ATOLL RESEARCH BULLETIN No. 125
RECONNAISSANCE GEOMORPHOLOGY OF RANGIROA ATOLL, TUAMOTU ARCHIPELAGO by D . R. S t o d d a r t
w i t h L I S T OF VASCULAR FLORA O F RANGIROA by M a r i e - H 6 l k n e Sachet
Issued by
THE SMITHSONIAN I N S T I T U T I O N Washington, D. C . , U . S . A.
March 30, 1969
Contents
Page I . General D e s c r i p t i o n
A . I n t r o d u c t i o n B . Acknowledgements
C . Gross form of Rangiroa D . Climate and t i d e s
11. The Northern o r Windward R i m A. General c h a r a c t e r i s t i c s B . Reef f l a t p r o f i l e s 111. The Western o r Leeward R i m
A. General c h a r a c t e r i s t i c s B . Reef f l a t p r o f i l e s
I V . The Southern o r Swellward R i m A. General c h a r a c t e r i s t i c s B . Reef f l a t p r o f i l e s
V. Problems of Rangiroa Geomorphology A. C o n s o l i d a t e d sediments
B . S u r f a c e f e a t u r e s o f t h e seaward r e e f f l a t s C . Unconsolidated sediments
D . T e n t a t i v e geomorphic h i s t o r y of Rangiroa Notes on land v e g e t a t i o n of Rangiroa
References
L i s t o f v a s c u l a r f l o r a o f Rangiroa, by ~ a r i e - H e ' l d n e S a c h e t
F i g u r e s ( f o l l o w i n g p . 44) F i g . 1 Location of Rangiroa A t o l l
F i g . 2 Rangiroa A t o l l
F i g . 3 Rangiroa r a i n f a l l 1959-65: monthly t o t a l s Fig. 4 Rangiroa r a i n f a l l 1960-65: s i n g l e - d a y maxima F i g . 5 Schematic diagram of n o r t h e r n r i m with i s l a n d s
F i g . 6 Schematic s e c t i o n s of i s l a n d s on n o r t h and s o u t h rims Fig. 7 P r o f i l e s 1-6, n o r t h e r n r i m
Fig. 8 P r o f i l e s 7-8, w e s t e r n r i m
F i g . 9 Schematic diagram o f s o u t h e r n r i m with i s l a n d s F i g . 10 P r o f i l e s 9-11, s o u t h e r n r i m
P l a t e s
1 T i p u t a , n o r t h s h o r e : seaward s h i n g l e r i d g e , smooth e r o s i o n ramp,
and beachrock and r u b b l e . R e l i c t beachrock on r e e f f l a t i n background.
2 T i p u t a , n o r t h s h o r e , west end, l o o k i n g towards Reporepo: s t e e p s h i n g l e beach, e r o s i o n ramp, and conglomerate beachrock.
3 T i p u t a , n o r t h s h o r e : s h i n g l e beach w i t h massive beach-foot conglomeratic beachrock. Note t h e narrowness of t h e r e e f f l a t . 4 T i p u t a - T e p a e t i a h o a :
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view from t h e lagoonward end, on T i p u t a ,towards t h e seaward r e e f .
5 T i p u t a , n o r t h s h o r e : b e a c h - c r e s t v e g e t a t i o n hedge of T o u r n e f o r t i a and tiedyotis
,
showing d e f o l i a t i o n , w i t h coconut palms.6 T i p u t a , s o u t h s h o r e , e a s t end, looking westwards: low sandy lagoon beach with coconut p l a n t a t i o n .
7 T e p a e t i a , s o u t h s h o r e , west end, n e a r e n t r a n c e t o hoa:
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outcrop of i s l a n d conglomerate on a sandy beach. Note t h e absence of a lagoon r e e f .8 Avatoru: markcage a t west end of t h e i s l a n d , looking n o r t h . 9 T e p a e t i a , n o r t h s h o r e , e a s t end, view e a s t : edge o f seaward r e e f
f l a t a t low w a t e r : pink a l g a l zone on t h e l e f t (covered with w a t e r ) , d i s s e c t e d orange zone i n t h e c e n t e r ; d r y i n g r e e f f l a t t o t h e r i g h t . 10 T e r e i a o , west s h o r e , view s o u t h : edge o f seaward r e e f f l a t a t low
w a t e r : d i s s e c t e d pink a l g a l zone.
11 T e r e i a o : massive conglomerate a t t h e f o o t of t h e seaward beach.
1 2 T e r e i a o : f r e s h c o r a l s h i n g l e on t h e c r e s t o f t h e seaward beach.
13 T e r e i a o : lagoon-side markcage with Cladiwn.
14 Maeherehonae: l a r g e lagoon-side mare/cage w i t h sandy lagoonward sand s t r i p i n t h e foreground.
15 Maeherehonae: view towards t h e lagoon from t h e sum111it of t h e dunes, a c r o s s t h e mar8cage. Pemphis, Pandanus and coconuts i n t h e f o r e - ground.
16 Maeherehonae: a r e c e n t l y - e x c a v a t e d
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marae on t o p of t h e lagoon-side dunes.17 Maeherehonae: conglomerate p l a t f o r m , with Pemphis a c i d u l a , a t w a t e r - l e v e l between t h e dunes.
18 Maeherehonae, west s h o r e , n o r t h e r n end: s t o r m b l o c k s on t h e seaisard r e e f f l a t , photographed from t h e seaward beach. Note tile human f i g u r e on t h e l a r g e s t b l o c k .
19 Porahu, seaward s h o r e :
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f e o s e p a r a t e d by a narrow moat from t h e seaward s h o r e of t h e i s l a n d .20 Porahu: i s l a n d w a r d margin of t h e
-
f e o shown i n P l a t e 1'3. Note tile narrow b a s a l e r o s i o n p l a t f o r m .21 Porahu, west s h o r e : i s l a n d conglomerate unconforinably o v e r l y i n g t r u n c a t e d r e e f c o r a l s . Seaward
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f e o i n t h e background.22 P e a r i : seaward r e e f edge with e x f o l i a t i n g r e e f rock on t h e r e e f f l a t 23 P e a r i : e x f o l i a t i n g r e e f rock on t h e seaward r e e f f l a t .
24 Peari-Baihoa hoa:
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i s l a n d conglomerate outcropping i n t h e hoa-
w a l l s w i t h seaward-
f e o i n t h e background.25 Vaihoa-Tuoto h o a :
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s m a l l sand cay w i t h Pandanus perched a g a i n s t t h e seaward f e o .-
26 U t o t o :
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f e o with Pempllis and perched b e a c h , looking e a s t w a r d s . 27 Utoto: d i s s e c t e d seaward margin of t h e-
f e o .IIECONNAISSANCE GEOMOI%PHOLOGY OF RANGIROA ATOLL, TUA- RIOTU AKCHIPELAGO
by D . R . S t o d d a r t
I . GENERAL DESCRIPTION A . I n t r o d u c t i o n
liangiroa ( F i g u r e 1) i s t h e l a r g e s t a t o l l i n t h e Tuamotu Archipelago, and s i n c e t h e i n s t i t u t i o n of a r e g u l a r a i r s e r v i c e , t h e most a c c e s s i b l e ; y e t i n common w i t h o t h e r Tuamotuan a t o l l s i t h a s r a r e l y been v i s i t e d by s c i e n t i s t s and i s b a r e l y mentioned i n t h e l i t e r a t u r e . Dana (1849) p u b l i s h e d b r i e f n o t e s follor.iing t h e Wilkes E x p e d i t i o n ; b u t t h e only f u l l account i s t h a t by Agassiz (1903). A g a s s i z ' s d e s c r i p t i o n s a r e , however, verbose and i m p r e c i s e , and marred by m i s i n t e r p r e t a t i o n s o f major a t o l l f e a t u r e s . I n t h e s t u d y o f Tuamotuan a t o l l s , a major r e f e r - ence p o i n t i s p r o v i d e d by t h e r e p o r t s o f t h e 1952 P a c i f i c S c i e n c e Board E x p e d i t i o n t o R a r o i a A t o l l , l e d by N . D . Newell. Major r e p o r t s r e s u l t - i n g from t h i s e x p e d i t i o n a r e t h o s e of Newel1 (1953, 1956) on geology and geomorphology; Doty and o t h e r s (1954), Doty and Morrison (1954), and blorrison (1954) on p l a n t and animal s y s t e m a t i c s and ecology; Harry
(1953) on i c h t h y o l o g y ; and Danielsson (1954, 1956) on c u l t u r e and econoinic l i f e . More r e c e n t l y , Ranson (1962 and e a r l i e r p a p e r s ) h a s p u b l i s h e d a c c o u n t s o f l l i k u e r u and o t h e r a t o l l s , and d e t a i l e d work h a s been c a r r i e d o u t i n connection with t h e French weapons t e s t s a t Mururoa a t t h e e a s t e r n end o f t h e a r c h i p e l a g o (Lalou and o t h e r s 1966, Chauveau and o t h e r s 1967). S i n c e A g a s s i z ' s work a t Rangiroa, T e r c i n i e r (1956) h a s p u b l i s h e d b r i e f l y on s o i l s , and O t t i n o (1965) and Garanger and
Lavondes (1966) have i s s u e d f i r s t r e p o r t s on c u l t u r e h i s t o r y . The a t o l l h a s been r e c e n t l y v i s i t e d by Dr. M.-H. S a c h e t i n 1963, and by
P r o f e s s o r A . G u i l c h e r and t h e p r e s e n t w r i t e r i n 1965. Aubert de l a
R G ~
(1964) h a s p u b l i s h e d a g e n e r a l account of t h e a t o l l s o f t h e Tuamotu Archipelago, though he d i d n o t v i s i t Rangiroa.
