ATOLL RESEARCH BULLETIN No. 142 THE UNINHABITED CAYS OF THE CAPRICORN GROUP, GREAT BARRIER REEF, AUSTRALIA by S. B. Domm Issued by THE SMITHSONIAN INSTITUTION Washington, D. C., U. S. A. February 16, 1971 #." wi I sop' $ I ,-- ,' I 1 - I < - - - A Wreck *---- - I , , 1 - _ I \\ '.!.:one Tree Mast head ,---.. ; '. -> \.____-- /7 //' - __- ,,'"'\, Fig. 1. Capricorn Group THE UNINHABITED CAYS OF THE CAPRICORN GROUP, GREAT BARRIER REEF, AUSTRALIA by S. B . Domm INTRODUCTION The islands and reefs comprising the Capricorn Group (Fig. 1) l i e between 30 and 50 miles out from the port of Gladstone on the coast of Queensland, Australia. This area is near the southern end of t he Great Barrier Reef (bet ter cal led Reefs) of Australia and consists of nine coral cays or islands, two of which are inhabited (Heron Island and the lighthouse a t North Reef). There are also four reefs having no vegetated cays on them, however these may contain sand or coarse rubble banks tha t are usually awash a t high t i d e . Broomfield Reef has a small sand bank on i t s lee which appears t o be growing i n s i z e and even a t high t i d e r i s e s about a foot above the water. [An ear ly and very interest ing, though incomplete, description may be found in the f i r s t chapter of Jukes 1847-Eds .] . Heron Island i s one of t he larger islands of the Capricorn Group and because a t ou r i s t resor t and a biological research s t a t i on (operated by the Great Barrier Reef Committee) are located on it, it i s t he s i t e of considerable ac t iv i ty . For the past two and a half years the author has worked a t the Heron Island Research Station and every opportunity has been taken t o v i s i t the other islands i n t h i s region. Now is a good time f o r a general survey of t h i s group of islands t o be made, each year more people v i s i t them as the romance of v i s i t i n g or l iv ing on a coral island is ever a l ive . I t is important t o know what e f fec t man i s having on these islands i n order t o i n t e l l i gen t ly conserve and maintain them i n t h e i r natural s t a t e . They belong t o the future as well as t o the present. The nine islands of the Capricorn Group a l l l i e i n an area about 30 miles square with Heron Island approximately in t he center. A l l of the islands are s i tuated on reefs which r i s e from a submarine platform. Nowhere i n t h i s area does the depth of water exceed about 35 fathoms and around most of the islands i t is much shallower. The platform extends t o about 18 miles eas t of Heron Island where t he 100-fathom contour can be taken as the seaward margin. Since Heron Island i s the focal point for a l l a c t iv i ty i n t h i s region, distances and direct ion w i l l be taken from it. A l l of the islands of the Capricorn Group are of the type termed "cays" i n t ha t they were formed from wave transported material which has now become s tab i l ized by vegetation and beachrock. In discussing the d i f fe ren t islands I am only concerning myself with the major t rees , however the f l o ra includes a l so many species of herbs, shrubs and grasses (see references). The major t rees are: Tournefortia argentea Scaevola taccada Casuarina equise t i fo l ia Pisonia grandis Pandanus tec tor ius On a l l of the larger sand islands these t r ee s are charac te r i s t ica l ly found growing i n zones. On the smaller islands these zones become l e s s apparent due t o the e f fec t of the wind and probably moisture [and sa l in i ty ? Ed.]. Figure 2 i s a diagrammatic sketch of a typical island of the larger type showing the usual zones of dominant t rees . A l l of the sand islands of the Capricorn Group are used extensively by the Green Turt le (Chelonia mydas) and the Loggerhead Turtle (Caretta caret ta) t o lay t h e i r eggs. The nest ing turt.les often do much damage t o the marginal vegetation by digging up the area. The Capricorn Group and the adjacent but more southerly Bunker Group together form a d i s t i nc t province with many similar features, but most important of a l l i s tha t they are the southern terminus of a vast complex of reefs and islands extending f o r over a 1,000 miles--the Great Barrier Reef of Australia. BIRDS OF THE CAPRICORN GROUP Wedge-tailed Shearwater o r Mutton Bird (Puffinus pacificus) Reef Heron (Egretta sacra) - Silver Gull (Lams novaehollandiae) Brown o r comm?ii?6ddy (Anous s tol idus) Black o r White-capped, Noddy (Anous minutus) Crested Tern (Sterna bergi i ) Lesser Crested Tern (Sterna bengalensis) Black-Naped Tern (Sterna sumatrana) Bridled Tern (Sterna anaethetus) Roseate Tern (Sterna dougall i i) Fairy Tern (Sterna nereis) Black Oystercatcher (Haematopus fuliginosus) Pied Oystercatcher (Haematopus ostralegus) L i t t l e Pied Cormorant (Phalacrocorax melanoleucos) Bar-Tailed Godwit (Limosa lapponica) Eastern Curlew (Numenius madagascariensis) Lesser Knot (Calidris canutus) Turnstone (Arenaria interpres) Mongolian Plover (Charadrius mongolus) Golden Plover (Pluvialis dominica) White-Breasted Sea Eagle (Haliaetus leucogaster) Brown Booby, or Gannet (Sula -- leucogaster) - Lesser Frigate Bird (Fregata a r i e l ) White-eye, or Silver-eye (~0s-s l a t e r a l i s ) Sacred Kingfisher (Halcyon sancta) Banded Rail (Rallus phil ippensis) Bar-Shouldered Dove (Geopelia humeralis) This list i s not comprehensive since various species of migratory waders and others may pass through t h i s area unnoticed by the author. Furthermore, birds are occasionally seen from the mainland but these seldom i f ever remain i n permanent residence on the island (Northwest Island may be an exception). DESCRIPTION OF CAYS Northwest Island Northwest Island (Fig. 3 , Plates 1-2) is the largest of the Capricorn Group and i s located about 18 miles t o the northwest of Heron Island. I t i s composed of sand and s i tua ted on the western end of a long reef which has no lagoon; the is land i s aligned approximately southwest by northeast. The major t rees are found i n the zones charac te r i s t ic of t h i s region with Tournefortia and Scaevola plus Casuarina and scattered Pandanus g r n t h e perimeter and enclosing a. very e x t e n s i w t of Pisonia. There i s a grove of Casuarina on the western end of the island. The only exposure of beachrock is on the northeast corner and here much of the sand i s being washed away from behind the beachrock. For so large an is land there i s very l i t t l e beachrock. On the southwest corner act ive erosion is taking place and here one finds many uprooted t rees (mostly Casuarina) and some c l i f f i n g of the beach margin. There i s evidence of erosion on other par t s of the island. Northwest supports a very large population of White-capped (Black) Noddies, and Mutton Birds (Shearwaters) and many different types of waders can be found here. Also of i n t e r e s t i s tha t t h i s is land i s often the home of various types of mainland b i rds , the more obvious being: Currawong, Peewee, White-Faced Heron and Cuckoo-Shrike. Introduced chickens appear t o be increasing i n number and there are a t l eas t several cats on the is land. Because i t i s a large i s land on a long reef , Northwest provides a very good lee i n southeasterly weather, a lso, since it possesses two sheds whose roof drainage provides a t l ea s t some fresh water it is popular with v i s i t o r s and i s more often inhabited than the other is lands. Masthead I s l and Masthead I s l and (Fig. 4 , P l a t e s 3-8) is the second l a r g e s t uninhabited i s l a n d of the Capricorn Group, about 12 miles southwest of Heron I s l and . I t i s somewhat pro tec ted by the presence of Pomaise Reef on t h e south- west and t h e much smal ler Erskine I s l and and Reef 3 miles t o t h e nor th- e a s t . Masthead i s a long r a t h e r narrow i s l a n d a l igned approximately east-west and i s s i t u a t e d on the western end o f a long narrow r e e f which has no lagoon. Masthead is covered by a dense vegeta t ion t h e major t r e e s o f which are zoned i n t h e c h a r a c t e r i s t i c manner of t h i s region. The i n t e r i o r contains a mature f o r e s t of -Pisonia while t h e perimeter i s colonized by the usual Tournefort ia and Scaevola with s c a t t e r e d Pandanus. Casuarina a re found i n well-developed groves on t h e ends of t h e i s l a n d and about halfway along the northern s i d e . This l a s t ex tens ive grove is remarkable f o r the s t r a i g h t l i n e of i ts t r e e s and may have once continued f u r t h e r but has been eroded back as t h e whole i s l a n d appears t o have s h i f t e d i t s western end t o the south; f u r t h e r evidence of t h i s i s found i n seve ra l i s o l a t e d outcrops of o ld beachrock now found a d is tance from the beach. Beachrock i s wel l developed a l o ~ g t h e southeas t (windward) corner where a l s o some erosion appears t o be tak ing p lace . The l e e of t h i s i s l a n d seems t o be bui ld ing up as low dunes a r e being colonized by grasses . An i n t e r e s t i n g f e a t u r e of Masthead is t h e presence of a type o f cactus o r pr ickly-pear which appears t o be gradually gaining a foothold on the i s l a n d and probably o r ig ina ted from seedl ings o r p l a n t s washed ashore. I f such a p l a n t as t h i s continues t o spread i t could d r a s t i c a l l y upset the ecology of the i s l and . The usual s e a b i r d s frequent t h i s I s l and and I have seen hundreds of White-Capped Noddies and Mutton Birds, nes t ing S i l v e r Gulls , Bridled Terns and Crested Terns. The western Casuarina grove i s a favored spo t with campers. Tryon I s l and Tryon I s l and (Fig. 5 , P l a t e 9) is loca ted about 15 miles t o t h e northwest of Heron Is land and about 5 miles from Northwest I s l and . I t is r a t h e r small , f a i r l y narrow and al igned about 65 degrees. Tryon is composed e n t i r e l y of sand and i s r a t h e r f l a t with a s l i g h t r idge along the western s i d e . I t i s covered by a dense vegeta t ion i n which the major t r e e s assume the zones found