 (F ig . 2 B ) . By June, low water s p r i n g s were centered around noon, by mid-July, they had migrated t o about 1000, and f i n a l l y i n l a t e December they were centered about 0300. Thus, a t t h e p r e s e n t t ime a t l e a s t , low water s p r i n g s occur i n mid-day dur ing t h e e a r l y summer and dur ing mid- n i g h t i n the win ter . Glynn (1968) d i scusses t h e e f f e c t s of a s i m i l a r t i d a l p a t t e r n on mass m o r t a l i t i e s of r ee f organisms i n r a t h e r p r o t e c t e d env i~onmen t s i n Puer to Rico. According t o t h e World At l a s of Sea Surface Temperatures, Uavy Hydrographic Of f i ce (1944), t h e o f f sho re s e a su r f ace temperatures i n t h e v i c i n i t y of S t . Croix range from 25' C i n February t o 28' C i n Ju ly . Behind t h e r e e f s and r idges , e s p e c i a l l y dur ing q u i e t weather , t h e s e va lues a r e o f t e n modified and the lagoon normal maximum range e s p e c i a l l y n e a r shore i s about 23-30? C. The e a s t end o f S t . Croix i s r a t h e r dry wi th a y e a r l y r a i n f a l l o f about 30 inches , much of t h i s being concent ra ted i n t h e r a iny season from June t o December. Occasional very heavy r a i n s may reduce t h e s a l i n i t y below 35O/00 nea r shore where i n t e r m i t t e n t s t reams e n t e r t h e lagoons. In the v i c i n i t y o f t he r e e f s and r i d g e s , s a l i n i t i e s probably only very r a r e l y go below 340/00. DISTRIBUTION OF ALGAL RIDGES AND CORAL REEFS The l o c a t i o n s and depth p r o f i l e s of t h e major r ee f and r idge environments on t h e s h e l f o f e a s t e r n S t . Croix a r e shown i n f i g u r e s 1, 3, 4 . We have only v i s i t e d a s i n g l e - l o c a l i t y on Lang Bank, and it cons i s t ed l a r g e l y of a rubble-covered carbonate pavement wi th only s c a t t e r e d head c o r a l s and very few Acropora palmata. A s I w i l l elabo- r a t e below, it would appear t h a t Lang Bank, now a t 9-18 meters depth, - - - was cons t ruc ted on t h e s h e l f edge a t 25-30 meters dur ing t h e Holocene. We have only seen the western p a r t s of t h e Meandrina pavement on t h e e a s t e r n s h e l f (Fig. 9) , and i t s ex tens ion over t h e whole s h e l f remains t o be examined. The two channels t h a t were c u t ac ros s Long Reef on t h e south c o a s t (Fig. 1) extend i n t o t h e i n d u s t r i a l complex t h a t now occupies KrauseLagoon. The e a s t e r n channel which e n t e r s t h e Hess Ref inery , ex- poses t h e e n t i r e Holocene, t he Holocene-Pleistocene con tac t , and t h e uppermost 7 meters of t h e P l e i s tocene . Puch of our time-frame and s t r a t i g r a p h i c i n t e r p r e t a t i o n o f t h e c o r a l r e e f system on S t . Croix i s based on t h i s s ec t jon . Fig. 3. "Shelf" sec t ions on eas te rn S t . Croix, see f igure 1 f o r loca t ions . . . . . , . . . , 8 9 St. C roix Nautical M~ les . ' Inner s h e l f b a r r i e r c o r a l r e e f s and a l g a l r i dges on e a s t e r n S t . Croix )31 a l g a l r i dges , 0 t o +O. 5 m , and i n c i p i e n t r i dges 0 t o -1 m; " c o r a l ~ a l " r e e f f l a t s 0 t o -1.5m; dominantly Acropora palmata r e e f s , -1 m t o 10 m near E a s t P o i n t , -1 m t o -6 m i n western a r e a s ; deeper Montastrea a n n u l a r i s and Dip lo r i a spp. f o r e r e e f o f t e n sepa ra t ed from A. palmata r ee f by an i r r e g u l a r band of 11. 6 miles to Hess c e r v i c o r n i s , lower l i m i t on s h e l f a t depths of -15 t o 25 meters ; l leandrina hard ground, -25 m; sand, -2 t o -6 meters i n lagoon, -15 t o -25 meters on s h e l f . 2 I o I 2 k~lorneters 1 I 3 I Buck Island Channel Sect ion through nor thern b a r r i e r r e e f a s shown i n Sediment depths i n lagoon and Buck I s l a n d channel i n Figs . 7-9) a r e based on probes. Holocene + Ple~stocene overburden 8 3011 Caribbean Sea Sec t ion through Long Reef o f f Krause Lagoon - southern p a r t o f BB shown i n F igs . 1, 3; t h i s s e c t i o n i s based on a d ive i n t h e 60-foot deep s h i p channel through t h e r e e f . E leva t ions , depths and s c a l e a r e based on t h e USGS topographic s h e e t s and C&GS c h a r t 905. I n t h i s and t h e fol lowing f i q u r e s : / R 7 v e r i f i e d basement; 77777 assumed basement o r i n t e r f a c e . Fig. 4. (and 60- C bo- o t' m > 40- 2 2% 5 L 20- 40- L g a--20 u 80. 100- + 30 - 20 overburden 8 so11 -10 . -10 -30 Fig. 7 . Sec t ion through b a r r i e r r ee f and Beach a l g a l r i dge j u s t e a s t o f Grassy Po in t (F ig . 4 ) . Sediment depths between r ee f and r i d g e based on probes; i n d i c a t e s hard basement) g r e a t e r depth shown Under Long Reef the she l f i s about 12 meters deep, and j u s t ou t s ide the reef it i s about 13 meters deep (Figs. 5 , 1 2 ) . No ra i sed p la t form, r a i sed t e r r a c e o r r a i s e d biohermal s t r u c t u r e is evident beneath t h i s r ee f . Elsewhere, where information was obtained on basement depths in- s ide and ou t s ide , bu t not d i r e c t l y beneath the reef s t r u c t u r e (see e .g . , the reef sec t ions shown i n f igures 6-9) , I have assumed t h a t the reef s t r u c t u r e i s l y i n g on a more o r l e s s gently-sloping pre-Holocene topog- raphy and n o t on r idge o r mound l i k e reef o r aeol ian s t ruc tu res . The she l f i t s e l f i s r e l a t i v e l y shallow (10-15 m) a t i t s western ends both i n the western Buck Is land Channel on the nor th (Fig. 6 ) and along the south shore from Krause lagoon west. Further e a s t , around Grassy Po in t (Fig. 4) the she l f l i e s a t about 15-18 meters under the r ee f and about 20 meters ou t s ide (Fig. 7) , while t o the north o f f Boi ler Bay, a shallow she l f i n the Bay slopes t o about 20 meters j u s t of fshore and has given r i s e t o a t r ip le - ree f complex (Fig. 8 ) . A t Eas t Po in t , ou t s ide the r e e f , the she l f i s about 24 meters i n depth and, although the re i s no d i r e c t evidence, presumably it i s 18-20 meters under the reef i t s e l f (Fig. 9 ) . The r e e f s ly ing on the inner she l f from P u l l Point i n the nor th and Long Reef i n t h e south eastward t o East Po in t show a general p a t t e r n of decreasing maturi ty. In the west the r ee f f l a t s a re r e l a t i v e l y broad; they decrease i n width eastward and o f f Boi ler Bay i n the north and Grapetree Bay i n the south they become fragmented. From I saac Po in t around Eas t Po in t t o Lamb Point , reef f l a t s a r e v i r t u a l l y absent . Thus it appears t o be she l f depth (i. e . , pos i t ion of the pre-Holocene surface r e l a t i v e t o Holocehe sea l e v e l s ) t h a t has influenced the r a t e and p a t t e r n o f development of the inner she l f b a r r i e r reef system on S t . Croix. The p a t t e r n of development i n Long Reef (discussed below), a s shown by c14 da t ing and c o r a l paleoecology, ind ica tes why t h i s has happened. The reason f o r the west t o e a s t she l f s lope may l i e i n carbonate sedimenta- t i o n p a t t e r n s , i . e . , a general e a s t t o west reduction i n energy along with increased carbonate sediment load, a t high sea l e v e l s tands during t h e l a t e r P le is tocene , though s t r u c t u r a l cont ro l i s a l s o poss ib le . The imme- d i a t e she l f su r face probably i s an age equivalent t o the massive very shallow marine formation a t about 2 meters he igh t from H m ' s Bluff t o Cane Bay on the northwest shore of S t . Croix. We have two c o r a l da tes on t h a t formation both a t 20,000 years B.P. However, these a r e probably both "dead", a f f e c t e d by more recent carbon, and a r e l i k e l y about 120,000 years B.P. i n age. A s I d iscuss below, higher sea l e v e l s tands probably would no t have allowed the development of massive Holocene reef systems on t h i s s h e l f , much a s the present s i t u a t i o n i n t h e southeastern p a r t of the northern Virgin I s l and she l f (see Adey and Burke, 1975). The p s i t i o n s o f the co ra l r e e f s a s wel l a s the major a l g a l r idges and the l a r g e r s u b l i t t o r a l o r " inc ip ien t " a l g a l r idges a re shown i n f ig - ure 4. A s was pointed o u t above, the re a r e small r idges on the nor th s ide of Buck I s l and and on the south shore ou t s ide of Spring Bay and a t Vagthus Point . These were not s tudied i n d e t a i l and w i l l receive only occasional reference here. Fig. 9 . o I 2 7 k~lorneters 4 I (ft) (m) to 184 meters Sect ion through r e e f a t Eas t Po in t (F ig . 