Because o f s h o r t a g e of t i m e , o b s e r v a t i o n s i n 1965 were l i m i t e d t o t h e s u p e r f i c i a l morphology o f t h e a t o l l r i m a t a s e r i e s o f sample l o c a t i o n s chosen t o i l l u s t r a t e d i v e r s e exposure environments. These l o c a t i o n s covered t h e n o r t h , west and s o u t h rims of t h e a t o l l . No ob- s e r v a t i o n s could b e made a t t h e e a s t e r n end o f t h e a t o l l , But Dr. Sachet s p e n t some time t h e r e i n 1963, and t h u s i n f o r m a t i o n i s a v a i l a b l e f o r p o i n t s on t h e e n t i r e p e r i m e t e r . 'I'his r e p o r t c o n s i s t s o f t h r e e p a r t s :
(1; a b r i e f g e n e r a l account of t h e g r o s s form and environment o f Rangiroa;
( 2 ) t h e s y s t e i n a t i c d c s c r i p t i o n of t r a n s e c t s around t h e a t o l l rim, w i t h emphasis on d e t a i l s of morphology; and
(3) a summary o f a t o l l geomorphology a s known a t p r e s e n t , w i t h
emphasis on t h e n a t u r e and form of c a r b o n a t e r o c k s and uncon- , . s o l i d a t e d s e d i m e n t s , and a s u g g e s t e d summary o f r e c e n t
geomorphic development.
B
.
AcknowledgementsThis s t u d y would have been i m p o s s i b l e without t h e h o s p i t a l i t y and a c t i v e h e l p o f M. Maurice Pomier, D i r e c t o r of t h e T i p u t a s t a t i o n o f t h e I n s t i t u t de Recherches pour l e s H u i l e s e t Ole'agineux (I.R.H.O.).
M. Pomier made i t p o s s i b l e t o v i s i t a l l t h e l o c a t i o n s d e s c r i b e d i n t h i s r e p o r t , and gave a g r e a t d e a l o f h i s time i n a s s i s t i n g t h e work.
D r . M a r i e - ~ k l & n e Sachet f i r s t drew my a t t e n t i o n t o Rangiroa, and was t i r e l e s s i n making arrangements f o r my v i s i t . F i n a l l y , I thank t h e Royal S o c i e t y of London, f o r t h e o p p o r t u n i t y t o v i s i t French P o l y n e s i a f o l l o w i n g t h e Royal S o c i e t y Expedition t o t h e B r i t i s h Solomon I s l a n d s i n 1965.
C. Gross Form o f Rangiroa
Rangiroa i s s i t u a t e d towards t h e northwestern end o f t h e Tuamotu Archipelago, a c h a i n o f a t o l l s 600 m i l e s long i n t h e c e n t e r o f t h e P a c i f i c Ocean ( F i g u r e 1 ) . According t o Menard (1964) t h e Tuamotus l i e on t h e s i t e o f p a r t o f t h e foundered Darwin R i s e . Rangiroa i s 125 m i l e s n o r t h e a s t of T a h i t i , t h e l a r g e s t o f t h e r e e f - e n c i r c l e d v o l c a n i c S o c i e t y I s l a n d s . Rangiroa i s i t s e l f l a r g e enough t o e n t i r e l y c o n t a i n t h e i s l a n d o f T a h i t i w i t h i n i t s lagoon: i t h a s a maximum l e n g t h of 54 m i l e s ( n o r t h w e s t - s o u t h e a s t ) and b r e a d t h of 23 m i l e s . Agassiz s t a t e s
(1903, 33) t h a t i t i s "somewhat pear-shaped" i n o u t l i n e . I n q u a n t i t a - t i v e terms t h e shape may be e x p r e s s e d i n a number of i n d i c e s , o f which t h e E l l i p t i c i t y Index i s most u s e f u l . Table 1 g i v e s c a l c u l a t e d shape i n d i c e s f o r Rangiroa and f o r a sample of 99 o t h e r a t o l l s . Rangiroa i s t h u s r a t h e r more e l o n g a t e o r e l l i p t i c a l t h a n t h e mean f o r a t o l l s s o f a r measured.
Table 1 --
Index Rangiroa A t o l l
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Mean v a l u e s f o r 99 a t o l l s ---
1 /I e 3.59 2.87 (modal c l a s s 1 . 5 - 2 . 0 )
F 0 . 2 2 0.35
Rc 0 . 4 3 0.57
Re 0 . 5 3 0.64
1/
S t o d d a r t (1966)The s i z e o f Rangiroa ( F i g u r e 2) i s i t s most immediately s t r i k i n g c h a r a c t e r i s t i c : t h e p e r i p h e r a l r e e f i s 137 m i l e s long, and t h e a t o l l h a s a t o t a l a r e a of 633 s q u a r e m i l e s . T h i s compares w i t h 189 m i l e s and
902 s q u a r e m i l e s , r e s p e c t i v e l y , f o r Kwajalein A t o l l i n t h e Marshall I s l a n d s , t h e w o r l d ' s l a r g e s t a t o l l . Dimentional s t a t i s t i c s f o r
Rangiroa, d e r i v e d from t h e 1:100,000 c h a r t by L t . d ' h g l e j a n C h a t i l l o n , Mission Hydrographique en Polyn6sie F r a n ~ a i s e 1959, t o g e t h e r with d a t a f o r R a r o i a and f o u r Marshall I s l a n d s a t o l l s , i n c l u d i n g Kwajalein, a r e g i v e n i n Table 2.
The p e r i p h e r a l r e e f f l a t s o f t h e a t o l l v a r y c o n s i d e r a b l y i n width, b e i n g g e n e r a l l y narrower on t h e windward and w i d e r on t h e leeward s i d e s . On t h e n o r t h e r n r i m t h e f l a t i s 350-700 yards wide; on t h e west r i m 475-700 y a r d s wide; and on t h e south r i m 825-1175 y a r d s wide. The g r e a t e s t width i s found a t t h e e a s t e r n end of t h e a t o l l , where n o r t h and s o u t h rims meet i n a s h a r p a n g l e . In common with o t h e r Tuamotu a t o l l s , much of t h e r e e f r i m i s covered with i s l a n d s . As a t Raroia, a t o l l land i s most continuous on t h e narrowest r i m , and l e a s t c o n t i n - uous where t h e r i m i s w i d e s t ; r i m dimensions i n b o t h c a s e s a r e s i m i l a r , though o r i e n t a t i o n i s n o t t h e same (Newel1 1965, 330). The 1959 c h a r t o f Rangiroa shows 241 i s l a n d s , of which a l l e x c e p t 10 a r e on t h e p e r i p h - e r a l r e e f r i m . I s l a n d s cover o n e - t h i r d o f t h e a r e a o f t h e p e r i p h e r a l r i m , compared with 35% a t R a r o i a , and c o n t r a s t i n g w i t h l e s s t h a n 10%
on some Marshall I s l a n d a t o l l s .