on most of the o t h e r i s l a n d s . The c e n t r a l P isonia f o r e s t is surrounded by a zone o f Tournefort ia and Scaevola with s c a t t e r e d Pandanus and Casuarina. On t h e western t i p t h e r e i s a grove of Casuarina. There i s a very l a r g e f i g t r e e on t h i s i s l a n d , a l s o a small palm. Otherwise the vegeta t ion , as on most of the o t h e r i s l a n d s , includes a few minor t r e e s and bushes, with some g rass . There a re seve ra l outcrops o f beachrock, with one extens ive development with a s t r i k e running out from t h e angle of t h e e x i s t i n g beach. These beachrock "spurs" a r e always good evidence t h a t t h e margins of the i s l a n d have migrated away, t h e sand being washed from he re t o be deposi ted elsewhere. On Tryon I s l a n d t h e r e i s an outcrop of what I take t o be f o s s i l beachrock high up on the beach i n the vegeta t ion . If t h i s rock is i n s i t u , i t represents beachrock t h a t was formed when sea- level -- was s l i g h t l y h igher than a t p r e s e n t , s ince beachrock a t p resen t only seems t o form a t the i n t e r t i d a l zone of t h e beach. There appears t o be l i t t l e p resen t e ros ion taking p lace on Tryon I s l and and t h i s may be due t o i ts r e l a t i v e l y p ro tec ted p o s i t i o n . North- west Reef and the sand t r a i n which continues almost t o Wilson I s l and would c e r t a i n l y influence t h e f o r c e of the seas reaching Tryon I s l and during sou theas te r ly weather. The e f f e c t of v i s i t o r s on t h i s i s l a n d doesn ' t appear t o be very extens ive , although campers on t h e nearby Northwest I s l and may f requent ly come across . Wreck I s l and Wreck I s l and (Fig. 6 , P l a t e s 10-11) l i e s about 8 miles NNE of Heron Is land and i s a small narrow i s l a n d a l igned approximately 240 degrees magnetic. Wreck Is land was the s i t e of a bore put down t o explore t h e s t r a t a underlying these r e e f s i n 1950. It i s composed e n t i r e l y o f sand and t h e r e i s wel l developed beachrock along t h e southern (windward) s i d e extending f o r about h a l f t he length o f t h e island.; much of t h i s beach- rock is now being eroded away. The su r face of the i s l a n d i s r a t h e r uneven i n t h a t along most o f t h e southern margin ( cen t ra l a rea ) t h e r e i s a high r idge (about 20 f e e t above the beach). This r idge forms a kind of "backbone" t o t h e whole i s l a n d and i s covered by vegeta t ion , indeed i t i s i n t h e l e e of t h i s r idge t h a t one f inds t h e small bu t wel l developed Pisonia f o r e s t which provides a d e l i g h t f u l camping s p o t . On a small i s l a n d l i k e t h i s some s o r t of p ro tec t ion from t h e p r e v a i l i n g southeas t winds and occasional storms i s necessary i f the r a t h e r f r a g i l e P isonia t r e e s a r e t o a t t a i n anything approaching t h e i r mature he ight . The r idge i t s e l f is densely covered by bushes and e s p e c i a l l y along t h e margin fac ing t h e beach i s found the zone of Tournefor t ia and Scaevola t r e e s . On the ends of the i s l a n d the re a re small groves of Casuarina, t h a t on t h e nor theas t t i p being espec ia l ly well developed. Pandanus a r e found s c a t t e r e d about t h e e n t i r e su r face of the i s l a n d . On t h e northwest s i d e ( lee) of the i s l a n d t h e r e i s a c l ea red a r e a across the r ee f f l a t leading out from about t h e middle of t h e i s l a n d t o the reef margin. This was probably c l ea red during t h e time when t h e bore was sunk and provides an easy approach t o t h e i s l a n d i n sou theas te r ly weather when the depth of water i s uncer ta in . Some e ros ion i s taking p lace along t h e margins of t h i s i s l a n d and the sand ba r s extending from i t s ends a r e cons tan t ly s h i f t i n g . Due t o an unfortunate i n f e s t a t i o n by r a t s I expected t h e b i r d population t o be advers ly a f fec ted . However most of the b i r d s c h a r a c t e r i s t i c of t h i s a r e a can be seen here a t one time o r another and t h e i s l a n d seems t o support q u i t e a l a rge S i l v e r Gull populat ion. An i n t e r e s t i n g poin t i s t h a t few White-Capped Noddies n e s t on Wreck Is land while on Heron I s l and only a s h o r t d i s t ance away they n e s t i n thousands. Wreck I s l and appears t o be the favored nes t ing s i t e f o r t h e Loggerhead t u r t l e . Unfortunately t h e e f f e c t of v i s i t o r s has been such t h a t much of the i s l a n d ' s p r i s t i n e cha rac te r i s now gone. There i s an abundance of rus t ing t i n s of var ious types ( including an o ld water tank) s c a t t e r e d about, and t r e e s a re being cu t down i n t h e camping a rea . Wilson I s l and Wilson I s l and (Fig. 7 , P l a t e s 12-15) is about 9 miles north of Heron I s l and and 3 miles from Wreck I s l and , and i s located on