? I YARDS I 300
DISAWEARWCE O F
LIMIT OF VEGETATION 1962
V I I I . SUIvIivIARY OF PHYSIOGRAPHIC EFFECTS
I n t h e c o n c l u d i n g c h a ~ t e r s of t h i s r e n o r t t h e main p h y s i o g r a p h i c , v e g e t a t i o n a l and economic changes on r e e f i s l a n d s r e s u l t i n g from H u r r i c a n e H a t t i e w i l l b e b r i e f l y and s y s t e m a t i c a l l y o u t l i n e d . R e f e r e n c e s a r e
g i v e n t o c a y s which w e l l d i s o l a y t h e t o p i c s d i s c u s s e d , b u t no extended comparison i s made &h t h e few s t u d i e s of h u r r i c a n e e f f e c t s i n o t h e r p a r t s of t h e world.
Z o n a t i o n o f Damage
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F ' r o m t h e d e t a i l e d d e s c r i p t i o n s and maps of c a y s i n C h a p t e r s 4-7, t h r e e main p r i n c i p l e s c o n c e r n i c g p h y s i o g r a p h i c change d u r i n g t h e h u r r i - c a n e emerge. These a r e : f i r s t , damage i s d i s t i n c t i v e l y zoned away from t h e s t o m c e n t r e ; second, damage i s g r e a t e r on small t h a n on l a r g e ,
and on narrow t h a n on wide i s l a n d s , a t a n y g i v e n d i s t a n c e from t h e c e n t r e ; and t h i r d , damage i s more i n t e n s e on i s l a n d s s t r i p p e d of n a t u r a l v e g e t a - t i o n , o r where v e g e t a t i o n h a s been much a l t e r e d b y man. D i s c u s s i o n o f t h i s t h i r d ~ o i n t w i l l b e found i n C h a ~ t e r s 9 and 11.
The z o n a t i o n o f damage i s shown i n Figure 62, which i s n e c e s s a r i l y g e n e r a l i s e d . The zone of maximum o r c a t a s t r o ~ h i c damage e x t e n d s f o r 15-20 m i l e s n o r t h and s o u t h o f t h e h u r r i c a n e t r a c k , Morth o f t h e s t o r m c e n t r e winds were n o r t h w e s t e r l y , v e e r i n g n o r t h and e a s t d u r i n g t h e
p a s s a g e of t h e storm; s e a s were g e n e r a l l y n o r t h and e a s t e r l y , w i t h l o c a l n o r t h w e s t e r l y s e a s i n t h e n o r t h e r n b a r r i e r r e e f lagoon ( F i g u r e s 5-1C and 1 3 ) . South of t h e storm t r a c k w i n d s were w e s t e r l y , b a c k i n g s o u t h a n d s o u t h e a s t a s t h e storm ~ a s s e d . Over t h e g r e a t e r p a r t o f this a r e a , s e a l e v e l r o s e c o n s i d e r a b l y , r e a c h i n g a maximum of a t l e a s t 1 5 f e e t above normal t o t h e imniediate n o r t h of t h e storm t r a c k ( F i g u r e 12-13), and winds t h r o u g h o u t t h e zone p r o b a b l y reached s u s t a i n e d s p e e d s o f up t o 150 mph.
A number of s m a l l c a y s d i s a p p e a r e d i n t h i s zone (Cay Glory, Paunch Cay, Cay Bokel, S t . Georgevs E a s t Cay and o t h e r s ) ; some were c ~ m ~ l e t e l y s t r i p p e d of l a r g e r ~ l a n t s ( s e r g e a n t Vs, Goff t s and Z n g l i s h cays) ; t h e physiograohy was i n a l l c a s e s much a l t e r e d , m a i n l y by m a r g i n a l e r o s i o n , s t r i p p i n g of s u r f a c e sand, and c h a n n e l - c u t t i n g ; and g r e a t damage was caused t o human h a b i t a t i o n s , l i g h t h o u s e s and j e t t i e s . Mangrove was c o m p l e t e l y d e f o l i a t e d , l o s t b r a n c h e s and w a s i n p l a c e s uprooted ( ~ r o w n e d Cays, T u r n e f f e ~ s l a n d s ) .
North and s o u t h of t h i s c e n t r a l a r e a i s a second zone, 15 m i l e s o r more wide, s u b j e c t t o l e s s extreme though s t i l l v e r y v i o l e n t wind and wave c o n d i t i o n s , b u t l a r g e l y u n a f f e c t e d by t h e s t o r m s u r g e , e x c e p t t o a n undetermined e x t e n t n o r t h of t h e s t o r m t r a c k , as a t Cay Caulker. Physio- g r a p h i c changes i n t h i s zone were g e n e r a l l y minor, b e i n g r e s t r i c t e d t o s h o r . e l i n e r e t r e a t and c l i f f i n g , and &nor n e a r s h o r e e r o s i o n and d c ~ o s j t i 011,
b u t V ~ g ~ + . ~ f , i n n , n - = p a . - i , l l t y -.:nrc.>mxt. t . r e e s , s u f f c r e d heavy rl~rnage- I n p l a c e s , as a t Cay Caulker, where t h e vogCtat.inn bad been p a r t l y c l e a r e d b e f o r e t h e storm, waves were a b l e t o c r o s s t h e c a y and e x c a v a t e s c o u r h o l e s and c h a n n e l s . Otherwise a s a t Tobacco, South Water and Chapel Cays t h e dominant e f f e c t w a s t r e e f a l l . IvIangrove i n t h i s zone escaped defo- l i a t i o n i n t h e c e n t r e of l a r g e i s l a n d s , and i n e a r l y 1962 was a l r e a d y b e g i n n i n g t o r e c o v e r l e a v e s on t h e s i d e s o f i s l a n d s i n t h e l e e of h u r r i - cane winds and waves.
bL t h i r d zone i n c l u d s s t h e cays bztw5en P l a c a c i a and Gladden S p i t , c e n t r a l b a r r i e r r e e f lagoon, whcre t h e main h u r r i c a n e winds blt:w from t h e south w i t h r e s t r i c t e d f c t c h a c r o s s water 15-25 fathoms dezp. On n o s t of t h c s e i s l e n d s t h e vegetational e f f e c t s werc i n s i g n i f i c a n t , b u t a l l had sand o r s h i n g l e d e p o s i t e d on t h e i r s o u t h and z a s t s h o r e s ( a s a t Trappts, Laughing Bird, S c i p i o and Colson Cays). Vegetation damage Ms only con- s i d c r a b l c on Bugle Cay, which had been c l e a r e d f o r coconuts: consid(:rable shoreline r e t r e a t accompanied t h e d e s t r u c t i o n of t h c v z g e t a t i o n . This zone l i e s 30-40 m i l z s s o u t h of t h e h u r r i c a n e t r a c k ; it i s n o t d u p l i c a t e d t o t h e n o r t h i n B r i t i s h Honduran waters, where th;. b a r r i c r r e e f lagoon i s very shallow and t h e c a y s few i n number; t o t h i s e x t e n t it a p p e a r s t o depend on l o c a l c o n d i t i o n s .