There a r e two p a s s e s through t h e r i m , b o t h on t h e n o r t h o r
windward s i d e . Avatoru, t h e w i d e s t , i s 400 y a r d s wide, with depths of up t o 10 fathoms; i t b i f u r c a t e s lagoonward. T i p u t a Pass i s 330 y a r d s wide, w i t h l e a s t d e p t h s o f 9 fathoms; i t a l s o h a s a p a t c h r e e f with
i s l a n d on t h e lagoon s i d e . The p a s s e s a r e c h a r t e d a t a s c a l e of
1:15,000 i n Admiralty Chart 1175. In both p a s s e s t h e r e a r e s t r o n g and complex c u r r e n t s ; Agassiz found t h e s e t o reach 4-1/2
-
8 k n o t s (Agassiz 1903, 33-34). There i s l i t t l e information on t h e lagoon f l o o r .Agassiz (1903, 36, 45) took soundings between Avatoru and Fenuaroa, and found a h a r d , f a i r l y f l a t f l o o r , r e a c h i n g 14-16 fathoms i n t h e c e n t e r , with a wide a r e a l e s s t h a n 5 fathoms n e a r t h e s o u t h r i m . Within t h e two p a s s e s t h e f l o o r l i e s a t depths o f 10-15 fathoms. S c a t t e r e d soundings
d u r i n g t h e 1959 s u r v e y gave f i v e r e a d i n g s of more t h a n 30 fathoms, with a maximum o f 34.4 fathoms ; t h i s compares w i t h a maximum o f 3 0 . 1 fathoms a t Raroia. There seem t o be few lagoon p a t c h r e e f s a c t u a l l y b r e a k i n g s u r f a c e i n t h e c e n t r a l p a r t of t h e lagoon, b u t a g r e a t e r number r i s i n g t o about 5 fathoms. Patch r e e f s a r e c o n c e n t r a t e d i n t h e narrow e a s t e r n p a r t of t h e lagoon, and a l o n g t h e western s i d e . Apart from t h e i s l a n d s on p a t c h e s n e a r t h e p a s s e s t h e r e a r e seven s m a l l i s l a n d s on lagoon p a t c h r e e f s .
Soundings on t h e o u t e r s l o p e s of t h e a t o l l average about 600 fathoms a t d i s t a n c e s o f 2 t o 4 n a u t i c a l m i l e s from t h e r e e f . Slopes a r e s t e e p e s t t o t h e n o r t h ( 1 i n 3 t o 1 i n 4 ) , e a s t ( 1 i n 3 . 6 ) , and s o u t h (1 i n 4 ) , and l e a s t s t e e p along t h e west s i d e ( 1 i n 6 t o 1 i n 7 ) . Soundings by Agassiz show t h a t t h e o u t e r s l o p e s on b o t h n o r t h and
s o u t h s i d e s of t h e a t o l l a r e s t e e p e s t n e a r t h e r e e f s , and become g e n t l e r a t depth.
A t o l l P e r i m e t e r Length Breadth T o t a l a r e a Lagoon a r e a Reef a r e a I s l a n d a r e a Number miles m i l e s m i l e s s q . m i l e s s q . m i l e s s q . m i l e s s q . m i l e s o f
i s l a n d s
RANG I
ROAL/
137 5 4 2 3 633 583 50 16.6 241Raroia- 21 56 2 7 9 154 131 23 8 280
Kwajalein- 3/ 189 80- 41 20- 41 902- 4/ 880 ( c . 22) 5 . & 9 2- 4/
Ronge l a p 3/ 9 3 33 20 426- 4/ 396 34 2.5- 4/ 5
&/
Eniwetok- 3/ 72 25 20 395- 41 36 0 32 2.5- 4/ 39- 4/
Bikini- 31 66 26 15 267- 41 243 30 2.8 2 9- 41
Sources :
L/
1:100,000 c h a r t by L t . d ' h g l e j a n C h a t i l l o n , 1959.2
'
Newell 1956, 329-330.3
'
Emery, Tracey and Ladd 1954, 22.41
Fosberg 1956, 34 and Figure 10.D . Climate and t i d e s
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The f o l l o w i n g summary o f wind c o n d i t i o n s i n t h e Taumotus i s taken from t h e Geographical Handbooks S e r i e s : P a c i f i c I s l a n d s , volume 2
(1943), p . 195-196.
"The t r a d e winds blow o v e r t h e a r c h i p e l a g o throughout t h e y e a r b u t a r e o f t e n d i s t u r b e d ; t h e y a r e most r e g u l a r from May t o October and t h e n blow f r e s h l y . T h e i r mean d i r e c t i o n i s e a s t , drawing t o e a s t - s o u t h - e a s t from June t o October, and t h e n t o e a s t - n o r t h - e a s t from November t o May. A t t h i s l a t t e r p e r i o d t h e y a r e o f t e n l i g h t , and may a l t e r n a t e w i t h heavy u n s t a b l e s q u a l l s from n o r t h t o s o u t h - w e s t , t h e s e s q u a l l s b e i n g more f r e q u e n t by n i g h t t h a n by day. S t r o n g winds from t h e s o u t h - e a s t a r e l i a b l e t o occur from May t o September, b u t e s p e c i a l l y i n August, when t h e y a r e a p t t o l a s t from f o u r t o e i g h t days. From November t o May, g a l e s from n o r t h t o south-west a r e r e l a t i v e l y f r e q u e n t , e s p e c i a l l y i n J a n u a r y t o March. I n J a n u a r y a g a l e a r i s i n g i n t h e north-west u s u a l l y s h i f t s t o n o r t h and n o r t h - e a s t ; a t o t h e r t i m e s t h e tendency i s f o r i t t o s h i f t from north-west t o s o u t h - w e s t .
"The Tuamotu group i s s u b j e c t t o h u r r i c a n e s , which occur a s a r u l e towards t h e end of t h e y e a r , o r i n J a n u a r y o r February. Normally, however, t h e y occur l e s s t h a n once i n two y e a r s . H u r r i c a n e s ( t r o p i c a l cyclones) of p a r t i c u l a r f o r c e have been recorded i n September 1877, February 1878, J a n u a r y 1903, and February 1906."
C l i m a t i c d a t a f o r t h e I.R.H.O. S t a t i o n a t T i p u t a , Rangiroa, a r e a v a i l a b l e s i n c e 1959. The mean annual r a i n f a l l f o r 1959-64 was 57 i n c h e s (1449 nun), b u t t h i s f i g u r e conceals c o n s i d e r a b l e v a r i a t i o n from y e a r t o y e a r . R a i n f a l l t e n d s t o b e h i g h e s t i n November-January, when i t i s a l s o l e a s t r e l i a b l e , and lowest i n April-May and again i n September. The s c a t t e r diagram of monthly r a i n f a l l o v e r t h e p e r i o d of r e c o r d (Figure 3) shows t h i s v i v i d l y . The range of a c t u a l monthly r a i n f a l l s recorded i n each month even over t h i s s h o r t p e r i o d g i v e s a s i m i l a r i m p r e s s i o n , a s shown i n Table 3. E x c e p t i o n a l l y h i g h monthly f i g u r e s appear t o r e s u l t l a r g e l y from t h e r a i n f a l l r e c e i v e d d u r i n g s i n g l e s t o r m s : i n each of t h e y e a r s of r e c o r d t h e r e was one storm y i e l d i n g more t h a n 100 mm o f r a i n f a l l i n a s i n g l e day, w i t h a maximum of 1 5 9 . 1 nun i n one day i n January 1965. These e x c e p t i o n a l storms a r e w e l l - d i s t r i b u t e d throughout t h e y e a r (Figure 4 ) , though with minima i n May and September.
Other c l i m a t i c d a t a a r e l e s s complete. Mean annual t e m p e r a t u r e i n 1963 was 27.7'C and i n 1964, 27.6'C. The v a r i a t i o n i n monthly means i s approximately 3'C: from a mini~num of 25.7-26.2" i n J u l y t o a maximum o f 28.8-29.3"C i n February-March. I n s o l a t i o n i s g r e a t e r t h a n 200 hours p e r month i n a l l months e x c e p t November and December, and t o t a l s between 2500 and 3000 hours p e r annum (1963: 2671 h o u r s ; 1964: 2893 h o u r s ) . No wind o r p r e s s u r e r e c o r d s a r e k e p t a t Rangiroa.
T a b l e
&'
--
J a n Feb Mar APT May Jun J u l Aug S e p t Oct Nov De c
- - - - - - - -
A: 370.8 292.5 224.5 212.5 340.2 248.0 9 8 . 8 1 7 1 . 0 3 4 . 7 2 8 6 . 5 4 9 6 . 1 321.4 B : 6 2 . 5 4 4 . 3 5 4 . 1 27.0 1 1 . 9 9 . 0 1 1 . 9 25.0 2 4 . 7 1 6 . 0 3 6 . 4 3 3 . 7 .
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S o u r c e : R a i n f a l l r e c o r d s , I . R . H . O . , T i p u t a . A. Maximum r e c o r d e d f o r e a c h month, mm, 1959-65 B. Minimum r e c o r d e d f o r e a c h month, mm, 1959-65Newel1 (1956) summarized s e a c o n d i t i o n s a f f e c t i n g t h e Tuamotu a t o l l s a s f o l l o w s :
(1) P r e v a i l i n g t r a d e winds from t h e e a s t g i v e heavy s e a s on t h e n o r t h e a s t o r windward s i d e s o f a t o l l s .