the western s i d e of a very small r e e f . Wilson I s l and has seve ra l i n t e r e s t i n g fea tures ; i t is made up of a mixture o f sand and coarse rubble, and it i s a r e l a t i v e l y la rge i s l a n d i n comparison t o i t s r e e f , which ind ica te s t h a t t h e s i z e of the r ee f i s no t a completely l imi t ing f a c t o r i n determining the s i z e of a cay. The d i s t r i b u t i o n of the major t r e e s is r a t h e r d i f f e r e n t on Wilson Is land than on t h e o t h e r sandy i s l a n d s . The dominant t r e e s a r e Pandanus which forms an extens ive grove along the e a s t e r n s i d e and Casuarina which i s abundant along t h e northwest and southwest s i d e s . A cyclone seve ra l years ago d id much damage t o t h e Pandanus and now many a r e uprooted o r have t h e i r tops blown o f f and a r e dead, present ing a r a t h e r bleak appearance. Wilson i s r a t h e r open i n t h a t the vegeta t ion f o r t h e g r e a t e r p a r t i s s c a t t e r e d and t h e r e is no Pisonia f o r e s t , i n s t e a d t h e Pisonia occur as s c a t t e r e d t r e e s . Tournefort ia and Scaevola a r e a l s o present . Very wel l developed beachrock e n c i r c l e s h a l f of the i s l a n d from t h e southeas t t o the northwest , i . e . on t h e windward s i d e . Much of t h i s beachrock has been broken up by wave a c t i o n and l i e s high on t h e beach i n grea t s l a b s . Wilson Reef is not l a rge enough t o p ro tec t t h e cay from t h e fo rce of storm waves during a high t i d e , although some p ro tec t ion i s gained from Wreck Reef and a l s o the nearby Bloomfield Reef (2 miles t o t h e n o r t h e a s t ) . Wilson Island i s very easy t o approach as the cay i s almost adjacent t o the lowest par t of the reef and even during low water there i s suf f ic ien t depth over the reef for a small boat t o be brought i n t o the beach. This makes Wilson a popular is land for guests from the Heron Island Hotel t o v i s i t and therefore it must be the most frequented of the uninhabited islands of the Capricorn Group. Evidence of these v i s i t s can be found i n the crude tables and small rubbish t i p which detract from i t s beauty. I t does not appear t o support a very large b i rd population. One Tree Island -- One Tree Island (Fig. 8, Plates 16-20) is about 1 2 miles eas t - southeast from Heron Island and i s the most exposed of the islands of the Capricorn Group. I t has many unique features: i t i s the only is land tha t i s found on the eastern end of i t s reef , a l l the r e s t are found on the western end ( lee) ; another unique feature ( for the Capricorn Group) i s tha t One Tree is constructed of coral rubble, there is prac t ica l ly no sand on it. The reef of One Tree appears t o be s l i gh t ly higher than tha t of others i n the group and a l so contains a deep and well developed lagoon. Towards the center of the is land there i s a small pond of brackish water. The zonation of dominant t r e e s found on most of the other is lands i s absent from One Tree. Here the t r e e s consist of scat tered Tournefortia and Scaevola, e i t he r as s ingle t r ee s o r i n small clumps, found along the perimeter. There are also several small groves of ra ther wind-blown Pisonia and some well developed Pandanus. Between these scat tered groves there is a thick vegetation of low bushes. The shape of One Tree i s a manifestation of the forces acting upon it; it i s rounded towards the southeast and the lee is drawn out i n t o two "horns" pointing down wind. Along the perimeter of One Tree are very high ridges of rubble formed by storm waves, and indeed the i s land seems t o consist of a se r ies of r idges. The coral l ine rubble is of a very coarse nature and consists of fragments of coral and reef rock. One Tree appears to support a r ich and varied b i rd fauna and especially the open area next t o the pond i s , during the nesting season for the Terns (spring), the scene of much ac t iv i ty . Crested, Lesser Crested, Roseate and Black-Naped Terns a l l vie with each other f o r nesting s i t e s around the pond while Bridled Terns rear t h e i r young among the rubble along the margin of the is land just above the high t i d e mark. One Tree Island has i n the past few years been the location f o r considerable s c i e n t i f i c work and various studies are currently being carried out there , yet I am always impressed by the lack of evidence of these v i s i t s . There i s a kind of lonely beauty about t h i s rugged, ra ther desolate is land tha t i s di f ferent from a l l the other islands of the Capricorn Group. Erskine I s l and Erskine I s l and (Fig. 9 , P l a t e s 21-24) is the smal les t of t h e Capricorn Group and is loca ted about 7.5 miles southwest of Heron on t h e northwest s i d e of a small r e e f . Erskine i s composed e n t i r e l y of sand and appears r a t h e r high f o r i t s small s i z e . I t may rece ive some p ro tec t ion from the nearby Wis tar i and Masthead