F i n a l l y , t h c c a y s of Gloverqs Reef and t h e southern b a r r i e r r e e f and lagoon s u f f c r o d no v e g e t a t i o n a l and l i t t l e o r no physiographic damage,
3 . ~ . W t f r m i n s i g n i f i c a n t s h o r e l i n e r e a d j u s t m m t s ( a s a t Long Cay North, Gloverqs Fieef), which may o r may n o t have r e s u l t e d from t h e h u r r i c m 3 i t s e l f . The cays of G l o v c r t s Reef l i e only 40 milts s o u t h of t h e storm t r a c k and t h e s m a l l dcgrei! of damag;? may r e s u l t from t h e f a c t t h a t dmlinant water movements were p a r a l l c l i n d i r e c t i o n t o t h c s o u t h c p s t r e e f of t h e a t o l l , on which t h c c?ys stand. A t Punta Gorda, s u s t a i n e d g a l e f o r c e winds were i n s u f f i c i e n t t o cause d e f o l i a t i o n of mangrovc o r physiographic changes t o t h e cays o f f Punta Ycacos. The zone of no damage i s n o t
found on t h e B r i t i s h Honduras Coast n o r t h of t h e h u r r i c a n e t r a c k ,
except i n t h e n o r t h e r n part of t h e Bay of C h e t m a l , and on t h e mainland Yucatan c o a s t n o r t h of Boca Bacalar Chico, b u t t h e s e a r e a s were not i n v e s t i g a t s d .
E r o s i o n s 1 E f f e c t s Underwater
1. Damage t o r e e f . D e s t r u c t i o n of l i v i n g c o r a l s was g r e a t e s t on t h e b a r r i e r r e e f immediately n o r t h of t h e storm t r a c k , where up t o 80$ of r'eef c o r a l s d i s a p ~ e a r e d , preslmably t r a n s ~ o r t e d by wavc a c t i o n i n t o d e e m r water. The c x t e n t of reef damage has been f u l l y d i s c u s s e d i n 'Chapter
3
and Figure 1 4 and w i l l not be r e ~ e a t e d h e r e o I n t e r n s of t h ezonation of cay d m a g e , maximum r e e f damage occurred i n Zone I, depending l a r g e l y on exposure t c waves, and t o a l e s s e r e x t e n t i n Zones I1 and 111;
o n l y i n Zone 111, where damage was moderate, was any l a r g e amount of reef m s t e r i a l thrown above high t i d e l e v e l . Reef damage i n Zone I V was n e g l i g i b l e .
2. Submarine Slumping. The only example of submarine slumping d i s - covered was th3.t a t Crickozeen Creek, on t h e west s i d e of Turneffe
I s l a n d s , between t h e mangrove rim and t h e cdge of t h e bank, where physio- graphic cvidcnce i n d i c a t e s r o t a t i o n a l m o v e m c n t of a s e c t o r of bottom s e d i - ment ( c h a p t e r
6 ) .
It i s n o t c l e a r why such slumping should b e s o r e s t r i c t e d i n occurrence, nor what f a c t o r s caused slumping a t t h i s one p o i n t .3.
Erosion of scour h o l e s and channels. No s i g n i f i c a n t change was caused t o underwater topography by t h e h u r r i c a n e ; even where l a r g e s c a l e d e s t r u c t i o n of r e e f c o r a l s occurred, t h i s o n l y extended t o t h e removalof a t h i n l i v i n g veneer of r e e f m a t e r i a l from t h e s u r f a c e of r e e f p a t c h e s , Th? p a t c h e s t h e m s z l v ~ s , and t h e reef f l a t s , s u r v i v e with v e r y l i t t l e changz. lhrhcrc such s h o d a r e a s a r e covered w i t h Thalassi?", i t i s a s h p l e m a t t e r t o observe t h e d i s t r i b u t i o n of scour h o l c s and c h a n n , d s c u t through t h e v e g e t a t i o n mat. Such f e a t u r c s can be s s e n a t Goff9s and Big Calabash Cays; t h e y a r e shallow and u n i m ~ o r t a n t fe a t u r e s , w i t h s t w p and overhanging s i d e s , 2nd i t i s c l e a r t h a t t h e v e g e t a t i o n has pre- vented f u r t h e r erosion. One i s n o t j u s t i f i e d i n assuming t h a t wnere t h e v & g e t a t i o n mat does not e x i s t , e r o s i o n h3s been g e n e r a l , on t h e same
s c a l e a s t h e c u t t i n g of t h e scour h o l e s and channels; i t may be t h a t l o c a l weaknesses i n t h e T h a l a s s i a m a t have simply c o n c e n t r a t e d e r o s i o n a t t h e s e ~ o i n t s , Comparison of a i r photographs b e f o r e and a f t a r t h s s t o m , even i n ? r z a s whsre rzef damagc was extremely sovore, r e v e a l s v e r y l i t t l e change indeed i n shallow-water v e g e t a t i o n p a t t e r n s ( c o g o on t h e e a s t s i d e of Turneffc, a t Calabash Cays and Cockrcach c a y s ) , Channel c u t t i n g h a s only been corlsidcrable wher;: water movement h a s been r e s t r i c t e d by isl?.nds, a s between t h c mangrove cays of t h e Drowned Cays, b a r r i m -
r e e f , o r through t h e narrow c r e c k s on t h c west s i d c of T u r n c f f e I s l a n d s . The overdeenened channels may be dozens cf y a r d s long and s c v z r a l fathoms deep, with s t e e ~ and overhanging s i d e s and meandering c o u r s e s w i t h i n t h e g e n e r a l meandering p a t t e r n of t h e c r e e k s themselves, Where n o t s o con- s t r i c t e d , channels arc? v-shaped i n p l a n and g e n e r a l l y c u t i n t o t h e la'goon- edge of r e e f - f l a t s ; none have c u t back more t h a n a few y a r d s i n t o t h e r e e f - f l a t s u r f a c e , and t h e i r depth i s r o s t r i c t e d . C o n s t r i c t e d channels a r c now occunied by f a s t t i d a l streams, which p r o b ~ ~ b l y l i m i t i n f i l l i n g ; u n c o n s t r i c t e d channels g m e r a l l y c o n t a i n f a i r l y s t i l l water, and may be expect5d t o f i l l i n f a i r l y r a p i d l y . Unconstricted c h ~ r m c i s a r e &nor features; a l l l a r g e channels, such a s t h o s z n e a r t h e edge of t h e c o a s t 2 1 s h e l f between Cays Chapel, Caulker 2nd i h b e r g r i s , a r e i n f a c t l i t t l e - o r un-altered nre-hurricane f e a t u r e s .