(2) Southern Ocean s w e l l b r e a k s h e a v i l y on t h e s o u t h o r swellward s i d e s of a t o l l s .
(3) Occasional h u r r i c a n e s o r t r o p i c a l storms s t r i k e a t o l l s i n t h e i r northwest o r stormward q u a r t e r .
T h i s framework may be c o n v e n i e n t l y used f o r c a t e g o r i z i n g t h e rims o f Rangiroa.
According t o Wiens (1962, 214) t h e s p r i n g t i d e range a t Rangiroa i s 2 . 1 f e e t ; t h i s compares c l o s e l y with a l i t t l e under 2 f e e t a t R a r o i a (Newel1 1956, 329).
11. THE NORTHERN OR WINDWARD RIM A. General C h a r a c t e r i s t i c s
The n o r t h e r n r e e f f l a t v a r i e s i n width from 350 t o 700 y a r d s , r e a c h i n g a maximum o f 1050 y a r d s a t i t s e a s t e r n end. I t i s approx- i m a t e l y 50 m i l e s long, and i s surmounted by i s l a n d s f o r some 85% o f t h i s l e n g t h . Five of t h e i s l a n d s a r e more t h a n a mile l o n g : most of t h e r e s t a r e between 200 and 600 y a r d s i n l e n g t h , b u t show c l e a r e v i d - ence o f b e i n g d i s s e c t e d fragments of formerly l o n g e r i s l a n d s . The two deep-water p a s s e s (ava)
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a r e found on t h e n o r t h e r n r i m , t o t h e west of t h e i s l a n d s o f Avatoru and T i p u t a , r e s p e c t i v e l y ; on each o f t h e s e two i s l a n d s t h e r e i s a v i l l a g e o f about 300 people. A t t h e lagoonward e n t r a n c e o f each p a s s t h e r e i s a r e e f patch with a small i s l a n d . The c h a r a c t e r i s t i c s o f t h e f l a t a r e d e s c r i b e d , f i r s t , with an i s l a n d , and second, w i t h o u t .(1) Reef r i m with i s l a n d .
The c h a r a c t e r i s t i c s o f t h e r e e f r i m with i s l a n d a r e shown
s c h e m a t i c a l l y i n Figures 5 and 6A. A c r o s s - s e c t i o n o f r e e f r i m with i s l a n d f a l l s i n t o t h r e e main d i v i s i o n s : (a) t h e seaward f l a t , (b) t h e i s l a n d , and (c) t h e lagoon r e e f . The width o f t h e seaward f l a t v a r i e s from 50 t o 200 y a r d s , and occupies roughly o n e - f i f t h o f t h e width of t h e r i m . I t c o n s i s t s o f a r a i s e d o u t e r r i m , with s u r g e
channels and l i v i n g P o r o l i t h o n t o seaward ( t h e " a l g a l r i d g e "
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s e n s u l a t o ) : and a b a r e rock f l a t , p a r t l y d r y i n g a t low w a t e r . D e t a i l e d-
t r a n s e c t s of t h e seaward f l a t a r e g i v e n i n S e c t i o n I I B .The i s l a n d s (motu) a r e g e n e r a l l y 250 t o 400 yards wide.
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They a r e b u i l t almost e n t i r e l y of s h i n g l e , w i t h massive s h i n g l e and r u b b l e r i d g e s r i s i n g t o h e i g h t s of 10-15 f e e t above t h e r e e f f l a t on t h e s e a - ward s i d e . Lagoonward o f t h e s e ramparts t h e s u r f a c e f a l l s f a i r l yr a p i d l y t o a l e v e l o f 6-8 f e e t above t h e f l a t , and d e c l i n e s lagoonwards.
The l a g o o n s h o r e i s formed by s t e e p r i d g e s o f f i n e r s h i n g l e . The lower p a r t o f t l i e s e a w a r d s h o r e a l m o s t i n v a r i a b l y c o n s i s t s of an e r o s i o n ramp 06 worn, smooth c o n g l m e r a t e , s l o p i n g s e a w a r d s a t a n g l e s o f 5 - 7 " ( P l a t e s 1 and 2 ) , and i n many p l a c e s o v e r l a i n b y t o p o g r a p h i c a l l y r e c o g n i z a b l e b e a c h r o c k ( P l a t e 3 ) . 'The b e a c h r o c k , w i t h d i p s o f 3-7' (maximum r e c o r d e d
11-20') n o r m a l l y f o l l o w s t h e s h o r e , b u t i n p l a c e s t r e n d s a c r o s s t h e s e a - ward f l a t a t a n a n g l e t o t h e s h o r e t o form o u t l i e r s up t o 80 y a r d s
d i s t a n t from i t . Seaward b e a c h a n g l e s v a r y c o n s i d e r a b l y : t h e b e a c h f a c e o f r u b b l e b e a c h e s may r e a c h 1 4 - 1 8 " , though a n g l e s a s s m a l l a s 6 - 1 / 2 " were measured on s m a l l p a t c h e s o f s a n d b e a c h . 011 t h e l a g o o n s h o r e , though t h e r e l i e f i s much l e s s , a n g l e s remain s t e e p : l a g o o n - f a c i n g s l o p e s o f s u c c e s s i v e b e a c h r i d g e s a t T i p u t a h a v e a n g l e s o f 10-13', w h i l e t h e b a c k s l o p e o f t h e r i d g e s r e a c h e s 19' i n p l a c e s . A t some p o i n t s t h e l a g o o n r i d g e s a r e b e i n g o v e r s t e e p e n e d by e r o s i o n , t o g i v e a n g l e s as h i g h a s 39' on t h e l a g o o n s h o r e o f b l a h e r e t e t i a e . I n many i s l a n d s , p a r t i c u l a r l y t o w a r d s t h e w e s t , t h e s t e e p lagoon r i d g e s e n c l o s e a n a r e a o f s t a n d i n g w a t e r o r marshy g r o u n d ( t a i r u a
m):
a t A v a t o r u t h i s nieasures 170 x 700 y a r d s ( P l a t e 8 ) . A f t e r h e a v y r a i n t l i e l o w - l y i n g a r e a s u r r o u n d i n g tlie p o o l s may b e c o v e r e d w i t h two t o t h r e e i n c h e s o fs t a n d i n g w a t e r f o r a p e r i o d o f d a y s . I n a d d i t i o n t o t h e p o o l s s u r f a c e x l e p r e s s i o n s a r e a l s o found i n t h e form o f t a r o p i t s ( m a i t e ) ,
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f o r i ~ i e r l y u s e d f o r c u l t i v a t i o n b u t now s e r v i n g a s d e p o s i t o r i e s f o r o r g a n i c r u b b i s h a n d t h e growing o f b a n a n a s and o t h e r p l a n t s .The i . s l a n d s a r e s e p a r a t e d by s h a l l o w , n a r r o w c h a n n e l s known a s h o a ( P l a t e 4 ) . These a r e c l e a r l y e r o s i o n a l , and r e p r e s e n t b r e a k s i n
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f o r m e r l y more e x t e n s i v e i s l a n d s . I n t h e w a l l s o f i s l a n d s t r a n s e c t e d by h o a , - c o n g l o m e r a t e r o c k i s exposed u n d e r l y i n g t h e i s l a n d c l a s t i c s and r i s i n g above t h e l e v e l o f t h e r e e f f l a t . T h i s c o n g l o m e r a t e forms a l e d g e i n t h e w a l l s , and a l s o f l o o r s t l i e h o a i t s e l f . C h a n n e l s c u t i n-
t h e c o n g l o m e r a t e by w a t e r p a s s i n g from s e a t o lagoon a r e e x t e n d i n g s e a w a r d by lieadward e r o s i o n , o f t e n t e r m i n a t i n g i n s n i a l l w a t e r f a l l s w i t h p l u n g e p o o l s . E r o s i o n o f t h e c o n g l o i n e r a t e i s g e n e r a l l y f r o n t e d by a f l a t e r o s i o n ramp r e p r e s e n t i n u tlie e x t e n t o f m a r i n e p l a n a t i o n . The l i e i g l i t o f t l i e c o n g l o m e r a t e v a n e s : b e t w e e n 'Tiputa and T e p a e t i a , '?