r e e f s bu t t h i s i s doubtful s i n c e t h e southeas t is unobstructed and t h e only e f f e c t of these two r e e f s may be t o channel the t i d a l stream i n such a way t h a t it flows p a s t Erskine a t an increased r a t e . The p a t t e r n of vegeta t ion (major t r e e s ) i s more simple he re than on most of the o the r sand i s l ands because Erskine is s o small t h a t i t is no t s u f f i c i e n t l y p ro tec ted from t h e wind t o enable a c e n t r a l P isonia f o r e s t t o grow. Here one f i n d s a wel l developed f r i n g e vegeta t ion of Tournefort ia and Scaevola with many t r e e s i n t h e cen te r of t h e i s l and . On t h e l e e s i d e (north) of t h e i s l a n d t h e r e i s a small grove o f very s tunted and wind-pruned Pisonia seldom exceeding 10 f e e t i n he ight . There a re no Pandanus o r Casuarina t r e e s on Erskine. Beachrock appears t o have been very wel l developed i n the p a s t on t h e northern s i d e ( l ee ) but now it l i e s p i l e d up i n g r e a t s l a b s high on the beach. On s o small a reef t h e i s l a n d i s r a t h e r vulnerable and subjec t t o i n t e n s e wave ac t ion during a s t r o n g wind with a high sp r ing t i d e , y e t very l i t t l e a c t i v e eros ion appears t o be tak ing p lace on Erskine. Few White-Capped Noddies o r Mutton Birds use t h i s i s l a n d f o r nes t ing nor does it appear t o be used as extens ive ly by nes t ing t u r t l e s as the o the r sand i s l a n d s . DISCUSSION From the above "Descriptions" i t can be seen t h a t t h e i s l a n d s of the Capricorn Group, with t h e exception of One Tree, a r e a l l very s i m i l a r : i n t h e i r composition (sand), i n t h e i r l oca t ion of t h e r ee f ( lee) and f o r the l a r g e r ones i n the zonation of t h e i r major t r e e s . Even though i n r e a l i t y they a r e nothing more than permanent vegetated sand banks they a r e , under the present s e t of condi t ions , s t a b l e f ea tu res , s t a b l e i n t h a t they e x i s t i n a s t a t e of equil ibr ium with t h e forces ac t ing on them. Wind-driven waves during a sp r ing high t i d e w i l l very o f t en cause eros ion on the windward s i d e ; t h e sand t h a t i s removed may be l o s t t o the i s l a n d o r merely t r anspor t ed t o t h e l e e sandspi t found on many of the i s l a n d s . Exposed r o o t s , dead and uprooted t r e e s , and c l i f f e d beach margins a t t e s t t o the f a c t t h a t t h e foregoing condit ions a r e not in f requen t . The margins of most i s l a n d s tend t o s h i f t pos i t ion , being cons tan t ly molded by t h e p r e v a i l i n g weather condit ions. I t i s poss ib le f o r a whole i s l a n d t o slowly migrate t o a s l i g h t l y d i f f e r e n t p o s i t i o n on i t s ree f and proof of t h i s can sometimes be found i n isolated outcrops of beachrock. These outcrops have a s t r i k e t ha t i s qui te dif ferent from the l i n e of the exis t ing beach, a l l of the sand having been washed away from behind the l i ne of rock which extends out onto the reef f l a t a t an angle t o the beach. During high t ides erosion begins t o remove the rocks and sometimes a l l one can see are scat tered remnants of a once continuous stratum. A l l of the islands are very low, seldom higher than about 10 or 15 f ee t above the high t i de level , yet covered by a dense vegetation and reinforced by p a r t i a l development of beachrock, they are able t o withstand any type of storm. However the most important fac tor contr i - buting t o the s t a b i l i t y of these is lands are the breakwater e f fec t of the surrounding reef and the ra ther protected posit ion of some of these reefs themselves ( re la t ive t o adjacent reefs ) . The prevail ing wind i s the southeast t rade (April t o October) and for days t h i s wind can blow a t from 15 t o 20 knots. As most of the islands are s i tuated on the lee of t h e i r reefs there i s a r e l a t i ve ly large expanse of reef i n f ront of the is land which has the e f fec t of at tenuating the force of the waves reaching it . The higher par t of any high t i d e ( i . e . par t e f fec t ive in island erosion) only l a s t s f o r about 2 hours before and 2 hours a f t e r the time of high t i d e ( t h i s being followed by the low t i d e 6 hours l a t e r ) , the actual time during which waves attack an island i s not very great . However, during a cyclonic storm a wind from the northwest combined with a high t i d e could do extensive damage t o an island as there would be much l e s s reef t o protect it. A l l of the cays of the Capricorn Group are located on a shallow submarine platform, therefore a strong oceanic swell i s seldom experienced. Less obvious but probably important i s the e f fec t the t i d a l stream has on reducing the force of the sea adjacent t o a reef . Here the t i d a l stream reverses its direction with the change of t i de and can flow a t r a t e s exceeding 2 1 / 2 knots (Australian P i lo t Vol. IV, 1962), however I believe tha t during mid-tide a t the time of spring the velocity of the t i d a l stream can be even greater and could act as a buffer t o reduce the impact of waves coming across the reef towards an island. Various surveys of the islands of the Great Barrier have been carr ied out i n the past (e.g. Spender, 1930, Steers, 1937) during which some o r a l l of the islands of the Capricorn Group were v i s i t ed b r i e f ly . From t h i s work evolved several c lass i f ica t ions of islands and t h e i r reefs, but the first r e a l i s t i c c lass i f ica t ion was presented i n 1950 (Fairbridge, 1950). Using it the two categories relevant t o the Capricorn Group are as follows: Type 2 . Vegetated Sand Cay. Moderately s tabi l ized, generally with beach rock. In t h i s group I would include a l l the sand islands of the Capricorn Group. Type 3. Shingle Cay, with o r without vegetation. Moderately s tab i l ized and widely dis t r ibuted, generally found on smaller more exposed reefs . In t h i s category I would put One Tree Island. A l l of the cays of the Capricorn Group are composed of material derived from the reef upon which they have formed. This material i s e i ther i n the form of sand or e l s e coarse fragments of reef rock (shingle o r rubble), coral skeletons, mollusk she l l s , e t c . A l l o r most of the above has an organic or igin; the sand is made up of small fragments of reef rock, t e s t s of foraminifera, Halimeda fragments, and small par t ic les of coral skeletons, e t c . Reef rock i s a conglomerate of the larger fragments derived from the reef cemented together with a calcareous cement produced by the fusing of small sand or s i l t - s i zed par t ic les . Wind driven waves and t o a lesser extent , t i d a l streams are responsible f o r the transportation and deposition of the material. Once the deposit is above the level of the highest t i de s the wind i s important i n forming and shaping i t . The genesis of the sand islands can be explained by the operation of two factors : (1) The mass transportation of sand across the reef f l a t from the windward areas towards the lee by wind-driven waves moving across the reef f l a t during high t i de and t o a lesser extent by the t i d a l stream. During low t i d e sand is transported by drainage currents as well as by wind-driven wave action. Most reefs are s l i gh t ly lower a t t h e i r western ends (lee) and since the highest par t of the whole reef is generally the outer r i m or margin (reef cres t ) when the level of a dropping t i d e f a l l s below t h i s r i m a l l of the water contained within the reef f l a t w i l l drain towards the lee . The sand transported t o the leeward pa r t of a reef would be deposited here when the current looses some of i t s velocity as i t meets an obstruction o r enters the shallow water of the lee reef f l a t . Once a sand bank i s formed the process of depositing sand continues u n t i l equilibrium i s reached and erosion begins t o have a negative e f f ec t . During low t i d e there i s some drainage out along the sides of a reef and on the windward areas, association with the so- called "groove and but t ress system1' often found here (Fig. 10). (2) The second fac tor i n the genesis of the sand cays i s the e f fec t of cross-swell depositing sand on the lee of the reef . The southeast swell upon encountering a reef is refracted around the reef , the waves approaching more or less normal t o the margin. The result ing turbulence w i l l cause sand t o be deposited i n a very localized area on the lee of the reef . The above only takes place during high t i de and periods of heavy swell (Fig. 10). The cays consisting of coarse coral l ine shingle o r rubble have a dif ferent mode of or igin . One Tree i s the only is land of t h i s type i n the Capricorn Group, although Wilson has much coarse material i n i ts composition and several reefs have rubble banks tha t are exposed during half t i de on t h e i r windward s ides , e.g. Heron and the nearby Wistari. Shingle cays are formed by the action of waves accumulating material and washing it up on the reef i n to ridges or banks. An examination of One Tree Island discloses a s e r i e s of concentric ridges more o r l e s s pa ra l l e l t o the margin and extending well i n to the island. I t was formed as successive ridges of rubble accumulated, each tending t o fuse with the next u n t i l a s t ruc ture of some s i z e evolved, able t o support vegetation. These accumulations of rubble always tend t o form towards the windward sides of a reef because here the wave action is the strongest , and t h i s i s necessary i n order t o move t h i s type of material. Such highly i r regular fragments interlock, giving the cay some resis tance t o erosion. The question ar ises as t o where the rubble, and t o a l e s se r extent sand, comes from. On One Tree and the various rubble banks s i tua ted on the windward s ides of t h e i r reefs the area producing