4 ) . Basement depth es t imated from previous s e c t i o n s . From -2 t o -13 m, t h e r ee f i s dominated by Acropora palmata on undercut blocks of A. palmata pavement s epa ra t ed by r u b b l e - f i l l e d grooves and bas ins . A narrow rubble-sand band wi th gor- gonians occurs a t -13 m. The r a t h e r narrow deep fore-reef i s charac- t e r i z e d by Acropora c e r v i c o r n i s , to 67 meters Montastrea annu la r i s and - Dip lo r i a labrynth i formis a l t e r n a t i n g wi th a 40 pavement occas iona l ly having gor- gonians and baske t sponges. This zone degenera tes t o a rubble s lope and then t o a narrow sand channel 140- 120- 100- 80- 60- 40- BO- a t 2 3 m. ?he e a s t e r l y l i m i t of t h e - 20 East Point Meandrina hard ground i s n o t known. 60- 40- 10 Co n 0- 20 Q, 10 eandr~na hardground 40- shelf edge ,-+ 63- - 20 3 F E 9 naut miles from Last Point a 0 3 100--7? ch" b a r r ~ e r reef Inner barr~er -10 40- 60- - 20 Bo- 3 -40 Fig . 8. - 30 of f - ly ing b a r r i e r r e e f s . B o i l e r I Bay basement depths based on I - 20 D 1 I p robes and cores ( s ee t e x t ) , f o re - I Boi ler Bay I r e e f sediment depths e s t i m a t e s . I I I Symbols a s Fig. 7 . CORAL REEFS The "reef f l a t " shown i n f igures 4, 10 and 2 7 i s genera l ly dominated by Acropora palmata. While some colonies of 5. palmata a r e a l i v e and ex- tend t o near mean low water spr ings , a l a r g e p a r t of t h e f l a t o r pavement surface a t 0.5-1 m below mean low water i s constructed of a matrix of dead arms of A. palmata coated with crus tose c o r a l l i n e , e spec ia l ly E- gonioli thon megacarpum and t o a l e s s e r ex ten t Neogoniolithon imbricatum and Poroli thon pachydermum. The ou te r edge o r c r e s t of the f l a t a l s o tends t o have a high proport ion of Millepora complanata, and the back f l a t sec t ions , a s they deepen i n t o the lagoon ( a t 2-6 meters) , frequently a l s o have abundant Montastrea annu la r i s , Diploria spp. , P o r i t e s p o r i t e s , and the small form of Acropora cervicornis . The Acropora palmata fore- reef i s usually s t rong ly dominated by t h a t species. I t extends t o depths of about 13 meters a t East Point , the lower boundary gradually r i s i n g t o the west probably because of decreasing l i g h t due t o t u r b i d i t y and per- haps p a r t l y due t o lessened wave ac t ion . Further west on the Tague Bay r e e f , on the south of t h e Buck Is land Channel, and on the western p a r t s of the south shore, the lower depth l i m i t i s 5-8 meters. The upper p a r t s of the - A. palmata f o r e r e e f , e spec ia l ly i n h ighly tu rbu len t a reas , o f t en cons i s t s of extensive dead areas o r pavements, heavi ly coated with crus- tose c o r a l l i n e s , mostly t h e same species dominating on the reef f l a t s (Fig. 11). These pavements, which a r e e s s e n t i a l l y A. palmata frameworks extensively f i l l e d with crus tose c o r a l l i n e and apparently without major amounts of submarine cementation, can g ive r i s e t o t h e " inc ip ient" a l g a l r idges discussed below. I n the r e l a t i v e l y q u i e t Buck Is land Channel, an i r r e g u l a r band of Por i t e s p o r i t e s o f t e n extends from the base of t h e - A. palmata fore reef t o the sand channel f l o o r a t 10-12 meters. However, f u r t h e r e a s t and on the south shore, t h i s zone i s usual ly occupied by Acropora cervicornis . The A. cervicornis band can be extensive, tens of meters wide, o r on the o t h e r hand sometimes it e x i s t s only a s s c a t t e r e d patches. From the lower end of the - A. palmata fore reef t o the sandy s h e l f , the dominant c o r a l i s usual ly Montastrea annu la r i s , though Diplor ia spp. a r e common and sca t - t e r e d - A. ce rv icorn i s , wi th an occasional ;. palmata, a l s o occur. Occasionally a marked spur and groove p a t t e r n occurs i n the lower fo re r ee f and sometimes it extends p a r t l y i n t o the shallower 5. palmata zone (see Fig. 27). The lower boundary of t h e deeper fore-reef i s sometimes marked by a more o r l e s s abrupt drop of 1-2 meters t o the sediment in te r face . The f l a t , sandy she l f i s found d i r e c t l y below t h e Montastrea- Diploria deep fo re reef and t h i s genera l ly extends almost t o t h e shel f margin. Off East Point , however, a s shown i n f igure 4 , the sand band i s narrow, and the she l f beyond is coated with a pavement o r hard ground. The dominant co ra l on t h e hard ground i s Meandrina meandrites, although Montastrea cavernosa, S ide ras t rea s ide rea , Diploria s t r i g o s a , Dichocoenia s t o k e s i i and Zoanthus spp. a l s o occur. Approximately 75% of the surface i s a coral-bare pavement with abundant gorgonians and sponges. The sub- surface nature of the hard ground was not s tudied . The wrapped-back westward-curving p a t t e r n of the hard ground and sand boundary seen i n f igure 4 suggests t h a t wave and current ac t ion on the she l f gradually moves much of the carbonate sediment produced on the hard ground westward. Fig. 10. Northern b a r r i e r r e e f o f f western Tague Bay ( s ee F ig . 4 ) . This a e r i a l photograph was taken looking n o r t h towards Buck I s l a n d channel a t an angle of 60-80' from h o r i z o n t a l . A s e c t i o n through t h i s a r e a was desc r ibed by Ogden e t a l . 1972. The r e e f i s about 140 m wide here . mostly living pavement lagoon, o u t ~ r par A. palmata area w i th abundant Montast rea ann Fig. 11. Aer ia l photograph of Lamb Inc ip ien t a l g a l r idge. P extensive Acropora palmata pavements. A small i n c i p i e n t a l g a l r idge lobe. No well-defined reef f l a t e x i s t s i n t h i s a rea . When t h i s combines with the probably much g r e a t e r quan t i ty of sediment pouring o f f the A. palmata and Montastrea r e e f , formation of a hard ground i s prevented by an excessive "bed-load" of carbonate sand and s i l t . Thus, the westward-widening sand channel o f f Eas t Point (Fig. 4 ) . The Meandrina hard ground i s espec ia l ly i n t e r e s t i n g and i n need of ex- tended study because o f i t s poss ib le importance t o shelf-building i n the Ant i l l e s . A d e t a i l of the Hess channel sec t ion through Long Reef i s shown i n f igure 12. This r ee f appeared t o be a r a t h e r loose aggregation of co ra l fragments, although t h i s might have resu l t ed from b l a s t i n g i n the channel. I n the upper p a r t of the sec t ion , the c o r a l was f requent ly coated with up t o 1 cm of c o r a l l i n e algae and Homotrema, however, t h e r e was no e v i - dence of e i t h e r inter-fragment connection of c r u s t s o r of secondary cementation. In the lower sec t ion , the re was no evidence of encrus ta t ion on the co ra l , while a considerable amount of surface boring was present . The dates shown were taken from the q u i t e "clean" cen te r s of l a rge co ra l pieces. The upper p a r t of the underlying carbonate basement i s well cemented and cons i s t s of coral l ine-coated P o r i t e s p o r i t e s and mollusc s h e l l frag- ments i n a'matrix of f ine-grained carbonate. The upper surface immedi- a t e l y below the ree f i s blackened t o a depth of 5-10 mm and p i t t e d much l i k e weathered carbonate surfaces subaer i a l ly exposed on S t . Croix. Ian Macintyre (Smithsonian I n s t i t u t i o n ) ran an x-ray d i f f r a c t i o n analys is on a blackened P o r i t e s p o r i t e s fragment from t h i s c r u s t f inding no aragoni te and with low-magnesium c a l c i t e dominant, f u r t h e r i n d i c a t i n g t h a t i s i s a subaer i a l ly weathered Ple is tocene surface. This Por i t e s - she l l member grades downward i n t o a poorly indurated carbonate c lay with s c a t t e r e d small s h e l l fragments having a t r a c e of intermixed non-carbonate minerals. The d - 4 ages shown i n f igure 12 a r e p l o t t e d a s a function of depth below presen t sea l e v e l i n f igure 13. The Neumann sea-level curve f o r Bermuda- (Neumann, i n m s . ) i s a l so p l o t t e d f o r reference , assuming negl i - g i b l e t e c t o n i c a c t i v i t y on S t . Croix during the Holocene. This reef was apparently not i n i t i a t e d u n t i l about 2500 years a f t e r sea l e v e l rose over i t s foundation, probably the time required t o c l e a r the Wisconsin rego- l i t h from t h e she l f surface and move it west beyond Krause lagoon. A t t h i s po in t , 4400 years B . P . , the water depth would have been about 7 meters, and a t t h i s r e l a t i v e l y tu rb id loca t ion the dominant c o r a l s would be Montastrea and Diplor ia spp. For about 1400 years t h e r e a f t e r , s ea l e v e l rose a t 2.4 mrn/year exceeding the r a t e o f r ee f growth, and the average water depth from 3000 t o 4000 years B.P. was increased t o about 9 meters. The r e s u l t i n g r a t e of reef growth f o r t h i s i n t e r v a l 1 . 