where tlie s e a w a r d b e a c h r i d g e s a r e n o t h i g h e r t h a n 8 f t . , t h e congloni- e r a t e s t a n d s t l i r e e f e e t above t h e i n n e r r e e f f l a t a t t l i c s e a w a r d e n d o f t h e l i o a ,
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f a l l i n g g r a d u a l l y t o 1 f t . a t t h e lagoonward e n d . The g e n e r a l d i p of t h e u p p e r s u r f a c e o f t h e r o c k i s a b o u t 0 . 5 ; a t t h e s e a w a r d e n d , s t r a t i f i c a t i o n p l a n e s i n t l i e r o c k d i p lagoonward a t up t o l o . No s e a w a r d d i p s were s e e n . hiuch o f t h e r o c k s u r f a c e i n tlie Xoa-
i s u n d e r g o i n g onion-ski.n s p a l l i n g o f p l a t e s l e s s t h a n 6 i n c h e s i n t h i c k n e s s , wliich once l o o s e ~ i e d a r e a t t i m e s t o r n o f f b o d i l y b y wave a c t i o n . N e i t h e r t l i e c o n g l o m e r a t e r o c k n o r t h e e r o s i o n ramp i m m e d i a t e l y t o s e a w a r d e x t e n d s beyond tlie g e n e r a l l i n e o f t h e s e a w a r d o r lagoonward s h o r e s o f i s l a n d s . I n p l a c e s s i m i l a r r o c k forms h o r i z o n t a l o u t c r o p s 1-2 f e e t h i g h and o f s m a l l e x t e n t on t h e l a g o o n s h o r e s o f i s l a n d s . The lagoonr>rard mouths of h o a a r e o f t e n a l m o s t c l o s e d b y s p i t s and b a r s-
o f f r e s h s m a l l c o r a l s h i n g l e . On M a h e r e r e t i a t a e a r e l i c t lioa was s e e n
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i n t h e p r o c e s s o f b e i n g r e c o l o n i s e d by v e g e t a t i o n , a f t e r b e i n g s e a l e d a t b o t h e n d s by beacli r i d g e s .
'The l a g o o n r e e f s on t h c n o r t h e r n rim a r e p o o r l y d e v e l o p e d . A r a t h e r s p a r s e g r o w t ! ~ o f c o r a l s c x t e n d s from t h e beacli f o o t t o t h e edge
of t h e lagoon s l o p e , a d i s t a n c e o f only 15-50 y a r d s , where t h e bottom f a l l s s t e e p l y t o t h e lagoon f l o o r a t d e p t h s o f 10-15 fathoiiis.
(2) Reef r i m without i s l a n d .
Reef r i m lacking i s l a n d was i n v e s t i g a t e d between Moao and Hararu.
Although no land i s shown on t h e c h a r t , t h e whole of t h i s s e c t i o n resembles a normal hoa.
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The seaward f l a t i s t h e same a s t h a t o f f i s l a n d s , w i t h seaward rim and b a r e r o c k - f l o o r e d r e e f f l a t , t o g e t h e r w i t h o c c a s i o n a l l a r g e r e e f b l o c k s . The p l a c e of t h e i s l a n d i s t a k e n by a p l a t f o r m of conglomerate rock, w i t h a g e n e r a l l e v e l o f l e s s than 2 f e e t above t h e f l a t . The seaward edge o f t h e conglomerate forms a low c l i f f o v e r l o o k i n g an a b r a s i o n p l a t f o r m . Channels through t h econglomerate a r e s h a l l o w e r and l e s s c l e a r l y d e f i n e d than i n t h e h o a , and
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a r e o f t e n d i s c o n t i n u o u s ; n e a r t h e lagoon e d g e , however, t h e y may become d e e p e r , V-shaped s c o u r n o t c h e s o n l y p a r t l y i n t e r r u p t e d by b a r s . Along t h e seaward edge of t h e conglomerate p l a t f o r m t h e r e i s no accwnulation o f r u b b l e o r s h i n g l e ; b u t along t h e lagoon edge t h e r e i s a zone 50
y a r d s wide o f high s t e e p s h i n g l e r i d g e s . The i n n e r p a r t o f t h e conglom- e r a t e p l a t f o r m and t h e lagoonward r i d g e s a r e covered with v e g e t a t i o n , mainly Pemphis a c i d u l a and S u r i a n a ~ n a r i t i m a growing d i r e c t l y on t h e b a r e rock s u r f a c e ; t h e p l a n t cover i s s u f f i c i e n t l y dense t h a t , s e e n from t h e s e a , t h e i n t e r - i s l a n d f l a t appears t o b e a v e g e t a t e d i s l a n d .
T h i s d e s c r i p t i o n a p p l i e s s p e c i f i c a l l y t o t h e 1-1/2 m i l e s long Maoa-Hararu gap, which i s one o f t h e l o n g e s t on t h e n o r t h e r n r i m ; whether o t h e r i n t e r - i s l a n d a r e a s on t h i s r i m a r e comparable remains t o b e s e e n .
B .
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Reef f l a t p r o f i l e sThe n o r t h e r n r i m o f Rangiroa was i n v e s t i g a t e d a t Avatoru, T i p u t a , T e p a e t i a , I \ , l a h e r e r e t i a t a e , and e a s t w a r d s t o Hararu i n t h e Moao-I-lararu gap. Measured p r o f i l e s o f t h e seaward f l a t a t t h e s e l o c a t i o n s a r e given i n Figure 7.
P r o f i l e - 1
Avatoru, n o r t h s h o r e , west end o f t h e i s l a n d . The r e e f f l a t has an average width of 170 y a r d s , and t h e seaward beach of 20 y a r d s . The f o l l o w i n g zones a r e r e c o g n i z e d :
(a) P o r o l i t h o n zone o f e n c r u s t i n g pink a l g a e . The a l g a e forin a ' p a i n t ' o v e r t h e s u r f a c e of s u r g e channels and i n t e r - c h a n n e l a r e a s ; t h e chan- n e l s themselves p a s s seawards i n t o t h e r e e f - f r o n t groove system. Apart from s m a l l and s c a t t e r e d P o r i t e s and Acropora i n t h e s u r g e c h a n n e l s , c o r a l s a r e a b s e n t . S l a t e - p e n c i l u r c h i n s a r e conspicuous on t h e a l g a l s u r f a c e . I n t h e channels t h e P o r o l i t h o n forms s m a l l , smooth-surfaced b o s s e s up t o 6 i n c h e s high and 6 i n c h e s i n d i a m e t e r ; t h e channels them- s e l v e s a r e 10-15 y a r d s a p a r t . From t h e i n n e r l i m i t of continuous
P o r o l i t h o n t o t h e wave break l i n e t h e zone i s 30-40 yards wide. A t low w a t e r t h e zone emerges during backwash, b u t i s covered by up t o 2 f e e t of w a t e r d u r i n g swash.
(b) Orange zone. T h i s name i s g i v e n t o a h i g h l y d i s t i n c t i v e zone o f e r o d e d r o c k a t t h e same l e v e l a s o r s l i g h t l y h i g h e r t h a n t h e P o r o l i t h o n z o n e ; i t c o n s i s t s o f rimiiled p o o l s and i ~ o l e s c o a t e d w i t h an o r a n g e a l g a . Zones ( a ) and ( b ) t o g e t h e r c o m p r i s e t h e o u t e r r i d g e o r , i n a wide
s e n s e , t h e a l g a l r i d g e , o f t h e s e a w a r d f l a t . The o r a n g e zone i s 20-25 y a r d s w i d e . I t s i n n e r boundary i s i r r e g u l a r , s e r r a t e d by e r o s i o n
c h a n n e l s foriiled by w a t e r p o u r i n g o v e r t h e l i p o n t o t h e f l a t i t s e l f . T h e r e a r e no growing c o r a l s i n t h i s z o n e .
( c ) Reef f l a t . A r o c k - f l o o r e d f l a t d e e p e n i n g from l a n d t o s e a . 'rlie i n n e r h a l f d r i e s c o m p l e t e l y a t low w a t e r , when t h e o u t e r p a r t s t i l l c a r r i e s up t o 1 . 5 f e e t . P o t h o l e s n e a r t l i e j u n c t i o n w i t h zone ( b ) con- t a i n a few s m a l l c o r a l s s u c h a s P o c i l l o p o r a and I ' o r i t e s , b u t o t h e r w i s e t h e r e a r e no growing c o r a l s on any p a r t o f t l i e f l a t . The p l a n e d r o c k s u r f a c e i s v e r y s p a r s e l y c o v e r e d w i t h a t h i n l a y e r o f m o b i l e s a n d . T o t a l w i d t h 100-110 y a r d s .