rubble o r shingle is very r e s t r i c t ed as it must be t o windward of the island. The normal l i f e and death processes of the animals and plants l iv ing on the windward of such a reef as One Tree could not account f o r the enormous amount of coarse coral l ine rubble found there . This i s especially t rue of reefs as f a r south as those of the Capricorn Group. I believe tha t the material made available t o form the cays, especially the coarse rubble o r shingle, was produced by wave action i n the past, when a drop i n the leve l of the sea l e f t the reef standing higher out of the water. Immediately t he sea would begin t o erode the reefs down t o about t h e i r present height and i n so doing would produce large quant i t i es of debris. Most s c i e n t i s t s concerned with the problem of sea level changes agree tha t there have been some marked changes i n the past , a t times the sea being higher than a t present and a t other times lower. However there i s no agreement as t o the exact height a t any given time, but it is generally accepted t h a t the l a s t change was a drop of a t l e a s t several fee t , probably more. A l l of the reefs are now ef fec t ive ly planed down, and although some i r r egu la r i t i e s probably ex i s t , most a r e s tabi l ized a t t h e i r present level and are covered by a veneer of l iv ing coral and other reef-dwelling organisms. The gradual destruction and depth of these organisms, t o be replaced by others, would furnish suf f ic ien t material t o maintain the exis t ing islands in a s t a t e of equilibrium, just balancing the loss due to erosion. HUMAN INTERFERENCE A t the present time human interference with most of the uninhabited cays of the Capricorn Group i s not extensive. This is probably due t o the number of v i s i t o r s being r e l a t i ve ly low, but a las the qual i ty of many of these v i s i t o r s i s a lso very low. Whether persons'going t o these islands do so i n t he i r boats (these would be the lesser number) or via a charter boat, they for the greater pa r t seem in ten t on enjoying themselves t o t he utmost, and regrettably, t o give l i t t l e thought t o preserving the p r i s t i ne character of the is land on which they find themselves. A s the islands become more accessible the increasing number of v i s i to rs , unless controlled, w i l l eventually destroy these areas of great natural beauty. Some of the cays are s t a t e parks, but a l l are worthy of preservation. The disappointment must be intense t o one who, a f t e r much planning and dreaming of a vacation on a coral island, f i na l ly lands upon a beach and sees f i r s t an open rubbish t i p , numerous empty beer bo t t l e s and some hideous tables and chairs made of beach flotsam. This same person i s very l i ke ly t o leave the island jus t as he found it! The'effect i s cumulative! Northwest Island was many years ago the s i t e of a cannery and evidence s t i l l remains of t h i s ac t iv i ty i n the form of an old shed and a few r e l i c s of bo i le rs and machinery. There i s a very small one-room cabin of more recent construction. The scat tered heaps of rubbish and t i n s a t t e s t t o the fac t t h a t Northwest Island i s v i s i t ed ra ther frequently. As it i s the la rges t of the group, the actual area i n t e r - fered with i s very small, and old boi lers , sheds, e t c . are i n an area about 200 fee t square. There a r e cats and chickens but t h e i r e f fec t on the natural f l o ra and fauna i s probably not s ign i f ican t on so large an island. Masthead Island i s a l so qui te large and i s v i s i t ed frequently, but probably less so than Northwest and there i s no fresh water and no sheds. The main camping area of Masthead is i n a del ightful grove of Casuarina on the western end of the lee s ide . Here the e f fec t of man is much less than a t Northwest and some crude furn i ture and perhaps a small rubbish heap are the only evidence of campers. There are no introduced animals but a t l eas t several clumps of a kind of cactus of prickly pear which may be spreading slowly. Tryon Island, with Masthead Island, is probably the l ea s t affected of the larger islands of t h i s group. Tryon i s very s imilar t o Masthead i n tha t again the camping area is on the western end among Casuarina t rees and the only sign of v i s i t o r s i s a crude tab le with some scattered t i n s and bot t les . Wreck Island has suffered the most from human interference. In 1950 an exploratory bore was put down, there were many men and some machinery on the island a t t h i s time. Later on, it was surveyed, possibly for a proposed a i r s t r i p , and much vegetation was cut down. During the summer of 1968 a party of campers l ived on Wreck f o r about a week and when they l e f t there was a large open garbage p i t , many rust ing t i n s , several ugly b i t s of furni ture and a l a t r i ne had been constructed i n the center of the most lovely grove of Pisonia. The p r i s t i ne