3 mm/yr, i s c lose t o t h a t obtained by Goreau and Land (1973) f o r reef growth a t 25 m on the nor th coas t of Jamaica. A t about 3000 years B . P . , sea l e v e l r i s e slowed to about 0.4 mm/year and continued reef accumulation shallowed water depths over the r e e f . A t about 1000 years B.P. Acropora palmata became es tab l i shed on Long Reef a t a depth of about 4.5 meters, reef up- ward growth became q u i t e r ap id (15 mm/year) and depths c lose t o the present reef f l a t (-1 meter) would have been reached a t about 500 years B.P. Reef f l a t accumulation a t t h i s r e l a t i v e l y q u i t e t u r b i d , westerly locat ion , with new r e e f s forming t o the e a s t , is probably very slow as shown i n f igure 13. East I Reef f l a t I Channel 2 0 - 30 - Montastrea - 10 Pleistocene Pleistocene (marl) 01 w - - - - - - - - - - - clay marl West Fig. 1 2 . view from above West wall of the ship channel cu t through Long Reef o f f the Hess O i l Refii~r a t Krause Lagoon. Collect ions were made independently by four divers and pooled l a t e r . Almost a l l samples taken from 4.5-13 m were Diploria ([rrml) and Montastrea (m). One Sideras t rea s iderea (1) was collected. Most of the samples taken above 4.5 m were Acropora palmata. A s discussed above, the inshore pre-Holocene she l f on southern and eas te rn S t . Croix s lopes i r r e g u l a r l y eastward. Even allowing l e s s time f o r reef establishment on these more e a s t e r l y shores , bu t considering t h e more rapid r i s e of sea l e v e l p r i o r t o 7000 years B.P. a t -12 meters, these r e e f s , a t the same growth r a t e s , would have been slower t o reach the surface than those t o the west. Thus, the genera l p a t t e r n of reef morphology, more mature t o the west and l e s s mature eastward. Applying the same reasoning t o the easternmost margin of the shel f and assuming no marginal pre-Holocene r idge , t h e she l f edge a t about -25 meters would have been covered by a r ap id ly r i s i n g sea l e v e l a t about 9500 years B.P. Taking the average depth of Lang Bank as about 45 f e e t and using a r a t e of 1 . 3 mm/year, the Bank could have been developed i n i t s p resen t proport ions by beginning i t s Holocene ree f growth within a few hundred years a f t e r being placed i n the marine environment. This i s i n agreement with the conclusion reached by Macintyre (1972) t h a t the sub- merged ( o r marginal s h e l f ) r e e f s of the eas te rn Caribbean a re Holocene (o r l a t e s t P le i s tocene) . These r e l a t i o n s a l s o i n d i c a t e t h a t i f sea l e v e l remains near i t s p resen t l e v e l f o r another 4-5000 years t h a t a shel f edge reef w i l l form a t Lang Bank, thereby depriving the inner r e e f s and r idges of wave action. kdey and Burke (1975) have indica ted t h a t pre-Holocene she l f depth i s one of the primary f a c t o r s con t ro l l ing reef presence and placement i n the eas te rn Caribbean. B a r r i e r r e e f s only occur where she l f depths a r e l e s s than 25 meters and the most mature systems where she l f depths a r e l e s s than 15 meters. When adversely comparing P a c i f i c a t o l l r e e f s with those of the Caribbean, t h i s should be borne i n mind. For example (Lalou, e t a l . , 1966) showed t h a t the pre-Holocene topography under Muroroa reef i s l a n d s was only a t 6-10 meters, and Wiens (1962) c i t e s fore-reef t e r r a c e depths o f l e s s than 20 meters f o r B ik in i , Eniwetok, Fongelap, Zohhoiiyoro and Raroia. This suggests t h a t perhaps major d i f ferences between P a c i f i c and Caribbean ree f development a r e n o t Holo- cene b io log ica l o r c l i m a t i c problems b u t Ple is tocene geological problems, i . e . , t h a t the eas te rn Caribbean pre-Holocene shelves a r e r e l a t i v e l y deep when compared t o those of P a c i f i c a t o l l s . Several patch r e e f s nor theas t of Buck I s l and r i s i n g from a sandy she l f a t about 10 t o 12 meters appear t o be anastornosing t h i c k e t s of massive Acropora palmata from bottom t o top. Also, recent ly Macintyre and Glynn (1974) d r i l l e d an i n t e r t i d a l r ee f f l a t o f f Caribbean Panama and found up t o 11 meters of Acropora palmata most of which accumulated from 6000 t o 2000 years B.P. The top 0.5-3 meters of t h i s r ee f was younger reef f l a t dominated by Millepora, Agaricia m, P o r i t e s fu rca ta and c o r a l l i n e a l g a l accumulation. This i n d i c a t e s t h a t given proper she l f depths, perhaps 5-15 meters, and favorable environmental condit ions, t h a t extens ive growth of 5. palmata can s t a r t s h o r t l y a f t e r submergence and develop reef s t r u c t u r e s dominantly of t h i s co ra l . Bolier Bay Algal Rdge ALGAL RIDGES In tu rbu len t a reas open t o the e a s t e r l y t rade wind sea , the upper- most Aczopora palmata fore r e e f , a t depths of 0 t o -2 m, i s f requent ly an i r r e g u l a r coral l ine-encrusted pavement (Fig. 11). Sometimes the o r i g i n a l o u t l i n e of the A. palmata is obvious and a broken branch w i l l show a c o r a l l i n e acc re t i zn of a few mm t o many cm on the dead cora l . In o the r cases , even though a vague o u t l i n e of the o r i g i n a l A. palmata arms i s s t i l l p resen t , one can break o f f p ro jec t ion a f t e r p ro jec t ion and f ind only c o r a l l i n e . In many of these a reas , e . g. , of f Fancy Mountain (Fig. 22) , Robin Bay, western Jack Bay, I saac Point (Fig. 19) and i n Lamb Cove, j u s t shoreward of these pavements, the - A . palmata shapes gradually d i s - appear t o be replaced by an i r r e g u l a r c o r a l l i n e pavement which s lopes up t o about low water l eve l s . Several Neogoniolithon species a r e important on these pavements and Poroli thon pachydermum i s sometimes conspicuous. I f the pavement extends t o near low water l e v e l s , Lithophyllum congestum may be abundant, build- ing small i r r e g u l a r heads extending above mean low water. Under over- hangs, and i n the abundant holes Lithothamnium r u p t i l e , Mesophyllum syntrophicum and sometimes Archeolithothamnium dimotum along with IIomotrema a r e a l s o responsible f o r considerable carbonate accre t ion . We have cored two of these coralline-A. - palmata pavements o r " i n c i p i e n t a l g a l r idges" near t h e i r c r e s t s a t about mean low water spr ings . During the f i r s t , a t Jack Inc ip ien t Ridge, our d r i l l t rans- mission f a i l e d a t 80 cm of near ly s o l i d c o r a l l i n e c r u s t with about 50% recovery. The upper p a r t o f t h e core i s q u i t e f r e s h i n appearance. Further down, although c r u s t c o r a l l i n e s a r e s t i l l dominant, much boring, and r e - f i l l of c a v i t i e s by sediment and minera l iza t ion has a l t e r e d the cores. The a l t e r a t i o n and minera l iza t ion of these a l g a l s t r u c t u r e s appears t o be s imi la r t o t h a t described f o r the Bermudian cup r e e f s by Ginsburg and Schroeder (1973), and I have n o t ca r r i ed ou t a d e t a i l e d examination of these processes i n t h e Cruzan r idges. The i n c i p i e n t a l g a l r idge d r i l l e d was on the o u t e r reef south of Fancy Mountain (Fig. 22) . Here we d r i l l e d through almost 2 meters of l i t t l e - a l t e r e d c o r a l l i n e , with a l i t t l e Lithophyllum presen t , b u t mostly c r u s t species with s c a t t e r e d Homotrema. Although occasionally the vermetid Dendropoma is important i n these a l g a l s t r u c t u r e s , i n c i p i e n t a s well a s high r idges , it i s more o f t en absent o r a very minor element. This d i f f e r s r a t h e r markedly from the Ginsburg and Schroeder (1.c.) desc r ip t ion of the Bermudian cup ree f s . From 2 t o 3 meters i n the Fancy i n c i p i e n t hole , with only about 10% re- covery only cora l was found. This was dominantly Acropora palmata although Diploria species and some