( d ) B e a c h - f o o t b e a c h r o c k : a iiiassive c o n g l o i n e r a t e , d i p p i n g s e a w a r d , f o r m i n g a p l a t e 5 y a r d s wide and 1 . 5 f e e t t h i c l t a l o n g t h e f o o t o f t h e b e a c h ; r e s t i n g on an i n c l i n e d r o c k s u r f a c e which p a s s e s u n d e r t h e beach s e d i m e n t s and which i s forined of a s i i i i i l a r conglomerate t o t h e b e a c h r o c k .
( e ) Beach f a c e : w h i t e , u n w e a t h e r e d c o r a l s h i n g l e , 14 y a r d s wide and r i s i n g t o 8 f e e t above t h e l e v e l o f t l i e i n n e r f l a t .
( f ) Beach c r e s t o f b l a c k e n e d s h i n g l e w i t h a h e d g e o f S u r i a n a m a r i t i m a . P r o f i l e - 2
' T i p u t a , n o r t h s h o r e , w e s t e n d o f t h e i s l a n d . The r e e f f l a t i s o n l y 90 y a r d s w i d e , and t h e r u b b l e b e a c h i s c o r r e s p o n d i n g l y h i g h e r and s t e e p e r .
( a ) l ' o r o l i t h o n zone o f smooth p i n k e n c r u s t i n g a l g a e b e t w e e n s u r g e c h a n n e l s , w i t h s i n a l l b o s s - l i k e g r o w t h s on c h a n n e l w a l l s . No growing c o r a l s ; a few p a t c h e s o f g r e e n a l g a e . \+'hen o b s e r v e d wave a c t i o n was g r e a t c r h e r e t h a n e l s e w h e r e on t h e r e e f e d g e : i n v e r y calin c o n d i t i o n s waves a t b r e a k i n g r e a c h e d h e i g h t s o f 1 . 5 t o 2 y a r d s , a t tiiiies t h r o w i n g s p r a y t o h e i g h t s of 6 y a r d s . The zone i s a t l e a s t 30 y a r d s wide.
(b) Orange zone o f e r o d e d r o c k w i t h l a r g e p o t h o l e s and some s h a r p r e s i d u a l p i n n a c l e s . No p i n k a l g a e . T h e r e a r e some c o r a l s i n t h e d e e p e r p o t h o l e s . The zone i s 10-25 y a r d s w i d e , and r i s e s 2 f e e t above t h e o u t e r r e e f f l a t .
( c ) Reef f l a t : a p l a n e d r o c k f l o o r w i t h a t h i n d i s c o n t i n u o u s c o v e r o f s a n d and no c o r a l s . S t r o n g l o n g i t u d i n a l c u r r e n t s move westwards
t o w a r d s T i p u t a p a s s t o remove w a t e r p o u r i n g o v e r t h e r a i s e d r i m . The f l a t d e e p e n s s l i g h t l y from l a n d t o s e a , and t h e i n n e r p a r t d r i e s a t low w a t e r , 35 y a r d s w i d e .
( d ) b e a c h - f o o t b e a c h c o n g l o m e r a t e : a r a t h e r h e a v i l y e r o d e d m a s s i v e c o n g l o m e r a t e , d i p p i n g s e a w a r d , 5 y a r d s w i d e .
(e) Beach f a c e , 12 y a r d s wide, r i s i n g s t e e p l y t o 10 f e e t above t h e f l a t ; b u i l t o f w h i t e s h i n g l e w i t 1 1 p o c k e t s o f f o r a m i n i f e r a 1 sand i n tlie l e e o f tlie beachrock.
( f ) Beach c r e s t of dark u n v e g e t a t e d s h i n g l e - mostly c o r a l fragments 3-9 i n c h e s l o n g , r e c o g n i s a b l e t o genus. The s u r f a c e s l o p e s g e n t l y uvwards from t h e t o n o f t h e beach f a c e t o a c r e s t 17 v a r d s i n l a n d . and t h e n f a l l s landwards t o t h e v e g e t a t i o n hedge o f T o u r n e f o r t i a a r g e n t e a and S u r i a n a m a r i t i m a .
P r o f i l e 3
-
'Tiputa, e a s t of P r o f i l e 2 , where t h e r e e f f l a t narrows t o a p p r o x i ~ ; i a t e l y 6 5 y a r d s .
(a) P o r o l i t l l o n zone and s u r g e c h a n n e l s . No c o r a l s
(b) Orange zone of eroded rock; zones (a) and (b) t o g e t h e r Form a rim a t l e a s t 30 y a r d s wide ( P l a t e 9 ) .
(c) Rock-floored r e e f f l a t , p o s s i b l y j u s t d r y i n g i n p l a c e s a t low w a t e r . Xo c o r a l s on t h e rock s u r f a c e . I n p l a c e s t i i e r e a r e l i n e a r r e s i d u a l s o f rock which a r e p r o b a b l y o l d p a t c h e s o f beachrock. Width 35 y a r d s .
(ci) Ueach-foot beaclirock ( P l a t e 3 ) : a sn!ootli i n c l i n e d p l a t e r i s i n g up t o 3 f e e t above t h e i n n e r f l a t l e v e l and p a s s i n g under bench sedim- e n t s . The rock c o n t a i n s l a r g e c o r a l f r a g m e n t s , b u t hand specimens liave tlie appearance of moderately i n f i l l e d beachrock. S t r u c t u r a l d i p seawards.
(e) Beach f a c e : a r a t h e r complex f e a t u r e 30-40 y a r d s wide, r i s i n g t o a maximum l i e i g h t of 15 f e e t above t h e i n n e r f l a t , mostly formed o f g r e y s h i n g l e 4 - 6 inclies long, with sand on t h e lower f a c e .
( f ) Beach c r e s t o f dark s h i n g l e , c o a r s e n i n g landwards. Declining i n h e i g h t towards t h e v e g e t a t i o n hedge which h e r e c o n s i s t s mainly o f ' T o u r n e f o r t i a a r g e n t e a , with coconuts f a r t h e r i n l a n d .
P r o f i l e 4 -
T e p a e t i a , n o r t h s h o r e , midway a l o n g t h e i s l a n d . The p r o f i l e i s narrow and comparable t o P r o f i l e 3.
(a) P o r o l i t h o n zone of pink e n c r u s t i n g a l g a e 2 5 y a r d s wide, w i t h s u r g e cliannels 5-10 y a r d s a p a r t .
( b ) Orange zone, width 30 y a r d s . The o u t e r t w o - t h i r d s o f t h i s zone i s s i m i l a r t o t h a t i n P r o f i l e s 1-3: smoothly rounded p o t h o l e s and r i d g e s a r e covered with an orange-colored a l g a . The i n n e r , landward t h i r d i s formed of s h a r p e r p i n n a c l e s and r i d g e s , w i t 1 1 a l o c a l r e l i e f of 3-6 i n c h e s .
(c) Bare rock r e e f f l a t , 25-30 y a r d s wide, l a c k i n g growing c o r a l s o r o t h e r l a r g e c o l o n i a l organisms, with o n l y a t h i n , d i s c o n t i n u o u s
sediment c o v e r .
(d) Beach-foot beachrock: a massive, very s t o n y conglomerate i n two d i s t i n c t bands, each with 2-3 l a y e r s , d i p p i n g seaward, 8-10 y a r d s wide, and r i s i n g up t o 1 . 5 f e e t above t h e i n n e r f l a t .
(e) Seaward beach f a c e : a s h i n g l e beach 15 y a r d s wide, w i t h most p a r t i c l e s l e s s t h a n 2 i n c h e s l o n g , and p a t c h e s of f o r a m i n i f e r a 1 s a n d , p a r t i c u l a r l y i n t h e lower p a r t .
( f ) Beach c r e s t , forming a wide f l a t of s h i n g l e , w i t h p a r t i c l e l e n g t h g r e a t e r t h a n 9 i n c h e s , mostly c o r a l s b u t with b l o c k s of beach conglom- e r a t e , presumably c a r r i e d by storm waves. The c r e s t s t a n d s 9 f e e t above t h e i n n e r f l a t l e v e l . Tlle o u t e r p a r t o f t h e c r e s t i s covered with a low hedge of S u r i a n a maritima: 35-40 y a r d s i n l a n d t h i s i s r e p l a c e d by a s o l i d s t a n d of G u e t t a r d a s p e c i o s a .
P r o f i l e 5 -
i - l a h e r e r e t i a t a e , n o r t h s h o r e , west end of t h e i s l a n d . The r e e f f l a t h e r e i s wide (160-180 y a r d s ) and t h u s comparable t o P r o f i l e 1.