S u b a e r i a l
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1. D e s t r u c t i o n of cays. A number of cays d i s a ~ p ~ a r e d altogether i n Zont 1, i n c l u d i n g Paunch Cay, S t , Georgegs E a s t Cay, Cay Glory, Cay Bokel, Big Calabash E a s t I1 Cay, Blackbird Cay, somc o f t h e Cockroach Cays, and Saddle Cay. A l l of t h e s e exccpt Paunch Cay were vegetated, though i n t h e c a s e of Cay Glory t h e v e g s t a t i o n only c o n s i s t e d of low-lying g r a s s e s , p r o s t r a t e p l a n t s and a v e r y young ooconut; o t h e r s had mature t r e e s ,
i n c l u d i n g t a l l mangroves and coconuts. P o s t were low-lying;
all
except S t . Georgevs E a s t Cay were predominantly sandy, w i t h .a l i t t l e nearshore rubble, and a l l were small. S t , Georgovs Z a s t , a s h i n g l e cay, w a s t h e l m g e s t i s l a n d t o disappear: b e f o r e t h e h u r r i c a n e it w a s 120 y a r d s long.2. Fiarginal Erosion. R e t r e a t of cay s h o r e l i n e s has t a k e n p l a c e i n Zones I, I1 and 111. A l l m a t e r i a l above s e a l e v c l and f o r s e v e r a l i n c h e s below it h a s been eroded away, i n a zone up t o 20 y a r d s i n width, g e n e r a l l y g r e a t e s t n e a r t h e h u r r i c a n e c e n t r e and on t h e s i d e of t h e cay f a c i n g main h u r r i c a n e winds ( e a s t t o t h e n o r t h of storm c e n t r e , southwest t o t h e s o u t h ) . This has l e f t a v e r t i c a l c l i f f , u s u a l l y 1-2 f e e t high, capped by a mat of coconut r o o t s from which a l l sand h a s been f l u s h e d . I n p l a c e s t h i s c l i f f e d s h o r e l i n e i s f a i r l y s t r a i g h t ; o f t e n t h e presence of near- s h o r e coconut b o l a s holding up promontories h a s given it an i n t r i c a t e
o u t l i n ? . C c c a s i o n a l l y one f i n d s a i l l a r - l i k e remnants of t h e old cay s t a n d i n g o u t s i d c t h e p r e s e n t c l i f f a n e , as a t S o l d i e r and Big Calabash Cays; t h e s e t o o a r e steep-sided and capped w i t h coconut r o o t s .
3.
D e s t r u c t i o n of unconsolidated s p i t s . This was probably u n i v e r s a l throughout Zones I-
111, b u t by e a r l y 1962 f r e s h s ~ i t s of similarg e n e r a l form had i n most c a s e s regrown a t o r n c a r t h e old l o c a t i o n , a s a t E n g l i s h Cay.
4 Channel-cutting. C u t t i n g of channels through cay s u r f a c e s was lirllited t o Zone I, i n t h e a r e a covered by t h e high sea surge, and t o cays w i t h coconut o r o t h e r t h i n ground v e g e t a t i o n . Scouring o f deep channzls occurred a t S t . Georgevs Cay, and shallower channels a t Mauger and Sandbore Cays, a l l a c r o s s narrow necks of land. I n c i p i e n t channels were seen a t Half Pioon Cay and Deadman V Cap, extending headward from t h e l e e s i d e of cays, and a t Cay Caulker, where roadways t r a n s v e r s s t o t h e seaward s h o r s were overdeepened i n t h e v i l l a g e .
5.
Scour h o l e s i n cay s u r f a c e s . Erosion of scour h o l e s by overtopping w a t e r s was widespread i n Zones I-
111. They were g e n e r a l l y developed a t t h e margin of some obstac1.e t o water flow, such as b u i l d i n g s (sandbore cay) o r t h e upturned b o l e s of t r e e s , u s i n g a s a nucleus t h e h o l e s l e f t by r o o t s . Holes a t Cay Caulker a r e e l o n g a t e and n o t a ~ p a r e n t l y r e l a t e d t o m y o b s t a c l e . A t Half Noon Cay s c o u r h o l e s were scen i n t h e l e e of s h i n g l e r i d g z s d e p o s i t e d by t h e h u r r i c a n e i t s e l f . Scour h o l e s have a l s o been seen a l o n g k e l e e s h o r e s of cays, where overtopping w a t e r s reach5d t h e l ~ g o o n (cay C h a ~ e l , Cay c a u l k e r ) .6 .
S t r i p p i n g of s u r f a c e sand. Where cays w;rc overtopped by t h e storm surge i n Zone I o r had t h e i r margins submerged by heayy wave a c t i o n i n Zones I1-
111, s t r i p p i n g of l o o s e s u r f a c e sand w 3 s almost u n i v e r s a l , though g e n e r a l l y confined t o a narrow m a r g i n a l s t r i p up t o30
y a r d s wide, immediately inlarid from t h e undercut cliffAin-?. D e c x a s e i n e l e v a t i o n was gent?rall.y l e s s t h a n 1 f o o t , but t h i s involved t h e removal of a l l o r most s u r f a c e sand and s o i l , r e v e a l i n g long coconut r o o t s and o c c a s i o n a l l yorange Thrinax r o o t s . Thesz a r e scmetimes combed i n t h e d i r e c t i o n of water movenent, as a t Deadman V and Big Calabash Cays. F r e q u e n t l y i n Zone I t h e coconut r o o t s form a, s u r f a c e mat s e v e r a l i n c h e s t h i c k , devoid of sand though l i t t e r e d with f r e s h d e b r i s ; i n Zones 11-111, however, t h e exposed r o o t mat i s much t h i n n e r and r e s t s on t i g h t l y packed sand, a s a t South Water Cay.