P o r i t e s as t reoides a l s o occurred. Immediately below the c o r a l l i n e cap, a ~ 1 4 date of 355 years B.P. was obtained i n a l a rge , clean core of A. palmata, thus providing an accre t ion rate i n t h i s i n c i p i e n t a l g a l r idge of 5.6 mm/year. Adey and Vassar (1975) have s tud ied crustose c o r a l l i n e accre t ion r a t e s on p l a t e s i n reef and r idge environments on S t . Croix, f inding values of up t o 5.2 mm/year. They a t t r i b u t e high accre t ion r a t e s i n turbulent environments t o reduced grazing ac t ion espec ia l ly by p a r r o t f i s h , and show t h a t i n q u i e t a reas where t h e s e f i s h can operate e f f e c t i v e l y n e t c o r a l l i n e accre t ion can be reduced t o 0.5 t o 1 mm/year. Once an i n c i p i e n t r idge bu i lds t o e l eva t ions near mean low water, branching Lithophyllum congestum of ten becomes dominant, a s it has on some of the i n c i p i e n t knobs o f f Lamb Cove. Steneck and Adey (1975) have shown t h i s p l a n t t o dominate highly i n tu rbu len t a reas a t - + 10 cm of m.1.w. spr ings and t o achieve average branch t i p growth r a t e s of 8 mm/year and accre t ion r a t e s o f 4 mm/year. Within 10-15 cm of the su r face of a Lithophyllum congestum head, the i n t e r s t i t i a l space between the branches i s o f t en f i l l e d with wave-driven sediment t h a t i s subsequently - - cemented by high magnesium c a l c i t e i n t o a hammer-ringing hard limestone. Except around holes o r channels, Lithophyllum congestum i s replaced by Poroli thon pachydermum a t r idge e leva t ions above about 20 cm above mean low water sp r ings . In areas o f e spec ia l ly high turbulence, the l a t t e r p l a n t i s capable of bui ld ing the r idge t o l e v e l s of about 50 cm above m.l.w.sp., i. e . , about 20 cm above mean high water spr ings (see Fig. 2 ) . ( ~ d e y and Burke (1975) have reported o t h e r a l g a l r idges i n the consider- ably rougher Martinique-Guadeloupe area t h a t reach heights of one meter above m.l.w.sp., approximately the he igh t o f the higher P a c i f i c a t o l l r idges. ) In the small , open cove j u s t t o t h e south of East Point , t h e i r r egu- l a r Acropora palmata pavement blocks a t depths o f 2 meters fuse i n t o lobed c o r a l l i n e pavements t h a t extend t o e l eva t ions of up t o about 30 cm above m . 1 . w . s ~ . These lobes, dominated by Lithophyllum congestum, with r a i sed rims and weakly-developed bowls behind and separated by rubble channels 1-2 meters deep, have n o t developed extensive overhangs o r i n t e r - lobe fusions. Although we have no subsurface information on t h i s small a l g a l r idge , i t s surface configurat ion as compared t o the well-developed r idges and t h e immaturity of the o f f l y i n g A. palmata reef suggests t h a t it i s the youngest of the Cruzan r idges with a thickness of only 2-4 meters and has e x i s t e d a s a high r idge f o r only a few hundred years . Since t h i s r idge provides an intermediate developmental s tage between the i n c i p i e n t r idge condit ion now f a i r l y abundant on the maturing 2. palmata r e e f s a t the eas te rn end of S t . Croix and t h e o l d e r high a l g a l r idges , it would be espec ia l ly des i rab le t o core it. I t is q u i t e rough however, and it would probably be necessary t o cons t ruct a l a r g e , hlgh platform from which t h e d r i l l i n g could be undertaken. I w i l l begin the discussion of the major high a l g a l r idges on S t . Croix with the complex o f f Robin Point on the south shore (Figs. 14-17). This r idge l i e s on a southwesterly p ro jec t ion of the reef system. I t i s open t o t h e e a s t e r l y sea, almost perpendicular t o the d i r e c t i o n of the wave t r a i n and i s general ly t h e roughest and most ac t ive of the Cruzan r idges . In the o u t e r l i n e , the re a r e some small indiv idual b o i l e r s o r cup r e e f s from 2 t o 3 m i n diameter wi th well-developed ra ised r i m s a l l around and marked c e n t r a l depressions. Other b o i l e r s range up t o about 30 m i n diameter with the highly r a i s e d r i m s being present only on the seaward margins. I n the l a t t e r case , t h e c e n t r a l bas ins a r e o f t e n e i t h e r open on the back o r with a s l i g h t r i m , and a r e r e l a t i v e l y deep, about 1 m , o f t e n with l a r g e Diploria heads. These l a r g e r r idge elements a r e formed by s e r i e s of fused individual b o i l e r s (Fig. 15) . The l a r g e s t of these , platform reef t ransected by sec t ion AA', i s about 120 m long and averages about 40 m i n width. I n p laces , t h e junction between fused b o i l e r s i s marked by t h e s t i l l r a i s e d r i m s and by a conspicuous s l o t occasionally wide enough f o r an arm o r pole. In o the r p laces , the re i s l i t t l e d i r e c t surface evidence of the fus ion, except f o r the obvious inward curving of the two b o i l e r s where they meet marginally. However, underneath i n t h i s case, the re i s usual ly a q u i t e open channel wide enough f o r a d ive r t o s w i m through. Many o f these ou te r r idge elements are honeycombed with such caverns, the rounded wal ls being coated with many l a y e r s o f shade c o r a l l i n e spec ies a s well a s Homotrema and Squamariacea. The waters immediately around the high a l g a l r idges o f t e n a r e r e l a t i v e l y deep, 3-6 meters, and i n addi t ion t o rubble and sand patches , i n some o f the channels, the pavements o f t e n support a community of Diploria spp., Millepora and Montastrea annu la r i s . I n t h i s environment, these c o r a l s tend t o be s c a t t e r e d b u t q u i t e l a rge . I n the deeper a r e a a t 5-6 m i n f r o n t of Robin Ridge, some Diplor ia colonies 1-2 m i n diameter extend near ly t o the surface . We were able t o d r i l l t h ree holes on Robin Algal Ridge, but none of these extended t o t h e basement. The deepest of these , on the o u t e r p a r t of the high r idge on sec t ion AA, broke o u t i n t o an underlying cavern a t about 3.5 meters and was terminated. The core i n t h i s case was dominated by branched heads of Lithophyllum congestum t o a depth of about 2 meters with c r u s t species and abundant Homtrema below. Using the Neumann s e a l e v e l curve (Fig. 1 3 ) , t h i s i n d i c a t e s t h a t t h i s r idge has been growing a t o r nea r mean sea l e v e l a t l e a s t s ince 3300 years B.P. The remaining two Robin cores a r e s h o r t , 1 m, and were placed i n the back r idge area on sec t ion AA'. The surface o f t h i s "r idge" i s now about one m below m . 1 . w . s ~ . and has l i t t l e c o r a l l i n e on i t s surface. Diadema an t i l l a rum i s abundant and the p i t t e d , scraped and gray carbonate surface i s obviously being removed by grazing. However, below severa l cm, t h e cores a re dominantly crus tose co ra l l ine . This suggests t h a t t h i s s t r u c t u r e was once a high r idge , perhaps o l d e r than t h e o u t e r s e r i e s , t h a t was subsequently blocked from the required wave ac t ion by the development of the younger o u t e r s e r i e s . Shoreward o f t h i s second l i n e , the re is a t h i r d s e r i e s of r idge- l ike s t r u c t u r e s which may a l so be degenerating high r idges . Present ly , seaward and southeas t of Robin Ridge an a c t i v e Acropora ~ a l m a t a reef i s developing. This i s shown cn both f igures 14 and 16. The surface of t h i s reef i s s t i l l 2-4 meters below sea l e v e l and only occasionally do waves break on it. However, it has l i k e l y already reduced some of the wave energy del ivered t o the r idge complex and i s probably p a r t l y responsible f o r the degeneration of the back r idge system. If t h i s r ee f has a growth r a t e comparable t o t h a t of Long Reef, a well- developed r e e f f l a t w i l l have formed wi th in 6-800 years and Robin Ridge wilL be i n f u l l degeneration. I saac Algal Ridge o f f I saac Po in t i s , i n a rea , one of the smal les t high a l g a l r idges. However, being q u i t e exposed, it has r i m he igh t s , about 50 cm above m.l.w.sp., equal t o any known on Robin Ridge. There a re a few off - ly ing individual b o i l e r s i n the southwestern p a r t of the Fig. 14. Aer i a l photograph of Robin a l g a l r idge . Compare wi th map (Fig. 1 6 ) . Note A. palmata r e e f b u i l d i n g everywhere seaward of t h e r i d g e except i n t h e narrow sand channel (mid-r ight) . Fig. 15. Large s c a l e a e r i a l photo of the c e n t r a l sec t ion of Robin r idge . (See Fig. 16 f o r loca t ion of MI.) The fused "boi ler" o r ig in of a l a r g e p a r t of t h i s r idge i s evident here. See f igure 17 f o r depths. Figure 16 Robin Bay 1 Rod Bay I Fig. 16. Robin a l g a l r idge complex and the surrounding cora l communities. Indica ted sec t ions a r e shown i n Figure 16. Ridge e leva t ions r e l a t i v e t o m.1.u. sp. : 1 +15 t o +50 cm, -10 t o +15 cm, -100 t o -10 cm, m -200 t o -100 cm. The following symbols a r e used on a l l of maps below: \ - =- l i v i n g Acropora palmata iA~F$-living P o r i t e s p o r i t e s 0 collapsed blocks of a l g a l r idge dead standing -!