(a) P o r o l i t h o n zone o f pink a l g a e w i t h many c l o s e - s e t g r o o v e - l i k e s u r g e cliannels a v e r a g i n g o n l y 4 y a r d s a p a r t . Nodular P o r o l i t h o n i s conspicuous a t lower l e v e l s .
(b) Orange zone, 25 y a r d s wide: t h e landward edge p o t h o l e d and grooved, and c l e a r l y undergoing e r o s i o n .
(c) Rock-floored r e e f f l a t with a t o t a l width of 115 y a r d s . I t s o u t e r p a r t l i e s 2 f e e t below t h e l e v e l of t h e orange zone, b u t r i s e s land- ward. No c o r a l s .
(d2) I n p l a c e s on t h e r e e f f l a t t h e r e i s a d i s c o n t i n u o u s zone of rock up t o 1 . 5 f e e t h i g h , jagged and eroded, forming narrow l i n e a r bands p a r a l l e l with t h e s h o r e . No s t r u c t u r e i s a p p a r e n t , b u t t h e s e a r e probably r e l i c t bands of beachrock.
(g) The i n n e r p a r t of t h e r e e f f l a t i s formed by a dark-colored bare rock p l a t f o r m , d r y i n g a t low w a t e r , 20-25 y a r d s wide, and o f e r o s i o n a l o r i g i n . I t s t a n d s up t o 1 f t . above t h e g e n e r a l l e v e l of t h e r e e f f l a t , b u t t h e r e i s n o t s h a r p l i n e of demarcation between t h e two.
(d) Beach-foot beachrock: a conglomerate rock o v e r l y i n g t h e e r o s i o n p l a t f o r m ; 4-5 y a r d s wide, up t o 1 . 5 f e e t h i g h , and d i p p i n g seawards.
(e) Beach f a c e , 20 y a r d s wide and 6 f e e t high froin t h e i n n e r f l a t t o t h e f i r s t v e g e t a t i o n hedge. The lower p a r t of t h e f a c e i s sand and f i n e g r a v e l ; t h e upper p a r t o f mediun s h i n g l e .
( f ) Beach c r e s t : a f l a t expanse o f v e r y c o a r s e blackened r u b b l e 40- 50 y a r d s wide and up t o 9 f e e t above t h e f l a t , s e p a r a t e d from t h e beach f a c e by a narrow hedge of S u r i a n a m a r i t i m a and d i s a p p e a r i n g i n -
land under a t h i c k e t o f T o u r n e f o r t i a , G u e t t a r d a , Scaevola and Hedyotis.
P r o f i l e 6
-
M a h e r e r e t i a t a e , n o r t h s h o r e , e a s t end of t h e i s l a n d . The r e e f f l a t i s a g a i n wide and comparable with P r o f i l e 5 .
( a ) P o r o l i t h o n zone: a s i n P r o f i l e 5 . (b) Orange zone: a s i n P r o f i l e 5.
(c) Rock-floored r e e f f l a t , 90 y a r d s wide, 1.5-2 f e e t below t h e l e v e l of t h e orange zone i n i t s o u t e r h a l f , and r i s i n g towards t h e s h o r e . There a r e some l a r g e r e e f b l o c k s o f s t o r m - t o s s e d o r i g i n , up t o 5 f e e t h i g h and 18 f e e t long on t h e f l a t from M a h e r e r e t i a t a e e a s t w a r d s ; one r e e f block i n t h i s p r o f i l e s t a n d s 110 y a r d s from t h e r e e f edge. There a r e no growing c o r a l s on t h e f l a t .
(g) Tile i n n e r p a r t of t h e r e e f f l a t i s covered with c o r a l r u b b l e t o form a b o u l d e r c a r p e t up t o 25 y a r d s wide, o v e r l y i n g t h e e r o s i o n a l r o c k p l a t f o r m s e e n i n P r o f i l e 5 .
(d) Beach-foot beachrock, h e r e a degraded conglomerate outcrop only 5 y a r d s wide, w i t h no c l e a r forin o r s t r u c t u r e , and l a r g e l y covered w i t h r u b b l e b l o c k s up t o 1 . 5 f e e t i n d i a m e t e r .
(e) Beach f a c e and c r e s t . The u n v e g e t a t e d beach a r e a h e r e has a t o t a l width of up t o 170 y a r d s . The o u t e r f a c e , 6 f e e t h i g h and 12 y a r d s wide, i s b u i l t o f c o a r s e s h i n g l e of 3-12 i n c h c a l i b r e ; i t i s topped by a d i s c o n t i n u o u s f r i n g e of S u r i a n a and T o u r n e f o r t i a . A 50 y a r d wide back s l o p e o f g r e y r u b b l e , h o r i z o n t a l and u n v e g e t a t e d , i s followed by a s t e e p r i s e t o a second r i d g e , 6 f e e t h i g h e r t h a n t h e f i r s t , of blackened r u b b l e . The backslope of t h i s h a s a s l i g h t landward s l o p e ,
and i s formed o f rough l a r g e c o r a l b l o c k s , t i g h t l y packed. The v e g e t a - t i o n hedge ( T o u r n e f o r t i a , G u e t t a r d a , Hedyotis and some Pandanus) l i e s 60-100 y a r d s back from t h e second beach c r e s t .
111. THE WESTERN OR LEEWARD RIM A. General c h a r a c t e r i s t i c s
The w e s t e r n r i m of Rangiroa i s 17 m i l e s l o n g : of t h i s d i s t a n c e 60% i s covered w i t h i s l a n d s . The b a s i c p a t t e r n i s s i m i l a r t o t h a t on t h e n o r t h e r n r i m , w i t h t h e d i v i s i o n i n t o seaward f l a t , i s l a n d , and lagoon r e e f . T y p i c a l c o n d i t i o n s were s e e n a t M o t u t i i and T e r e i a o
( S e c t i o n I I I B ) . IIere t h e seaward f l a t , with a t o t a l width of about 120 y a r d s , c o n s i s t s o f an o u t e r r a i s e d rim, bounding a rock-floored r e e f f l a t which towards t h e seaward s h o r e o f t h e i s l a n d i s covered by a p l a t f o r m of conglomerate r o c k . The i s l a n d s a r e approximately 500 y a r d s wide; f i v e of them a r e c o n s i d e r a b l y l o n g e r t h a n t h e r e s t , which
a r e s e p a r a t e d by h o a .
-
The seaward beach r i d g e s , f a c i n g away from t h e p r e v a i l i n g winds, a r e lower, l e s s s t e e p , and b u i l t of f i n e r d e b r i s t h a n t h e r a n ~ p a r t s on t h e n o r t h e r n r i m . '13~s a t P l o t u t i i t h e beach of f i n e s h i n g l e r i s e s t o 4 - 7 f e e t above t h e f l a t and h a s a mean s l o p e of 6-9". I n p l a c e s where t h e s h o r e approaches c l o s e r t o t h e r e e f edge, llowever, a s on p a r t s of T e r e i a o , t h e seaward r i d g e i s h i g h e r (up t o10 f e e t ) , s t e e p e r ( i n p l a c e s up t o 20'1, ant1 b u i l t of c o a r s e r s h i n g l e . Lagoonward of t h e seaward beach c r e s t , t h e s u r f a c e d e c l i n e s f a i r l y g e n t l y towards t h e lagoon s h o r e : t h e seaward p a r t i s b u i l t of blackened g r a v e l , t h e lagoonward p a r t o f sand. The lagoon beach i s
low and narrow, of e i t h e r sand o r f i n e s h i n g l e , and may e n c l o s e a narrow pool o r marshy a r e a ( P l a t e 1 3 ) . The main c o n t r a s t i n i s l a n d form between t h e n o r t h e r n and western r i m s , t h e r e f o r e , i s i n t h e absence of s a n d - s i z e m a t e r i a l i n t h e f o r ~ ~ i e r and i t s iiiiportance i n tile l a t t e r . A f u r t h e r major d i f f e r e n c e i s i n t h e n a t u r e of t h e lagoon r e e f . On t h e western rim t h e lagoon r e e f s f a c e 10-20 m i l e s of open lagoon, i n c o n t r a s t t o t h e s h e l t e r e d leeward lagoon r e e f s of t h e n o r t h e r n r i m . On t h e west r i m t h e lagoon r e e f s form a sandy s h e l f a t l e a s t 1 f o o t deep a t low w a t e r , edged w i t h l i v i n g c o r a l , and up t o 200 y a r d s wide.