7. Erosion of c o n s o l i d a t e d d e p o s i t s . Beachrock has been remarkably suc- c e s s f u l i n r e s i s t i n g e r o s i o n , e s p e c i a l l y underwatero However, a t Half Moon Cay, where t h e conglomerate platform i s much p i t t e d and eroded, wave
smoothing i s apparent on t h e s u r f a c e , where s e v e r a l l a r g e p r o j e c t i n g blocks have been t o r n away. I n t h e s o u t h e a s t bay t h e o u t e r margin of t h e beachrock has s u f f e r e d c o n s i d e r a b l e damage; l a r g e p l a t e s were broken o f f and thrown onto t h e s h o r e o r u p t i l t e d i n t h e water a g a i n s t t h e
unbroken beachrock. The o n l y c o n s o l i d a t e d d e p o s i t s t o d i s a p p e a r were i n c i p i e n t beachrocks of small e x t e n t , such a s a beachrock seen i n 1961, n e a r t h e n o r t h p o i n t of Northern Cay. C l e a r i n g of a l g a l mats on beachrock a l s o r e v e a l e d much f r a c t u r i n g , which may have e x i s t e d b e f o r e t h e h u r r i c e n e ,
8. U ~ r o o t i n g of coconut t r e s s i n Zones I
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I1 could l e a v e h o l e s i n t h e cay s u r f a c e UD t o 3 y a r d s i n d i m e t z r and 3 f e e t i n depth, which were l i a b l e t o s c o u r i n g by waves n e a r t h e s h o r e . These h o l e s may r e a c h t h e watert a b l e , and many a r e now f i l l e d w i t h w a t a r , g i v i n g s u r f a c e a pock-markcd a ~ ~ e a r a n c e , a s a t Long Cay, Lighthouse Reef.
9.
F i n a l l y , c l i f f i n g and i l n d e r c u t t i n g of sand a r e a s could t a k e p l a c e w e l l above s e a l e v e l d u r i n g v i o l z n t wave? a c t i v i t y , a s on t h e s o u t h w s t sand r i d g e a t Half Noon Cay, where u n d z r c u t t i n g and r i d g e r e t r e a t h a sformed a v e r t i c a l c l i f f
4
f e e t h i g h j u s t below t h e r i d g e c r e s t and 20 y a r d s from t h e s e a .D e p o s i t i o n a l E f f e c t s
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Underwater
1. D e l t a s and d s p o s i t i o n cones of sand a t t h e mouths of s c o u r c h a m e l s between cays, a s a t Drormed Cays: and s t t h e leeward end of c r e e k s , a s on t h e west s i d e of Turneffe, a r e found i n Zones I
-
11.2 , D e p o s i t i o n cones a r e a l s o found a t t h e leeward ends of s c o u r c h a n n e l s c u t t h r o u g h cay s u r f a c e s . They a r e b e a u t i f u l l y developed a t S t . Georgegs Cay, Zone I.
3 . Shoa.iing i n n e a r s h o r e a r e a s , by d e p o s i t i o n of m a t e r i a l crcdt-d frcm c a y s u r f a c e s , e s p e c i a l l y sand. T h i s i s w e l l s e c n i n t h c Sandbore Cay lagoon, Zone I.
4.
S c a t t e r i n g of l a r g e b l o c k s on r e e f f l a t s . I n view of t h e g r e a t d e s t r n c - t i o n of r e e f s it i s s u r p r i s i n g t h a t s o - c a l l e d qnegro-heads? o r r e e f - b l c c k s throw up o n t o r e e f - f l a t s d u r i n g t h e storm a r e almost non-existent. Imme- d i a t e l y a f t e r t h e storm i t i s s a i d t h a t more small r e e f d e b r i s was v i s i b l e above s e a l e v e l , a t k a s t on t h e n o r t h e r n b a r r i e r r e e f , b u t b y e a r l y 1962 most o f t h i s had d i s a p p e a r e d . Only a t Big Calabash Cay, T u r n e f f e , andCary Cay, c e n t r a l b a r r i e r r e e f lagoon, a r e any l a r g e b l o c k s now s t r a n d e d on r e e f - f l a t s .
5.
D e p o s i t i o n of r u b b l e c a r p e t s . This i s much more widespread i n Z o n s I-
111, though t h e amount o f d e p o s i t i o n i s s t i l l small c o m ~ a r e d w i t h J a l u i t . C a r p e t s a r e b e s t developed a l o n g t h e o l d r e e f c r e s t , where t h e yr i s e t o w i t h i n a few i n c h e s of t h e s u r f a c e , o c c a s i o n a l l y emerging a s low r i d g e s , and t e r m i n a t i n g lagoonward i n s t e e p f a c e s . They a r e b e s t seen a l o n g t h e b a r r i e r r e e f , as a t Cay Glory and C a r r i e Bow Cay, and on t h e T u r n e f f e e a s t r e e f s . I n p l a c e s t h e s h i n g l e l a c k s t h i s well-defined r i d g e form and i s spread i n a t h i n n e r , w i d e r c a r p e t , w i t h i m b r i c a t e palmata s l a b s , a s a t G o f f f s Cay.
6. Ephemeral r e e f - c r e s t s h i n g l e r i d g e s c o n s i s t o n l y of t h e emergent por- t i o n s of t h e s e c a r ~ e t s , None a r e more t h a n
3
f e e t i n h e i g h t and most o n l y a few y a r d s long. Immediately a f t c r t h e h u r r i c a n e t h e y may have been more e x t e n s i v e t h a n when seen i n e a r l y 1962. The r i d g e s a t S o l d i e r Cay,C a r r i e Bow Cay and S k i f f Sand a r e p r o b a b l y i n t h e p r o c e s s of d e s t r u c t i o n .
7. Presumably t h e g r e a t amount of r e e f m a t e r i a l d e s t r o y e d i n Zone I h a s been swept i n t o d e e p e r w a t e r on t h e seaward ar.d lagoonward s l o ~ e s of
r e e f s ; much of it may l i e a t d e p t h s of 50 f e e t o r more, Nuch v e g e t a b l e m a t e r i a l h a s a l s o been dumped i n d e e p e r water: i n t h e n o r t h e r n b a r r i e r r e e f lagoon t r e e s a r e seen on t h e lagoon f l o o r and even emerge abovz t h e s w f a c e a t d i s t a n c e s of s e v e r a l m i l e s from cays. No i n v e s t i g a t i o n s were made of c o n d i t i o n s on lagoon f l o o r s o r i n d e e p e r water.