-?;dead o r P o r i t e s pavement B ~ a l e d o n i a bedrock rubble pavement Y l i v i n g Acropora ce rv icorn i s ;/ dead \/ \/ , , '( pavement l i v i n g Diplor ia spp. o S ide ras t rea s ide rea dead 0 P o r i t e s a s t r eo ides l i v i n g Montastrea annular is <-,> dead l i v i n g Millepora dead 0 sand beach rock V dominantly V dominantly Tha l l a s s i a g rass Syringodium grass 20 Fig. W Y W o short cores - G e e t e x t l I Sections of Robin a l g a l r idge complex shown i n f igures 14-16. r idge system, and i n t h e e a s t e r n corner only a p a r t i a l fusion of severa l b o i l e r s has taken place (Figs. 18, 1 9 ) . These have the t y p i c a l high, f l e shy algae covered r i m s , t he uppermost sec t ions being r e l a t i v e l y smooth- l y encrusted with Poroli thon pachydermum and the f r o n t o r lower areas dominated by Lithophyllum congestum. The higher p a r t s of r i m and fore- r idge areas have few Echinometra and tend t o be r e l a t i v e l y smooth. How- ever , the upper back r idge a r e a s a t approximately -20 t o +20 cm a r e heavi ly i n f e s t e d with these echinoids and t h e i r burrows, making walking very d i f f i c u l t . The elongate main sec t ion of the r idge has developed by a fusion of b o r l e r s . This i s q u i t e apparent i n t h e e a s t e r n sec t ion where one can s t i l l s w i m through some of the channels below. However, i n the c e n t r a l and western s e c t i o n s , the only obvious evidence f o r the o r i g i n a l b o i l e r s t r u c t u r e i s the presence o f sca t t e red closed caves a t l e a s t one of which extends back t o the r idge " f l a t " a s a very narrow s l o t . The r e s u l t of b o i l e r fusion over a long t r a c t has been the exclusion of wave ac t ion from the back r idge a reas , and a pronounced narrowing of the high r idge zone. The back r idge " f l a t " i s now a t about -1 m and i s dominated by P o r i t e s p o r i t e s and P o r i t e s as t reoides . I t should be pointed out here t h a t while the P a c i f i c a t o l l a l g a l reef margins a re c a l l e d r idges , they a r e more o r l e s s formed of b o i l e r s a s described here. This i s we l l shown by the massive, b o i l e r b u i l t r idge system described f o r Muroroa by Chevalier e t a l . (1968) and a s diagrammed by Emery, Tracey and Ladd (1954) i n t h e i r c l a s s i c t reatment of the Bikin i r idges. The southeas t corner o f I saac Ridge has a q u i t e low r i m , and the collapsed blocks j u s t t o the ou t s ide have apparently broken o u t of t h i s pos i t ion . On the eas te rn corner of t h i s a r e a , an unfused b o i l e r has f a l l e n over, with the upper su r face now r e s t i n g a t about 45'. The upper por t ions of t h i s conspicuous t i l t e d b o i l e r now p r o j e c t near ly a meter above low water and have t h e c h a r a c t e r i s t i c p i t t e d surface o f coas ta l subaer i a l eroding limestone (see Fig. 18) . J u s t ou t s ide of t h i s t i l t e d block, a new b o i l e r i s developing, having reached the i n c i p i e n t s tage j u s t below m . 1 . w . s ~ . This i n c i p i e n t b o i l e r can be t r aced seaward i n t o t h e Acropora palmata pavement on which it is developing. A s i n g l e core hole has been placed i n one of t h e eas te rn lobes of I saac Ridge. The dominant elements encountered a r e shown i n f igure 2 1 . A t about 8 meters, a mixture o f Caledonia and cora l fragments were en- countered which made d r i l l i n g v i r t u a l l y impossible with our r i g . I have i n t e r p r e t e d t h i s a s a Caledonia bedrock surface , b u t it has no t been es tab l i shed with c e r t a i n t y t h a t it i s no t a weathered, P le is tocene reef surface with te r r igenous pebbles and cobbles. A sand probe t o the out- s i d e of the r idge, a s shown, d id not reach a hard surface a t 15 meters, thus ind ica t ing t h a t the basa l s t r u c t u r e of the a l g a l r idge i s formed on a r a i s e d bench s t r u c t u r e a t 9 meters, a t l e a s t 6 meters and probably 8-10 meters above the o f f l y i n g s h e l f . A da te of 4040 years B.P. was obtained from a l a r g e , c lean core o f Acropora palmata taken a f o o t o r two above the basement. Unfortunately, t h i s hole was not p laced on a major r idge lobe, b u t r a t h e r on a minor lobe connecting two b o i l e r s . Thus, only 2.5 meters of c o r a l l i n e , mostly - L. congestum, was encountered here , and from 3.5-6 meters a s i n g l e massive Diploria head, of the type , tilted boiler Fig. 18. Photograph of Isaac Algal Ridge on a r e l a t i v e l y q u i e t day with a t i d e l e v e l of about i -20 cm. Note f igure standing (bent over) near the center . Fig. 19. Aerial photo of I saac Algal Ridge taken a t about 600 f e e t on an extremely q u i e t day. See f igure 20 f o r s c a l e . Fig . 20. I s a a c Algal Ridge and surrounding c o r a l communities. +25 t o +50 cm, 0 t o +25 cm, IEff3 -30 t o 0 cm. Symbols a s given f o r f i g u r e 16. @ Caledonia pebbles and cobbles . Isaac's Bay I: dominantly r' Actopora almata Core, A- rnl w sp. $ 6 s 2 W ---- A A P A A d . Cfct.ceou. Crust- Call l ine Ncogon~olithon L Parolothon Lnthophyllurn conprRum Poritc!, p o r l t s Poriteo asteo~des - - - _ _ A A Fig. 21. Sect ions across I saac Algal Ridge a s shown i n Figure 20. I n the cores I/;\',-I g. congestun, c r u s t c o r a l l i n e s , a c o r a l rubble. The sec t ion N1' i s an i n t e r p r e t a t i o n based on the su;-face c h a r a c t e r i s t i c s of the b o i l e r s making up t h i s sec t ion of the r e e f . normally encountered i n deeper i n t e r - b o i l e r channels, was cored. Below 6.5 meters, t h e sub-ridge s t r u c t u r e was dominantly A. palmata. Based on the su r face p lan of the r idge i n t h i s a r e a , an i n t e r p r e t i v e sec t ion p a r a l l e l t o the r idge f r o n t (AA') i s a l s o shown i n f igure 21. The depth-age re l a t ionsh ips o f I saac Ridge a r e shown i n f igure 13 and i n t e r p r e t e d a s follows: Sea l e v e l rose over t h e r a i sed beach o f f I saac P o i n t a t about 5700 years B.P. About 1600 years was required t o e s t a b l i s h s t rong reef growth on t h i s bench. Since sea l e v e l was only about 5 meters above the bench i n the eas te rn a r e a , Acropora palmata dominated and quickly b u i l t the reef t o wi th in 1 .5 t o 2 meters of sea l e v e l . There, c r u s t type c o r a l l i n e s b u i l t t he r idge t o sea l e v e l , a s an i n c i p i e n t r idge , by about 3200 years B.P. (determined by p o s i t i o n of Lithophyllum congestum i n the c o r e ) . From t h a t time u n t i l about 1300 years B.P., the r idge was dominated by L. congestum i n the area cored and probably had a he ight of 0-25 cm above m . 1 . w . s ~ . The l a s t p a r t of the r idge sec t ion cons i s t s o f c o r a l l i n e c r u s t species , ind ica t ing t h a t s ince about 1300 years B.P., t h i s p a r t of the r idge has maintained i t s p resen t he ight . The developing - A. palmata reef o f f I saac Algal Ridge is s t i l l r e l a t i v e l y deep, 3-4 meters, although d i r e c t l y t o the e a s t o f f I saac Bay a sec t ion of reef i s approaching the surface. Probably it w i l l be more than 1000 years y e t before I saac r idge w i l l be blocked t o the po in t of se r ious degeneration. Figures 22 and 23 sho3 the a l g a l r idge p a i r and i t s associa ted i n c i p i e n t r idges o f f Fancy Mountain. These r idges a r e both r e l a t i v e l y low and degenerating due t o wave-blocking by the A. palmata reef forming outs ide . The inner r idge being p a r t l y blocked b y t h e o u t e r r idge reaches maximum heights of only +26 cm above m . 