character of t h i s island has been a l l but destroyed and may never re turn again. Rats introduced probably a t the time of the bore are a nuisance and w i l l cer ta inly prey on young sea birds when available. Wilson Island was once very lovely with i t s extensive grove of Pandanus. Now the Pandanus are dras t ica l ly reduced due, so I understand, t o a f i r e followed by a cyclonic storm. The f i r e would almost cer ta inly have been s e t by v i s i t o r s . Now t h i s cay is v i s i t ed regularly by an excursion from the hotel on Heron Island and t h i s has i t s e f fec t i n a permanent rubbish t i p and some crude tab les . One Tree I s l and shows the l e a s t human in te r fe rence and y e t has been v i s i t e d by many persons, a high percentage of which would be s c i e n t i s t s . There i s almost a t o t a l absence o f o ld t i n s , bee r b o t t l e s , and t h e usua l rubbish t i p s . Erskine i s very small bu t un fo r tuna te ly t h e abundant evidence o f v i s i t o r s is he re i n t h e form of l i t t e r . Old t i n s , b o t t l e s and c r a t e s may be found s c a t t e r e d about. In the absence of any form o f con t ro l it i s important t h a t any person l i k e l y t o v i s i t t hese i s l a n d s be informed t h a t they a r e not t o be misused: t h i s should be presented i n an unmistakable manner. I t a l l depends on educat ing t h e i n d i v i d u a l t o r e spec t and t r e a t with cons idera t ion a reas of n a t u r a l beauty which belong t o a l l . REFERENCES Cribb, A. B. 1965. The marine and t e r r e s t r i a l vegeta t ion of Wilson Is land, Great Bar r i e r Reef. Proc. Roy. Soc. Qld. 77: 53-65. Fairbridge, R . W . 1950. Recent and Ple is tocene c o r a l r e e f s of A u s t r a l i a . Jour . Geol. 58: 330-401. Folk, R . L. 1966. Sand cays of ~ l a c r a ' n Reef, ~ u c a t i n , Mexico: Morphology. Jour. Geol. 75: 412-437. Fosberg, F. R. and Thorne, R . F. 1961. Vascular p l a n t s o f Heron Is land, pp. 5-13 i n Fosberg e t a l . , Heron Is land, Capricorn Group, Aus t ra l i a . A to l l Res. Bull . 82: 1-16. Gillham, M. E . 1963. Coral cay vege ta t ion , Heron Is land, Great Bar r i e r Reef. Proc. Roy. Soc. Qld 73: 79-92. Jukes, J . B . 1847. Narrat ive of t h e surveying voyage of H.M.S. Fly ... 2 v o l s . , London. MacGillivray, W.D.K. and Rodway, F.A. 1931. P lan t s on i s l a n d s o f t h e Bunker and Capricorn Groups. Rep. G t . Ba r r i e r Reef Comm. 3(7): 58-63. Musgrave, A., e t a l . 1926. The biology of Northwest I s l e t . Austr. Zool. 4: 199-280. Spender, M. 1930. I s land r e e f s o f t h e Queensland Coast. Geog. Jour. 76: 193-297. S tee r s , J . A . 1937. The c o r a l i s l a n d s and as soc ia t ed f ea tu res o f t h e Great Bar r i e r Reefs. Geog. Jour . 89: 1-28, 119-146. Fig. 2. Typical sand cay showing vegetation zonation a. Beach margin; b. Casuarina grove often found on ends of island; c. outer zone of Tournefortia, Scaevola and Casuarina; d. inner zone of Pisonia. Figs. 3-9: 3. Northwest Island. 4. Masthead Island. 5. Tryon Island. 6. Wreck Island. 7. Wilson Island. 8. One Tree Island. 9. Erskine Island. Fig. 10. Typical reef illustrating factors effective in genesis of a sand island. A. Reef at low tide a. general current from windward to leeward; b. lateral drainage across reef crest B. Reef at high tide c. wave fronts advancing on reef and being refracted around margins; d. cross swell on lee of reef due to refraction of waves. Plate 1. View along beach, lee s ide , Northwest Island. I Plate 2 . Old cannery building on Northwest I . P l a t e 3. Results of e ros ion , south s i d e , Masthead I . - - .- -. - Pla te 4. S t r a i g h t l i n e of Casuarina on dunes, north s i d e , Masthead I . Plate 5. Casuarina grove, west end, Masthead I . P la te 6. Spreading prickly pear, south s ide , Masthead I . Pla te 7 . Well developed Pisonia f o r e s t , i n t e r i o r of Masthead I . . ! i '. &A . - . . , _,. . ,,. ,. . P l a t e 8. Casuarina grove, e a s t end, Masthead I . P l a t e 9 . Eas t end of Tryon I P l a t e 10. Latr ine i n cen te r of camping area , i n P i son ia f o r e s t , Wreck P l a t e 13. Traces of weekly v i s i t s of t o u r i s t s from Heron I . on Wilson I . I . I P l a t e 14. Beachrock on windward s i d e of Wilson I P l a t e 15. Devastated Pandanus on Wilson I . P l a t e 16. Low vegeta t ion and Pandanus grove, One Tree ' I . I P l a t e 17. Coral rubble from which One Tree I . was formed. - .,..7mrm"s . ~ ~~~ P l a t e 18. North-facing embayment, One Tree I . . . P l a t e 19. Brackish pond i n cen te r o f One Tree I . P l a t e 20. Sea e a g l e ' s - n e s t , poss ib ly t h a t L recorded by Jukes . / i n 1840, I One Tree I . I P l a t e 21 . Erskine I . , from 1 mile o f f . 1 P l a t e 22. P i l e d up s l a b s of beachrock, no r th s i d e , Erskine I . P l a t e 23 . T o u n e f o r t i a and Scaevola above s l i g h t l y c l i f f e d beach margin, Erskine I . 1 P l a t e 2 4 . Erskine I . viewed across rubbly reef f l a t .