S u b a e r i a l
Submarine d e p o s i t i o n a l f e a t u r e s a r e of minor importance, and w i t h t h e exception of t h e S t . Georgeqs Cay d e l t a s , none a r e s p e c t a c u l a r . S u b a e r i a l d e p o s i t i o n a l f e a t u r e s a r e p h y s i o g r a p h i c a l l y s i g n i f i c a n t , more s o i n some a r e a s t h a n i n o t h e r s . They a r e n o t w e l l developed i n Zone I on t h e b a r r i e r r e e f , b u t a r e found i n t h e same zone on t h e e a s t s i d e of Turneffe; t h e y r e a c h t h e i r most t y p i c a l development i n Zone 111, where e r o s i o n a l e f f e c t s a r e minimal. P a r t of t h e d i f f z r e n c e i n Zone I l i e s i n v e g e t a t i o n : on t h e b a r r i e r r e e f t h e cays were low, small and c o ~ o n u t - c o v e r e d . With t h e coco- n u t s swept away, and submerged by t h e storm surge, t h e i s l a n d i t s e l f would ~ r e s e n t l i t t l e o b s t a c l e t o t h e passage of sediment. On t h e a t o l l s , however, most of t h e c a y s were d e n s e l y v e g e t a t e d and n o s t o f t h e v e g z t a t i o n r e m a i n ~ d i n p l a c e d u r i n g t h e storm, a c t i n g as a b a r r i e r t o sediment move- ment and r e s u l t i n g i n t h e piling-up of d e b r i s . To i l l u s t r a t e t h i s one may c o n t r a s t t h e mainly d e p o s i t i o n a l e f f e c t s a t Cockroach I1 ( p e l i c a n ) Cay,
covered w i t h Bursera-Cordia bush; and t h e d i f f e r e n t p i c t u r e a t Cockroach Cay i t s e l f , covered w i t h e a s i l y - d e s t r o y e d coconuts, where most Surface sand was s t r i p ~ e d , no s h i n g l e r i d g e s wsre b u i l t , and d e p o s i t i o n was l i m i t e d t o accumulation of sand i n t h e leeward bay. There i s a s i m i l a r c c n t r a s t between d e p o s i t i o n on t h e Cordia-Bursera covered south s h o r e of Half Moon Cay9 and e r o s i o n on t h e c o c o n u t - c o v ~ r e d southwest shore. F i n a l l y , i n
Z O I11 ~ one may c o n t r a s t t h e widespread d e p o s i t i o n on d e n s e l y v e g e t a t e d i s l a n d s such a s Colson, S c i p i o , Owen, Trappqs and o t h e r cays, and t h e c o n s i d e r a b l e e r o s i o n on Bugle Cay, which had been c l e a r e d f o r coconuts.
S u b a e r i a l d e ~ o s i t i o n t a k e s t h e f o l l o w i n g forms:
1. L i t t e r i n g of h e a v i l y eroded s u r f a c e s w i t h c o a r s e c o r a l rubble, o f t e n of s m a l l c a l i b r e , a s a t Cockroach Cay and Sandbore Cay, Zones I and 11.
2, Accumulation of c o r a l s h i n g l e and sand a g a i n s t v e g e t a t i o n b a r r i e r s : t h i s o c c u r s a t Cockroach I1 and Half Moon Cays (where d e b r i s i s p i l e d up t o a h e i g h t of 1 0 f e e t above s e a l e v e l ) , and on s e v e r a l of t h e c e n t r a l b a r r i e r r e e f cays, such as Owen and Laughing Bird Cays.
3 -
Deposition of wider, t h i n n e r c a r p e t s , g e n e r a l l y of sand, on t h e cay s u r f a c e , e s p e c i a l l y i n l a n d from t h e m a r g i n a l e r o s i o n zone a l r e a d y des-c r i b e d , i n Zones I1 and 111. T h i s f e a t u r e i s t y p i c a l l y s e e n a t South Water, T r a p p f s , S c i p i o and Colson Cays, The c a r p e t may b e up t o 30 y a r d s wide, wedging o u t seaward and t h i c k e n i n g landward, t e r m i n a t i n g on t h e landward
s i d e i n a s t e e p f a c e , o f t e n a r c u a t e i n plan. The sand b u r i e s t h e o l d cay s u r f a c e , which may r e t a i n i t s v e g e t a t i o n and s o i l s , t o d e p t h s of up t o 2 f e e t . Even where t h e sand i s d e e p e s t t h e t a l l e r v e g e t a t i o n may p r o t r u d e through it and s u r v i v e ( e s p e c i a l l y ~ h r i n a x ) ; where t h i n n e r , Hymenocallis i s o f t e n s t i l l v i s i b l e and l i v i n g , Such c a r p e t s were seen a t P l a c e n c i a
i n 1961 011 t h e day t h e y were deposited by Hurricane Anna.. i3urial of old s o i l h o r i z o n s by r ~ e t s , mostly of rubble, was d e s c r i b e d a t J a l u i t a f t e r Typhoon Onhelia
i"
B l ~ r n e ~ ~ s t o c k , e d i t o r , 1961) ; i n B r i t i s h Honduras r u b b l e c a r p e t s a r e d i s t i n c t l y r a r e , except where t h e y form p a r t s of r i d g e s p i l e d a g a i n s t v e g e t a t i o n b a r r i e r s , a s a t Cockroach Cays.4 ,
Deposition of s h i n g l e r i d g e s round t h e old cay shore, which may i t s e l f have s u f f e r e d e r o s i o n . Again, t h e s e a r e t y p i c a l l y dcvsloped i n Zone 111, where t h e r i d g e s may o r may not a d j o i n t h e o l d shore f o r a l l t h e i r l e n g t h . I n p l a c e s t h e main r i d g e may l i e some y a r d s t o seaward, e n c l o s i n g a low- l y i n g s h i n g l e c a r p e t o r onen w a k between it and t h e ercded shore ( ~ o l s o n , S c i p i o , T r a p p f s c a y s ) . I n t h i s case t h e c a l i b r e of t h e m a t e r i a l i s c o a r s e s t i n t h e r i d g e , and much f i n e r i n t h e enclosed zone. Thcse o f f s h o r e r i d g e s may p a s s l a t e r a l l y i n t o r i d g e s b u i l t a g a i n s t and on t h e s h o r e i t s e l f .5.