1 . w . s ~ . having an average e levat ion of +10 t o +17 cm. The o u t e r r idge averages +17 t o +23 cm. The core hole i n the inner r idge returned a c lean , 8 cm sec t ion o f Caledonia with p a r a l l e l top and bottom f r a c t u r e planes a t 8 meters. The boring then broke i n t o co ra l rubble and jammed with hard d r i l l i n g a t 9 meters. I have in te rp re ted t h i s a s shown i n f igure 23. While here a l s o the p iece o f Caledonia could have been a cobble on a weathered reef sur- face, t h e r e i s no evidence of a rounded nature on e i t h e r end of the core and bedrock seems the most l i k e l y i n t e r p r e t a t i o n . The c r u s t c o r a l l i n e i n the co re begins a t 6 meters, and L. congestum a t 4 meters, ind ica t ing t h a t the co ra l reef s t r u c t u r e developed on the ledge shown a t about 4800 years B.P. had become an i n c i p i e n t r idge by 4500 years B.P . and f i n a l l y a high r idge by 4300 B.P. (Fig. 13) . The care i n the o u t e r r idge i s dominated by L. congestum only i n the upper 1 . 5 ~ n e t e r s and becomes mixed with Millepora and then Montastrea a t 2.5 meters. Thus, the ou te r r idge i s considerably younger than the inner , s t a r t i n g a t about 3000 years B.P. I t s pos i t ion i n l i n e with t h e p resen t Acropora palmata reef c r e s t and i n the v i c i n i t y of a number of presently-forming i n c i p i e n t r idges suggests the presence of a secondary bench o r s e r i e s of benches, i n e l eva t ion somewhere between the high bench with the inner r idge a t 8 meters and the ou te r she l f a t approximately 15' meters. Fig . . . inc~p~ent algal r~dges ie algal pavements . . : . . . . ..' . . . . . . . . . b dominantly A.palmata 4 3 / F & & & P a ~ m a t a ~ n c ~ p ~ e n t reef f lat 6 . ridges .. 2 2 . N a p o f Fancy Mountain a l g a l r i d g e p a i r and i t s a s soc i a t ed i n c i p i e n t a l g a l r i dges and c o r a l communi.ties. Sec t ion shown i n f i g u r e 23. +15 t o +30 cm above m.l.b:.sp., -10 t o cm, -100 t o -10 cm. r ee f +15 Fig. 23 . Sect ion from shore a t Fancy Mountain, across Fancy a l g a l r idges t o the o u t e r s h e l f . Symbols a s previous f igures . I,nl i n f e r r e d head cora l s Diplor ia spp. and Montastrea annular is . Beach Algal Ridge, between Grassy and Grapetree Po in t s (Figs. 24-27) i s the longes t (over 0.5 km) and s t r a i g h t e s t r idge on S t . Croix. A l - though some narrow caverns a r e present extending back i n t o the r idge , the re i s l i t t l e surface evidence of b o i l e r amalgamation. I t i s probably the o l d e s t Holocene r idge on eas tern S t . Croix, 5000 years o r more, a l - though we do n o t have a core t o e s t a b l i s h t h i s . Beach Ridge i s not high, the maximum e leva t ion measured being 22 cm above m.l.w.sp., and it i s a l s o being blocked by an off - ly ing reef system. I n the eas te rn p a r t of the area the of f - ly ing ree f has already developed a r ee f f l a t and t o the westward it gradually decreases i n height . The a l g a l r idge behind i s inverse ly propor t ional i n i t s height t o t h a t of the r e e f . Behind the reef f l a t a reas , it has degenerated t o sub t ida l l e v e l s . Beach Algal Ridge apparently l i e s on a bench t h a t i s g r e a t e r than about 8 meters i n depth, b u t probably considerably l e s s than the she l f depth here of about 19 meters. Unless reef growth was p a r t i c u l a r l y favorable a t t h i s l o c a l e , which doesn ' t seem l i k e l y because of i t s em- bayed nature , the maximum bench depth would be 10-12 meters. This i n f e r r e d bench is conspicuously al igned with an i n f e r r e d f a u l t scarp on the i s l and (see Fig. 27) . Beach Ridge a l s o grades i n t o an A. palmata- Millepora r i c h ree f t o the west. This could r e s u l t from a s loping of the underlying bench t o the west t o merge with t h e s h e l f . The sand channel i n f r o n t of Beach Ridge i s q u i t e deep and abrupt. I t is mostly 8 t o 10 meters i n depth, but reaches 13 meters i n i t s western sec t ions . I t is bigger and much b e t t e r defined than any of o t h e r fo re r idge channels known on S t . Croix, perhaps because of the a s soc ia t ion with a pronounced scarp s t r u c t u r e . BOILER BAY ALGAL RIDGE A t the time of our a r r i v a l on S t . Croix i n mid-1972, the b o i l e r s o r "cup ree f s" of Bo i l e r Bay were genera l ly known t o the considerable number o f s c i e n t i s t s who had worked on the eas te rn end of the i s l and . The high a l g a l r idges , although e a s i l y v i s i b l e from the a i r , had not been noted, a s they can be q u i t e obscure t o the swimmer espec ia l ly with average wave condit ions and without low water spr ing t i d e s . Perhaps it is unfortunate t h a t our work was begun i n Boi ler Bay, a s t h i s r idge i s c e r t a i n l y the most a t y p i c a l on the i s l and . Some of the 32 holes d r i l l e d i n Boi ler Bay could perhaps have been more useful on the l a r g e r r idges . Except f o r Eas t Po in t a l g a l r idge , bhe Boi ler Bay r idge i s the only one associa ted with the head of a bay. The former case does no t even appear a s much of an exception as the cove i s q u i t e shallow and faces due e a s t . Most of the o t h e r a l g a l r idges a r e associa ted with p o i n t s , o r i n t h e case o f Beach r idge an apparent f a u l t scarp o r i en ted almost n0rma.l t o the wave t r a i n . Most o f the high r idges a r e a l s o associa ted wi th the point-forming Eas t End Member of the Caledonia Formation suggest ing devel- opment on benches c u t a t 10 t o 12 meters, perhaps a t the 30 t o 40 thousand B .P high l e v e l sea stand. The c e n t r a l b o i l e r s of the Boi ler Bay r idge a r e r e s t i n g on a 0.5 t o 1 .5 meter th ick l a g c o l l u v i a l deposi t (Fig. 28). The l a t t e r , a weakly Fig. 24. Coral Reef and a l g a l r idge complex between Grassy Po in t and Grapetree Point . high a l g a l r idge , [IID reef f l a t , a l g a l r idge a r e a , a Z!.cropora palmata fore r e e f , a Diploria-Eontastrea deep f o r e reef and back r idge , 0 sand, grass . Sect ion shown i n f igure 26. Scale $ Fig. 25. Beach Algal Ridge, a r e a i n i n s e t o f Fig. 24. 1 0 t o +22 cm above m.l.w.sp., -20 t o 0 cm, -100 t o -20 cm, -300 t o -50 cm. Otherwise symbols a s prev ious diagrams. Fig. 26. Sect ion of Beach Algal Ridge and off - ly ing reef complex a s shown i n f i g u r e 24. .pueT uo d x e ~ s q ~ t - 1 ~ 3 p a x x a s u ~ Y ~ T M a 6 p ~ x y-Seauaq y m a q paxxa3ur 30 I r q ~ n u ~ q u o ~ quaxedde 6 u r ~ o q s eaxe a b p p ~ e b ~ v yaeaa 30 ydex60qoyd Terxav ' L Z ' ~ T K ~ i g . 29. .Boiler Bay area (see Fig. 4) showing ridge c r e s t s 4 and drainage pa t t e rns ; -- . co l luv ia l lobe ,-: and of f ly ing b a r r i e r reefs . / / cemented probably Wisconsin rapid flow o r colluvium cons i s t s of unsorted, angular Caledonia boulders, cobbles and pebbles i n a s i l t - s a n d matr ix . With l a t e Holocene r i s e of sea l e v e l , t h i s deposi t was eroded rap id ly , leaving a l a g of boulders and cobbles, and now appears a s a 10 meter high b l u f f , with a wave c u t t e r r a c e behind the c e n t r a l r idge area . The b o i l e r s a r e s t r e t ched ou t across t h i s boulder-pebble l a g t e r r a c e between the two Caledonian r idges with a smaller s e t t a i l i n g ou t t o the west probably on a pebble s p i t developed from the colluvium. De ta i l s of the boi ler - r idge area and the surrounding c o r a l communi- t i e s a r e shown i n f igures 29-37. Depths and subsurface data a r e given i n f i g u r e s 39-44, keyed as t o l o c a l i t y i n f igure 38. The surfaces and even the h ighes t b o i l e r r i m s i n Boi ler Bay a r e p r e s e n t l y in fes ted with Echinometra, t h e i r burrows occupying about 30% o r more of the surface. Crustose c o r a l l i n e s only occupy about 30% of t h e remaining surface , being intermixed with algal-bored, dead c o r a l l i n e , peyssonnelid c r u s t s , Homotrema and the c r u s t s o r filamentous bases' of the abundant f leshy-leafy algae. The l a t t e r develop a l a rge and d ive r se standing crop and a r e discussed i n d e t a i l by Connor and Adey (1975). The crus tose c o r a l l i n e s t h a t a r e p resen t a r e dominantly Neogoniolithon spec ies , Poroli thon pachydermum not being common and Lithophyllum congestum being q u i t e r a re . Tenarea bermudense, a shade spec ies , can be a massive a c c r e t e r on the undersides of overhanging l i p s . Even though wave ac t ion i s present ly i n s u f f i c i e n t i n Boi ler Bay t o s u s t a i n e f f e c t i v e growth o f - L. congestum, the borings a t Shark Reef s l o t show t h a t t h i s p l a n t was the major cons t ructor o f these r idges (Fig. 40) j u s t a s it was on the p resen t high r idges. Adey and Vassar (1975) have shown t h a t pre- s e n t c o r a l l i n e accre t ion r a t e s on t h e c r e s t s of these r idges a r e reduced t o 1-2 mm/year. This i s apparently no t s u f f i c i e n t t o maintain the r idges i n the face of the massive Echinohetra boring. Long, overhanging l i p s appear t o be more c h a r a c t e r i s t i c o f the fore- r idge i n Boi ler Bay than the l a r g e r r i d g e s , perhaps because reduced wave s t r e n g t h allow them t o reach l a r g e r s i z e . In addi t ion t o overgrowing and fus ing with each o t h e r , b o i l e r s apparently fuse with Millepora heads and (Figs. 39 and 40) probably occas ional ly A. palmata colonies . Most of the b o i l e r s i n Boi ler Bay a re based on 6. palmata and some, e .g . , R-ubble Reef and Sand Reef Skerry (Fig. 44) , have c o r a l l i n e caps only 0.4-1 meter th ick (dominantly of &. congestum) making them l e s s than 2000 years o ld (see f i g u r e 11). A few o f the d r i l l e d b o i l e r s were found t o be based on Millepora, mostly those i n the f a r west a reas . A ~ 1 4 da te on c o r a l under West End Reef a t 2 meters gave 650 years B.P . However, Ni l lepora appears t o f requent ly lack the s t r eng th t o support a b o i l e r alone and severa l "exploded" Millepora-based b o i l e r s occur i n the area (see Fig. 34) . A s i n g l e b o i l e r was found with an apparent Diplor ia base (Fig. 43A). I n l a r g e areas around the Bo i l e r Bay r idges a r e l a t i v e l y bare pave- ment occurred a t depths of 1-4 meters. These have only s c a t t e r e d c o r a l s , usual ly S ide ras t rea s ide rea and P o r i t e s a s t r eo ides and a r e named accord- ing t o t h e dominant cora l . Crustose c o r a l l i n e s a re r e l a t i v e l y unimportant on these pavements occupying i n some cases a s much as 30%, b u t usual ly l e s s than 10-15% of the surface. Several ~ e o g o n i o l i t h o n species , Fig. 2 9 . Key map of b o i l e r area of Boiler Bay showi-ng the locat ions of l a rge sca le maps. Fig . 30. Westernmost b o i l e r , West End Reef. Algal r i d g e 0 t o +17 cm above m. 1 . w . sp. -30 t o 0 cm, -60 t o -30 cm, a pavement > 60 cm. Symbols otherwise a s preceding maps. X rhodo l i t h s ( c o r a l l i n e nodules) , t e r r i g e n o u s pebbles . See f i g u r e 42C f o r depths . Fig. 31. Twin Reef and palmata pa tch b o i l e r s . The Acropora palmata patch i s the l a r g e s t p resen t ly l i v e area o f 5. palmata i n the boiler-pavement area . See f igure 4 3 B f o r depths. Fig . 32. Landing Reef and Nub b o i l e r s . See f i g u r e s 4 3 B & C f o r depths . Fig . 33. S t i c k Reef and Breaker Reef. These a r e bo i l e r - fu sed r i d g e s , S t i c k Reef having some caverns , b u t t h e o r i g i n a l b o i l e r p a t t e r n i s n o t obvious. Considerable c o l l a p s e has occur red on t h e f r o n t o f Breaker. See f i g u r e 4 1 f o r dep ths . o 20 30 40 feet 0 *metors 5 Fig. 34. Rubble and Setback Reef areas. Rubble reef i s a young b o i l e r developed on a massive recent A. palmata pavement. Halfway between Rubble and St ick i s a Millepore-based bo i l e r t ha t has collapsed i n a r ing of broken blocks. See f igure 44B f o r depths . are .n S !l. Fig. 37 . East Reefs. This p a r t l y fused complex o f b o i l e r s began t o degenerate before completion of r idge formation. For depths see f igure 4 2 A . Fig . 38. Locat ions o f t r a n s e c t s , d r i l l co re s and c o l l e c t e d r i d g e o r pavement b locks i n B o i l e r Bay. Transec t 1 - Fig. 42B, T ransec t 2 - Fig . 43A, Transec t 3 - Fig. 42C, T ransec t 4 - Fig . 41, Transec t 5 - Fig . 43B, T ransec t 6 - Fig . 43C, Transec t 8 - Fig . 39, T ransec t 9 - Fig . 42A, Transec t 12 - Fig . 44A, Transec t 1 3 - Fig. 44B. Fig. 39. Sect ion through Shark Reef a l g a l r idge. The d e t a i l s of the nose a rea a r e based on a 60 cm wide s l o t chise led o u t o f the r idge f r o n t (Fig. 40) . The A. palrnata immediately underlying the c o r a l l i n e cap was dated a t 2900 years B.P. ( see Fig. 13). crus tose c o r a l l i n e cap. 0 I 2 3 4 5 6 1 8 1 I I I I I I feet from lip , / / / / la rge Millepore area below the overhanging l i p was determ species, mostly Distance from l ip (feet) 110 $30 90 80 70 60 50 40 30 2 0 10 0 10 20 30 40 50 60 70 r--. 4 -2 9 u Fig. 41. Sect ion through S t i c k Reef. This r idge has an obvious loba te charac ter and our cores have apparently penet ra ted the cavernous junction of severa l b o i l e r s . No standing l a r g e co ra l species were found beneath t h e c o r a l l i n e cap i n these cores. Caledonia pebbles and cobbles, perhaps pr imar i ly derived from colluvium, o v e r l i e t h e Caledonia basement under t h i s r idge . 40 jo 20 10 0 (meters) 10 I I I I I I East R e f Fig. 42. Sections through East , Reference and West End Reefs. The small westernmost bo i l e r s appear t o be Millepc3~-based, the Millepore having developed on a s p i t of pebbles extending down- t fvom the co l luv ia l lobe. 2 0 to p (meters) 10 2 0 I I I I I H D , ~ *,? I / I ' / I - - _ _ - - _ _ - ? - - - .__ ,,I - - _ _ 7 - - - - - ? - .- ---_- - - - - - - - - _ _ _ - - _ _ Blg Nub and Nub- Land~ng Channel $9. $80 t i 0 ,qo -. ---. . . - - .. Fig . 43. Sec t ions through Clean Cut, Big N u b -?-.- .. - - _ . - and West Landing b o i l e r s . The pave- .?-. '. --. ments a r e probed by d r i v i n g r e in - -_ - 7 . .. --. _ fo rc ing rods through them wi th a s ledge hammer. \ Rubble Reef Complex \ % Fig . 44. Sand Reef Skerry and Rubble Reef b o i l e r s . The - A. palmata base of t h e l a r g e r of t h e s e s t r u c t u r e s can be seen by snork l ing . Fig. 45. Generalized surface s t r u c t u r e of a l g a l r idge a rea i n Bo i l e r Bay. cora l l ine-const ructed a l - g a l r idges ; a cora l pavements, mostly P o r i t e s p o r i t e s and Acropora cervicornis (small v a r i e t y ) , soxe A . palmata; l i v i n g A. palmata colonies; dominantly l i v e c o r a l , mostly - - P. p o r i t e s and A. ce rv icorn i s areas . - - e s p e c i a l l y shade types, and a small brancher - N. westindianum occur on t h e pavements, bu t Hydrolithon bdrgeseni i i s a l s o o f t en conspicuous. Much of the su r face i s apparently "dead" and occupied by green o r red boring algae ( see Adey and Boykins, 1975 - Hawaii) . We have removed severa l blocks from these pavements i n i n t e r and back r idge areas (Fig. 38) , and i n a l l cases , the framework was found t o be e i t h e r P o r i t e s p o r i t e s o r Acropora c e r v i c o m i s with considerable Homotrema, secondary cement and perhaps 10-40% poros i ty . Two ages on cora l were obtained on these pavements, one a t 640 and the o t h e r a t 940 years B.P. Apparently p r i o r t o 600-900 years B.P. most of the b o i l e r a rea was densely-covered with a co ra l t h i c k e t much l i k e t h a t p resen t ly e x i s t i n g i n f a r eas te rn areas (Fig. 3 7 ) . The pavements a re more o r l e s s e a s i l y probed using re inforc ing rod and were cored i n some cases (Figs. 39, 43B and C) . Fore r idge pavements have not been cored o r probed and some a r e A. palmata frameworks. - In some a reas t h e back-ridge pavements support s c a t t e r e d rhodo l i ths , c o r a l l i n e encrusted nodules with cores o f c o r a l , terr igenous pebbles o r s h e l l . These a r e being s tudied separa te ly . I n summary, the b a r r i e r reef i n f r o n t of Boiler Bay has e f f e c t i v e l y begun t o block wave ac t ion i n t o t h e bay only during the p a s t 300-500 years. P r i o r t o t h a t time the bay was q u i t e open. About 4000 years B.P. r i s i n g sea l e v e l encountered the Bo i l e r Bay colluvium. Within 1000 yea r s , wave and cur ren t ac t ion had removed much of the weakly-consolidated colluvium leaving a l a g conglbmerate of Caledonia cobbles and pebbles. With the p a r t i a l p ro tec t ion of the bay, co ra l colonies e spec ia l ly - A. palmata were soon f lour i sh ing , and by 3000 years B.P. i n c i p i e n t a l g a l r idges and high b o i l e r s had begun t o form. This process continued up t o about 500 years B.P. with numerous small b o