Z x t e r s i o n of leewarc! s h o r e s by sand d e ~ o s i t i o n . T h i s was noted by mapping a t Cockroach Cay and Deadman I Cay along t h e g r e a t e r Dart of t h eleeward shore; minor e x t e n s i o n s of s h o r e l i n e s by d e r o s i t i o n was a l s o noted a t Half ibloon Cay and elsewhere,
I n c i d e n t a l P h y s i o g r a ~ h i c E f f e c t s
The most i m o r t a n t i n c i d e n t a l physiographic e f f e c t on cay physiography was t h e i n c r e a s e i n number of outcrops of cementcd sands. These have been d e s c r i b e d i n d i v i d u a l l y i n t h i s r e p o r t , and i n g e n e r a l terms i n
ARB
87, 106-109, They may be grouped a s f o l l c w s :I n t e r t i d a l beachrock
The name qbeachrockq i s here r e s t r i c t e d t o t h s narrow s t r i p s of cemen- t e d beachsands, which g e n e r a l l y d i ~ seawards, following t h e l i n e of t h e shore, a r e of v e r y l i m i t e d v e r t i c a l e x t e n t , and a r e c n a r a c t e r i s t i c a i l y found on r e t r e a t i n g beaches o r on r e e f - f l a t s marking t h e s i t e s of former s h o r e l i n e s . I n B r i t i s h Honduras beachrclck and beachsands c o n t a i n much Halinieda and e n c r u s t i n g r e d f o r a m i n i f e r a . New exDosures r e s u l t i n g from Hurricane H a t t i e were of t h r e e types: ( a ) o l d r e l i c t beachrock covered w i t h l a t e r sediments has been r e - e x ~ o s e d , a s a t Goffys Cay, C a r r i e Bow Cay, and p a r t i a l l y a t Paunch and Curlew Cays. (b) I n c i p i e n t beachrock noted i n 1960-61 h a s been revealed by shore r e t r c a t and s t a n d s away from p r e s e n t s h o r e a s t r u e r e l i c t beachrock; t h i s i s w e l l shown on t h e l e e s h o r e of South Water Cay and t h e s o u t h shore of Sandbore Cay. ( c ) I n o t h e r a r e a s shore r e t r e a t h a s revealed beachrock t h e e x i s t e n c e of which was not p r e v i o u s l y suspected. This i s w e l l shown on t h e lagoon shore of Sandbore Cay, and i n t h e p e c u l i a r islandward-dipping beachrock on t h e seaward shore of C a r r i e Bow Cay, None of t h i s beachrock s t a n d s above o r below i t s i n t e r t i d a l l o c a t i o n ; i t s p o s i t i o n r e v e a l s h o r i z o n t a l s h i f t s i n
s h o r e l i n e l o c a t i o n only. A t C a r r i e Bow Cay, two l i n e s of f r e s h beach- rock have been r e v e a l e d marking t h e p o s i t i o n of a former ephemeral sand- s p i t , which i n 1960 was s u b j e c t t o o $ e r t o p ~ i n g by waves: i t i s d i f f i c u l t t o s e e how f r e s h groundwater could havc played any p a r t i n t h e formation of t h i s rock, i n t h e manner R u s s e l l (1962) h a s described. Most of t h e new i n t e r t i d a l beachrock v a r i e s considerably i n degree of cementation, but a l l
c o n s i s t s of t y p i c a l beachsands, The r e v e r s e - d i p p i n g beachrock a t C a r r i e Bow Cay i s well-cemented, t h a t marking t h e ephemeral s p i t i s v e r y s o f t , Nany of t h e s o f t e r e x p o s u r e s , e s p e c i a l l y on seaward s h o r e s , may n o t s u r - v i v e a g a i n s t wave a t t a c k f o r a s u f f i c i e n t l y l o n g p e r i o d f o r t h e secondary c e m e n t a t i o n which R u s s e l l i n d i c a t e s i s n e c e s s a r y f o r complete l i t h i f i c a t i o n . No f r e s h beachrock anywhere i n B r i t i s h Honduras a p p r o a c h e s t h e t o u g h n e s s of long-exposed examples, such a s t h a t a t Half Moon Cay. Examination o f hand samples shows t h a t f r e s h l y exposed beachrock c o n s i s t s o f l o o s e l y bonded p a r t i c l e s i n a v e r y f r i a b l e cement, w i t h many open s p a c e s ; whereas i n o l d e r beachrock t h e g r a i n s a r e t i g h t l y cemented i n t o a s o l i d mass w i t h few i n t e r s t i c e s , e x c e p t where t h e g r a i n s a r e l a r g e , dominantly Halimeda, where i n t e r s t i c e s s t i l l remain open.
IEJhile it i s t r u e t h a t beachrock exvosures a r e o n l y a s s o c i a t e d w i t h r e t r e a t i n g b e a c h e s , a s Seymour S e m l l (1935, 511) and S t a n l e y G a r d i n c r
(1930, 1 6 ) r e c o g n i s e d , t h e B r i t i s h Honduras d a t a shows q u i t e c l e a r l y t h a t m a s s i v e beachrock may form on s t a b l e , and even a g g r a d i n g s h o r e s , a s on t h e
i n t c r i o r lagoon beach of Sandbore Cay. I n t h i s i n s t a n c e , t h e t h i c k c e s s of bsachrock on t h e s t a b 1 6 lagoon s h o r e much exceeds t h a t on t h e e r o d i n g seaward s h o r e , Beachrock c l e a r l y forms b e n e a t h beaches, n o t a t t h e s u r f a c e ; where i n c i p i e n t beachrock o u t c r o p s a t t h e f o o t o f b e a c h e s , it may b6
t r a c e d i n l a n d b e n e a t h beach sands, i n t h e same way i n B r i t i s h Honduras a s mapped by R u s s e l l e l s e w h e r e i n t h e Caribbean, The c a u s e of r e v e r s e d i p
i n beachrock i s unknown: i t s o c c u r r e n c e a t C a r r i e Bow Cay and Southwest Cay 11, Gloverqs Reef
(ARB 87,
97), s u g g e s t s t h a t it may b e a more wide-s p r e a d phenomenon t h a n p r e v i o u s l y r e c o g n i s e d , Widespread evidence of r e t r e a t of c a y s a c r o s s r e e f f l a t s s u g g e s t s t h a t r e v e r s e d beachrock may have been formed on lagoon beaches and exposed on seaward s h o r e s a s t h e c a y r e t r e a t e d c o m p l e t e l y a c r o s s i t ; b u t such a n i n t e r p r e t a t i o n n e e d s more sub- s t a n t i a l e v i d e n c e , The r o l e of beachrock a s a s t a b i l i s i n g element i n c a y physiography h a s o f t e n bcen s t r e s s e d : t h u s S t e e r s (1937, 1 2 ) w r i t e s t h a t :%he f o r m a t i o n of beachrock on a n y c o r a l i s l e t o f f t h i Queensland c o a s t i s a s t a b i l i s i n g f a c t o r , and t h e permanency of a n i s l e t p r o b a b l y depends more on t h i s f a c t o r t h a n on any o t h e r ' $ . The p r e s e n c e o f many e x p o s u r e s
of r e l i c t beachrock o f f p r e s e n t c a y s h o r e s shows t h a t t h i s p r o t e c t i o n i s a t b e s t i m p e r f e c t ; d u r i n g H u r r i c a n e H a t t i e no i n s t a n c e w a s s e e n where t h e p r e s e n c e of beachrock r e s t r i c t e d s h o r e r e t r e a t , when compared w i t h n e a r b y a r e a s l a c k i n g b e a c h r o c k , By c o n t r a s t , well-developed and u n d i s t u r b e d n a t u r a l v e g e t a t i o n a p p c a r e d much more e f f e c t i v e i n c a y p r e s e r v a t i o n .