i l e r s developing on Millepora and A. palmata colonies along with fusion of the major r idges . Appar- e n t l y about 1500 t o 2000 years B.P. wave ac t ion had begun t o be reduced around the b o i l e r s and dense t h i c k e t s of the f inger c o r a l s began t o de- velop. The r i c h e s t per iod , i n terms of co ra l and c o r a l l i n e algae devel- opment i n Boi ler Bay, must have been about 1000 years B.P. Since t h a t time and espec ia l ly during the p a s t 500 yea r s , degeneration, mostly a s a r e s u l t o f wave blockage by the ou te r b a r r i e r , has become progress ively more ser ious . SUMMARY AND DISCUSSION On the eas te rn she l f a reas of S t . Croix, Acropora palmata dominates shallow, tu rbu len t water , reef communities a t depths of from 1 t o 6 and up t o 12 meters depending probably p r imar i ly on water c l a r i t y . Once es tab l i shed t h i s co ra l i s capable of v e r t i c l e reef bui ld ing a t r a t e s of up t o 15 mm/year, q u i t e s u f f i c i e n t t o match t h e r a t e of sea l e v e l r i s e having occurred a t any time during the Holocene. However, a s sea l e v e l rose over the she l f a reas , A. palmata communities were no t immediately e s t ab l i shed , perhaps due t o wave removal of the sediments of the Ple is tocene rego l i th . By the time cora l colonies were es t ab l i shed on the o u t e r she l f a t depths of 25-30 meters, sea l e v e l s were from 8-10 meters above the developing r e e f s and the deeper and much slower-growing (1-2 mm/year) Diploria-Montastrea r ee f s could no t keep up with sea l e v e l r i s e r a t e s of from 2-5 mm/year. A t shallower depths, 12-20 meters, on the inner p a r t s of the shelves development followed the same pa t t e rn . The s i l t s t o n e s and sandstones of the Cretaceous Caledonia Formation form most of the basement rock of eas tern S t . Croix. These weakly meta- morphosed rocks of deep water volcanic o r i g i n probably underl ie the car- bonate s h e l f , and they r i s e r a t h e r abruptly from under the shel f near the p resen t shore l ine . Especia l ly o f f modern p o i n t s , narrow benches, prob- ably of Ple is tocene o r i g i n , appear t o be c u t i n t h i s Formation. Since the benches because of t h e i r r e l a t i v e e levat ion were probably quickly c leared of sediment a s sea l e v e l rose over them 5000-3000 years B . P . , co ra l colo- n i e s were soon es tab l i shed . These were e a r l y dominated by Acropora palmata and rap id ly b u i l t t o wi th in 1-2 meters of the e x i s t i n g sea l e v e l . Typical A. palmata colonies have a l a r g e proport ion of t h e i r surface a t any one time i n t h e dead b u t standing condit ion, perhaps due t o grazing o r d isease . This dead su r face , i n water l e s s than 1 - 2 meters i s quickly occupied by crus tose c o r a l l i n e , and i f major grazers of c o r a l l i n e , e spec ia l ly p a r r o t f i s h and Diadema, cannot opera te e f f e c t i v e l y due t o continuous water turbulence, the c r u s t s w i l l a cc re te a t r a t e s of up t o 6 mm/year. This acc re t ion would soon develop an "A. palmata pavement", and a t r a t e s of sea l e v e l r i s e o f the l a s t 6000 would bu i ld an i n c i p i e n t a l g a l mound o r r idge t o about mean low water. Lithophyllum congestum i s a rapid ly growing, branched c o r a l l i n e t h a t appears t o be confined, i n i t s massively branched form, t o q u i t e turbulent a reas near low water l e v e l s . A shallow subsurface a l g a l mound colonized by L. congestum would develop quickly i n t o the typ ica l , i n t e r - t i d a l , cup-shaped b o i l e r . A group of these b o i l e r s occurring together on a bench because of t h e i r outward growing l i p s would tend t o fuse with each o t h e r and with the surrounding cora l s t r u c t u r e s . Fusion r e s u l t s i n a reduction of wave a c t i v i t y on the l e e s i d e of the b o i l e r s , g r e a t e r grazing i n the back area and a narrowing of the fused b o i l e r s t o eventual ly form a r idge type s t r u c t u r e . From about 5000 t o 2000 B.P., a s e r i e s of these high a l g a l r idges developed on favorably s i t u a t e d benches on the e a s t e r n end of S t . Croix. Massive shallow b a r r i e r type r e e f s were not y e t developed, and the inner she l f a t t h i s time had only r e l a t i v e l y deep cora l r e e f s . In Boi ler Bay on the nor theas t corner of S t . Croix, a r a t h e r unusual Pleistocene col- luviurn, o r r ap id flow, of poorly consolodated boulders t o pebbles i n a s i l t y matr ix became exposed t o wave ac t ion about 3500 years B.P. Within 500-LQ0 yea r s , an A. palmata community, followed by a l g a l b o i l e r s and r idges asrreloped o n the l a g cobbles from t h i s colluvium. Beginning about 1000 years B.P. the deeper water co ra l r ee f s on the inner shel f were reaching c lose enough t o the su r face t o develop A. palmata communities. These i n turn have b u i l t r ap id ly a t r a t e s oT about 15 mm/year t o near p resen t sea l eve l s . Due t o a genera l west t o e a s t s lope of the s h e l f the more westerly r ee f s matured f i r s t and with gener- a l l y l e s s wave ac t ion have tended t o form broad reef f l a t s . In more e a s t e r n areas , t h e r e e f s have j u s t reached l e v e l s of -1 t o -3 meters, a few small reef f l a t s have formed, and i n some p laces , where s u f f i c i e n t wave ac t ion i s p resen t , i n c i p i e n t and young a l g a l r idges a r e forming on the b a r r i e r r e e f . This b a r r i e r system i s blocking wave ac t ion t o t h e o l d e r a l g a l r idges r e s u l t i n g i n t h e i r des t ruc t ion by grazing and burrowing organisms. I f sea l e v e l remains near ly constant f o r another 5000 t o 6000 years , the s h e l f edgereefs should reach the surface and develop new b a r r i e r r e e f s and a l g a l r idges . The p resen t b a r r i e r reef and i t s developing r idges w i l l then a l s o be deprived o f the required wave ac t ion and face gradual des t ruct ion by boring, burrowing and grazing organisms. Adey and Burke (1975) descr ibe the d i s t r i b u t i o n of b a r r i e r r e e f s and a l g a l r idges i n the eas te rn Caribbean. Also, they d iscuss i n some d e t a i l t h e major f a c t o r s t h a t have cont ro l led the Holocene development of these r e e f s and r idges and compare t h e i r development with t h e equiva- l e n t s t r u c t u r e s of severa l of the b e t t e r known P a c i f i c i s l ands . There i s perhaps no reason t o repeat t h a t discussion i n d e t a i l , bu t I w i l l t r y t o cover the s a l i e n t points . Rela t ive ly f l a t carbonate shelves have developed during the l a t e T e r t i a r y and Pleistocene t o the nor th , e a s t and south of most e a s t e r n Caribbean i s lands . While the depth of the surface o f these shelves r e l a t i v e t o present sea l e v e l i s q u i t e va r i ab le , the average depth, inshore and near the i s l ands , i s 12-20 meters. Within t h i s range, a s discussed above f o r S t . Croix, many extensive b a r r i e r r e e f s have de- veloped. These a r e p resen t ly reaching sea l e v e l and t o various .degrees a r e forming reef f l a t s . Areas with a genera l ly shallower she l f (e.g.-, the Grenadines) have an extens ive mature reef development. Other a reas (such a s the southeastern p a r t of the northern group of the Virgin Is lands) have deeper shelves and have very few mature r ee f s . I n a reas of considerable turbulence, a l g a l r idges , the equivalent of the i n c i p i e n t r idges on S t . Croix, a r e forming on these b a r r i e r s . However, a t depths shallower than 15 meters, r e l a t i v e l y small benches a r e a l s o l o c a l l y c u t i n t o t h e i s l a n d bedrock. These a r e e s p e c i a l l y s t rong ly developed on limestone capped i s l ands , bu t occur on l a t e Ter t i a ry volcanics a s well a s Cretaceous metamorphics. These benches, probably developed a t high sea l e v e l s tands during the l a t e Ple is tocene , a r e e f f e c t i v e l y l o c a l , shallow shelves and have a more mature reef system. Especia l ly i n the high wave energy S t . Eus ta t ius t o ~ a r b u d a t o Martinique a rea o f the l e s s e r A n t i l l e s , l a rge a l g a l r idges , t o g r e a t e r k l ~ a n 1 meter i n he igh t a r e developed on these shallow benches. 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