Cay Sandstone
Cementation of c a y s a n d s above s e a l e v e l was d e s c r i b e d by Kuenen
(1933,
86-88) and Seymour S e w e l l(1935 ,
502-512), b u t on a c c o u n t of i t s l e s s d i s t i n c t i v e c h a r a c t e r ; s t i r s and i ~ f r e q u e n t exposure it h a s n o t o f t e n been d e s c r i b e d . Moresby(1835,
l 5 9 ) , Seymour S e w e l l (1935,
502-512) and S t a n l e y G a r d i n e r (1907) noted it i n t h e IJIaldive I s l a n d s , where G a r d i n e r termed i t 19tuffejl; Kuenen a l s o n o t e d it i n t h e E a s t I n d i e s (1933; 1950, 434-435). Following H u r r i c a n e H a t t i e , t h e r e i s l i t t l e doubt t h a t rocke x p o s u r e s on t h e B r i t i s h Honduras c a y s f o r m e r l y i n t e r p r e t e d as p o s s i b l e r a i s e d beachrocks a r e i n f a c t cay s a n d s t o n e s , Such r o c k s a r e b e s t exposed a t Harry J o n e s P o i n t ard Big Cay Bokel, T u r n e f f e . The r o c k h a s a n a l m o s t h o r i z o n t a l upper s u r f a c e , b u t may d i p l a t e r a l l y a l o n g t h e boach w i t h
v a r i a t i o n i n t h e h e i g h t of t h e cay s u r f a c e i t s e l f . The upper s u r f a c e i s smooth when f r e s h , b u t becomes blackened, p i t t e d and eroded on exposure t o t h e a t m o s ~ h e r e . Undermining a f t e r exDosure a l o n g t h e s h o r e may l e a d t o t h e b r e a k i n g o f f of s l a b s of r o c k , which t h e n l i e on t h e s h o r e a t t h e a n g l e of t h e beach, and where t h i n may b e confused wiLh i n t e r t i d a l beach- rock. T h i s i s w e l l seen a t Northern Cay, Lighthouse Reef. T p i c a l l y , c a y s a n d s t o n e l a c k s t h e l a r g e unbroken Halimeda g r a i n s , l a r g e f r e s h r e d Homotrema, and d i s t i n c t i v e v e r t i c a l s i z e - g r a d i n g of i n t e r t i d a l beachrock.
I t s c o n s t i t u e n t s a r e f i n e r , and t h e cementation i n f r e s h samples i s super- f i c i a l and poor. A f t e r exposure it a p p e a r s t h a t a secondary cementation t a k e s p l a c e , a n a l o g o u s t o t h a t which R u s s e l l d e m o n s t r a t e s f o r i n t e r t i d a l beachrock (1962), r e s u l t i n g i n a tough, r i n g i n g r o c k ; t h i s was i n f a c t n o t e d l o n g ago by Moresby (1835, 400).
Cay sandstone i s normally e x ~ o s e d b y r e t r e a t i n g s h o r e s ; a t Cay Chapel i t h a s been r e v e a l e d by s t r i p p i n g of s u r f a c e sand a c r o s s t h e f r o n t s l o p e of' a beach ridge. Here it forms a n i n c l i n e d d a t e , d i p p i n g seaward though w e l l above s e a l e v e l , and s t i l l very f r i a b l e . A f t e r f o u r months of CxDcsure t h e s u r f a c e was e t c h e d i n t o hole-and-ridge p a t t e r n s o f t e n a s s o c i a t e d w i t h wind e r o s i o n i n d e s e r t s ( c o t t o n , 1942). Nearby, t h e same rock i s seen i n a beach s c a r p , covered w i t h 1 2 i n c h e s of uncemented sand and s o i l , w i t h v e g e t a t i o n . S i m i l a r though weaker c e m e n t a t i o n was s e e n a t Cay Caulker, i n o v e r d e e ~ e n e d ro a d s t r a n s v e r s e t o t h e seaward beach.
Nowhere was cay s a n d s t o n e seen t o b e o v e r l a i n w i t h more t h a n
3
f e e t of uncementcd sand. It i s presumably a s s o c i a t e d with p e r c o l a t i o n of r a i n - w a t e r t o t h e w a t e r - t a b l e , and w a s n o t s e e n on any cay without a knownf r e s h - w a t e r l e n s .
The recency o f t h e cementation, and f o r m a t i o n a t t h e p r e s e n t a l t i t u d e a r e beyond d i s p u t e , A t Harry J o n e s , cay sandstone c o n t a i n s Kaya p o t t e r y , Kost newly exposed s u r f a c e s c o n t a i n coconut r o o t s w i t h i n t h e rock m a t r i x , At t h e i n n e r edge of t h e exposed a r e a , t h e s e r o o t s may b e followed up
i n t o o v e r l y i n g u n d i s t u r b e d sand. F u r t h e r , cementation beneath cay s u r f a c e s i s l e s s advanced t h a n on exposed a r e a s : t h i s i s w e l l shown by a s c o u r h o l e c u t through t h e c a y s u r f a c e i n t o u n d e r l y i n g cay s a n d s t o n e a t Northern Cay, Lighthouse Reef ( F i g u r e 63). Here a weak h o r i ~ o n of cementation was exposed on a l l s i d e s of t h e scour h o l e , with uncemented sands both above and below. Outcrops of cemented m a t e r i a l , t o o r e s t r i c t e d t o d e t e r m i n e form, a s a t South Water Cay, a r e h e r e i n t e r p r e t e d a s cay s a n d s t o n e s , r a t h e r t h a n r e s o r t t o h m o t h e s e s of warping o r e u s t a t i c f l u c t u a t i o n s of sea
l e v e l . It should b e n o t e d t h a t t h i s r e - i n t e r p r e t a t i o n of t h e Harry J o n e s exposure renoves what evidence t h e r e was f o r movements of t h i s k i n d a t T u r n e f f e I s l a n d s (ARB
87,
109-111): no f e a t u r e o f t h e s e c a y s now r e q u i r e s a n y r e c e n t h i g h s t a n d of t h e s e a i n e x p l a n a t i o n o f it.Promenade Rock
T h i s name i s ilsed h e r e f o r t h e cemented mteri.al comprising a r e s t r i c t e d t o p o g r a p h i c f e a t u r e , t h e promenade, d c s c r i b c d by S t o e r s initially f r o m
t h e Great Barrier r t ~ f Tslands (1929, 252-256; 1937s 27, 119) and i n g r e a t e r d e t a i l f o r t h e Morant Cays, Jamaica (1940a, 39-40: 1940b9 309).
A t Morant Cays S t e e r s found p l a t f o r m s w i t h eroded, h o r i z o n t a l upper s u r - f a c e s , showing no a p p a r e n t d i p , 12-18 i n c h e s above s e a l e v e l , composed
of beach sands and s i m i l a r t o t r u e beachrock. The rock s u r f a c e appeared t o be recemented and was much h a r d e r t h a n t h e i n t e r i o r . Promenades always occurred on t h e windward s i d e s of cays. The s i m i l a r i t y of t h e s e prome- nades with t h e conglomerate ~ l a t f o r m a t Half Moon Cay h a s been p r e v i o u s l y noted (ARB
87,
1C7). During K u r r i c a n e H a t t i e , low-lying p l a t f o r m s of cemented sand and r u b b l e were e w o s e d b y s e v e r e s h o r e l i n e r e t r e a t a t Dead- man I and I1 Cays, Turneffe, where o r e v i o u s l y c n l y a s m a l l p a t c h of sup- p o s e d l y i n c i p i e n t beachrock had been seen. These resemble t h e Half Moon Cay exposures i n e v e r y t h i n g except d e g r e e of cementation. It a p p e a r s t h a t cementation of sands, s h i n g l e and r u b b l e must t a k e wlace beneath cays a t and s l i g h t l y above s e a l e v e l , and t h a t t h i s cemented m a t e r i a l h a s been exposed by e r o s i o n . The cernentation i s s u f f i c i e n t t o p r e s e r v e t h e form of t h e promenade, b u t n o t f o r t h e c o l l e c t i o n of specimens. A s i n t h e c a s e of beachrock and cay sandstone, it seems l i k e l y t h a t secondary cementation o c c u r s once t h e promenade i s ex-posed, t o t r a n s f o r m i t i n t o s o l i d r e s i s t a n t rock. Whether t h e Deadman exposures w i l l s u r v i v e long enough f o r t h e secondary cementation t o t a k e p l a c e has y e t t o b e seen, As i n t h e c a s e of cay sandstone t h e important c o ~ c l u s i o n i s t h a t promenades form b e n e a t h c a y s a t t h e i r p r e s e n t e l e v a t i o n s : and t h a t t h e y a r e n o t i n d i c a t o r s of r e l a t i v e change i n l a n d o r s e a l e v e l . T h i s e x p l a n a t i o n d o e s n o t p r e c l u d e t h e p o s s i b i l i t y t h a t e l e v a t i o n o r d e ~ r e s s i o n h a s occurred, through t e c t o n i c o r e u s t a t i c c a u s e s ; b u t it does mean t h a t without o t h e r evidence, pm'nenades and c a y sandstone e m o s u r e s a r e n o t i n themselves s u f f i c i e n t i n d i c a t o r sof such movements
.
S h i n g l e Rock
A t Cockroach Cay I1 t h e exposure of a v e r y c o a r s e s h i n g l e rock, csmented by a brownish cement, h a s been d e s c r i b e d i n Chapter 6. The cement a p p e a r s s u p e r f i c i a l l y s i m i l a r t o t h a t i n t h e c e n t r a l p a r t of Half Moon Cay, and i n t h e two f r e s h exwosures of t h e same rock on t h a t i s l a n d , A t Cockroach I1 t h e
resent
t o p o g r a p h i c form of t h e s h i n g l e rock i ss i m i l a r t o t h a t of t h e Cay Chapel c a y sandstone: i t l i e s on t h e f a c e of t h e beach r i d g e , i s r e s t r i c t e d i n a r e a w i t h v e r y s t e e p s i d e s , and d i p s
seawards. The main p o i n t of d i f f e r e n c e i s i n t h e brown cement. I n t h i s connection it i s i n t e r e s t i n g t o n o t e t h a t b o t h i s l a n d s where t h i s o c c u r s have s u b s t a n t i a l a r e a s of
-
Cordia-Bursera bush,Other Rock
F i n a l l y , s u r f a c e i n c r u s t a t i o n of sands, p e r h a p s r e s u l t i n g from inunda- t i o n by s a l t w a t e r d u r i n g t h e h u r r i c a n e , has been d e s c r i b e d a t Sandbore Cay, Lighthouse Reef. This i s v e r y f r i a b l e and t h i n , and t h e r e i s no
evidence t h a t l i t h i f i c a t i o n w i l l proceed s u f f i c i e n t l y t o form a s i g n i f i c a n t t o p o g r a p h i c f e a t u r e .
Apart from t h e Cockroach I1 s h i n g l e rock and t h e i n t e r i o r cementation a t Half Moon Cay, cemented m a t e r i a l s on c a y s a p p e a r t o c o n s i s t of c a y
sandstone, promenade rock, i n t e r t i d a l beachrock, and a few o t h e r exDosures which may b e i n t e r m e d i a t e between t h e s e groups. Seymour Sewell was f f b y no means f u l l y convincedrf (1935, 501) of t h e d i s t i n c t i o n between c a y sand- s t o n e and beachrock. Cay s a n d d o n e f o r m a t i o n i s a p p a r e n t l y a s s o c i a t e d
w i t h f r e s h w a t e r ~ e r c o l a t i o n ( ~ u e n e n , 195G,
434-5)
; and R u s s e l l (1962) h a s r e c e n t l y assembled evidence t o show t h a t primary cementation i n beach-rock formation i s a l s o a s s o c i a t e d w i t h t h e presence of a f r e s h w a t e r l e n s . While t h e r e a r e Inany examsles, some c i t e d i n t h i s r e p o r t , where i t i s d i f f i c u i t i f n o t i m p o s s i b l e t o a c c e p t t h e r o l e of f r e s h w a t e r i n beachrock formation, c h i e f l y on l o c a t i o n a l grcunds, it i s p e r t i n e n t t o e n q u i r e , i f R u s s e l l i s c o r r a c t ir, t h e c a s e s he d e s c r i b e s , what t h e e s e e n t i a l d i f f e r e n c e s a r e between i n t e ~ t i d a l beachrock and o t h e r r o c k s a s s o c i a t e d w i t h f r e s h
water b u t develoaed w e l l above high t i d e 1.evel. I f t h e r e i s , i n f a c t , no e s s e n t i a l d i f f e r e n c e , t h e n t h e u s e f u l n e s s of beachrock a s a r e l i a b l e i n d i - c a t o r of changes of l e v e l must be v e r y much reduced, i n s p i t e of t h e f a c t t h e f i e l d exposures of t h e two t y p e s of rock a r e c h a r a c t z r i s t i c a l l y corn- p l - e t e l y d i f f e r e n t
.
PATTERN OF HURRICANE DAMAGE TO SAND CAYS
CAY DESTROYED
@ MAJOR WYSIOGRAWK
DAMAGE VEGETATION
MAJOR VEGETATION BUT LITTLE
@ PHYSIOGRAPHIC DAMAGE
0 LITTLE VEGETATION CHANGE W l MUCH MARGINAL SAND AND SHINGLE DEPOSITON
0 NO SIGNIFICANT CHANGE DEPTH LESS THAN THREE FAT-S
0 20
WUTICAL MILES
e FIG. 62
HURRICANE SCOUR
HOLE
SMOOTH SURFACED U)NG
-
EXPOSED SANDSTONEEXPOSED BY MUCH ERODED
RECESSION OF SHORE CAY SANDSTONE TILTED BLOCK OF ERODED
SANDSTONE
SOFT SANDSTONE