A t t h e a r e a s i n v e s t i g a t e d , t h e i s l a n d s a r e s e p a r a t e d by !lea i n t h e s i d e s o f which conglomerate p l a t f o r m s o u t c r o p . These a r e c l o s e l y com- p a r a b l e t o s i m i l a r f e a t u r e s on t h e n o r t h e r n r i m . Thus a t M o t u t i i , where t h e seaward beach i s 6 f e e t h i g h , t h e conglomerate p l a t f o r m u n d e r l y i n g i t s t a n d s 1 f o o t above t h e f l a t . To t h e s o u t h , on T e r e i a o , wilere t h e seaward beach r e a c h e s 10 f e e t i n h e i g h t , t h e conglomerate p l a t f o r m r i s e s t o 3 f e e t a t i t s seaward edge. I n both c a s e s t h e rock s u r f a c e d e c l i n e s lagoonward. No conglomerate rock o r beachrock was s e e n on t h e lagoon sllore h e r e ; b u t a t 'I'ivaru, wlrere t h e lagoon s h o r e i s sandy and up t o 5 f e e t h i g h , t h e r e a r e outcrops of beacllrock t r e n d i n g a t a s l i g h t angle t o t h e modern beach.
This d e s c r i p t i o n a p p l i e s t o ' t y p i c a l ' i s l a n d s of t h e w e s t e r n rim.
This r i m , however, i s s u b j e c t t o l l u r r i c a n e s and r e s u l t i n g c a t a s t r o p h i c m o d i f i c a t i o n . Such changes were s e e n a t Maeherehonae, and may e x i s t e l s e w h e r e . Here t h e seaward f l a t ( S e c t i o n I I B ) , of normal width, i s l i t t e r e d f o r a d i s t a n c e of 1 m i l e with l a r g e r e e f b l o c k s ( P l a t e 1 8 ) . Most of t h e s e a r e 6-10 f e e t h i g h ; t h e l a r g e s t i s a huge fragment of r e e f rock 18 f e e t lligll, 18 f e e t long, and 15 f e e t wide. Tile seaward beach i s formed by a s h i n g l e rampart 6 f e e t h i g h , and t h e seaward hundred yards of t h e i s l a n d s u r f a c e by a s e a of b o u l d e r s and c o a r s e
r u b b l e , probably h u r r i c a n e - d e p o s i t e d . The r e s t of t h e i s l a n d s u r f a c e , almost t o t h e lagoon s h o r e , i s formed by a f l a t p l a i n , p a r t l y of
s t r i p p e d conglomerate rock (exposed by o v e r t i p p i n g wave s c o u r ? ) , p a r t l y of t h i n sand and g r a v e l with d i s c o n t i n u o u s v e g e t a t i o n .
The lagoon s h o r e of Maeherehonae i s of e s p e c i a l i n t e r e s t . As a t 'Tereiao i t i s sandy, f a c e s a wide lagoon sand f l a t edged w i t 1 1 r e e f , and e n c l o s e s an e x t e n s i v e marsh a r e a , up t o 75 y a r d s wide, with s t a n d i n g b r a c k i s h t o f r e s h w a t e r ( P l a t e 1 4 ) . A t a d i s t a n c e of 150-200 yards
from t h e lagoon s h o r e , however, t h e r e i s an e x t e n s i v e a r e a of h i g h dunes (one t e i t e i ) , probably r e a c h i n g a h e i g h t of 30 f e e t and t h u s forming t h e h i g h e s t land on t h e a t o l l ( P l a t e s 15 and 1 6 ) . Congloill- e r a t e rock paveiiients can be s e e n b e n e a t h t h e dunes ( P l a t e 1 7 ) , and i n
p l a c e s i n t h e p o o l s n e a r t h e lagoon s h o r e ; and f a r t h e r cemented hor- i z o n s can be s e e n i n swales ( o f t e n c o n t a i n i n g w a t e r ) w i t h i n t h e dune system i t s e l f . Gaps i n t h e dunes a r e f l o o r e d by s h i n g l e o v e r l y i n g t h e
conglomerate p l a t f o r m . No lagoon s h o r e dunes have been s e e n e l s e w h e r e . The dunes a r e v e g e t a t e d , with Pandanus and T o u r n e f o r t i a , b u t with no continuous ground cover except dead Pandanus l e a v e s . 'The Maeherehonae dunes t h u s r e p r e s e n t an a d d i t i o n t o t h e g e n e r a l i z e d i s l a n d t y p e shown i n F i g u r e s 5 and 6 .
No i n t e r - i s l a n d s e c t i o n s of r e e f f l a t ( o t h e r t h a n hoa) were s e e n
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on t h e west r i m .
B .
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Reef f l a t p r o f i l e s ( F i g u r e 8) P r o f i l e 7 -T e r e i a o , west s h o r e , n e a r t h e n o r t h end o f t h e i s l a n d .
(a) P o r o l i t h o n zone, up t o 60 y a r d s wide ( P l a t e 1 0 ) . Three subzones can be d i s t i n g u i s h e d : a lower p a r t with grooves i n t h e a r e a of wave- b r e a k ; a s e a w a r d - s l o p i n g s e c t i o n 30 y a r d s wide of r a t h e r i r r e g u l a r
topography, w i t h e r o s i o n a l p i l l a r s and p o t h o l e s and l o c a l r e l i e f of 1 f o o t , covered i n pink a l g a e b u t l a c k i n g s u r g e c h a n n e l s ; and a f l a t , r e g u l a r , uneroded pink a l g a l pavement 25 y a r d s wide, w i t 1 1 many s c a t t e r e d s m a l l e n c r u s t i n g P o r i t e s . The s u r g e channels a r e h e r e c l e a r l y much l e s s e x t e n s i v e i n t h e u p p e r , i n n e r p a r t of t h e P o r o l i t h o n zone than t h e y a r e on t h e n o r t h e r n r i m .
(b) Orange zone, 40 y a r d s wide. The o u t e r t e n y a r d s (b2) i s a s t r i p o f b a r e r o c k , w i t h o u t pink a l g a e b u t with orange coloration; t h e r e s t i s t y p i c a l l y i r r e g u l a r , with p o t h o l e s and r i d g e s .
(c) Ilock-floored r e e f f l a t 95 yards wide, w i t h no growing c o r a l s b u t w i t h a s p a r s e g r a v e l and sand cover.
( d ) Conglo~nerate rock p l a t f o r m , 45 y a r d s wide, r i s i n g up t o 3 f e e t above t h e i n n e r f l a t , and p a s s i n g landward b e n e a t h t h e seaward beach.
The rock i s h o r i z o n t a l and shows no seaward d i p ( P l a t e 1 1 ) .
(e) Seaward beach f a c e , up t o 7 f e e t high and 20 y a r d s wide, of f i n e white s h i n g l e .
( f ) Seaward beach E r e s t , of blackened g r a v e l ( P l a t e 1 2 ) , mean l e n g t h about 6 i n c h e s , f a l l i n g s l i g h t l y lagoonward, w i t h a covering of
G u e t t a r d a and T o u r n e f o r t i a and a seaward f r i n g e o f S u r i a n a and s c a t t e r e d S c a e v o l a . I-leight 4 - 9 f e e t .
P r o f i l e 8
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klaeherehonae, northwest s h o r e , n e a r t h e n o r t h end o f t h e i s l a n d . This p r o f i l e d i f f e r s c o n s i d e r a b l y from t h e o t h e r s r e p o r t e d h e r e , p r e - sumably a s a r e s u l t o f h u r r i c a n e e f f e c t s .
(a) Reef edge. The margin h e r e i s v e r y i r r e g u l a r and b r o k e n : t h e r e a r e no s u r g e channels and no a l g a l r i d g e o r s i m i l a r f e a t u r e . The s u r f a c e l i e s a t a lower l e v e l t h a n i n o t h e r p r o f i l e s , and i s covered w i t h dead P o c i l l o p o r a c o l o n i e s c o a t e d w i t h pink a l g a e , t o a d i s t a n c e of
30 y a r d s from t h e edge.
(c) Reef f l a t . The r e e f f l a t p r o p e r i s about 100 y a r d s wide. I t i s l i t t e r e d with l a r g e r e e f b l o c k s jumbled t o g e t h e r ; t h e l a r g e s t i s 18 f e e t h i g h , tile r e s t mostly 6-10 f e e t h i g h . Between t h e b l o c k s t h e s u r f a c e i s covered w i t h l i v i n g P o c i l l o p o r a growing s o c l o s e l y t o g e t h e r t h a t i t i s d i f f i c u l t t o s e e t h e u n d e r l y i n g s u r f a c e . To landward of t h e b l o c k s , i n calmer w a t e r , t h e P o c i l l o p o r a i s r e p l a c e d by e n c r u s t i n g Acropora and P o r i t e s covering perhaps 10% o f t h e t o t a l a r e a .
(e) Seaward beach, 25 y a r d s wide, b u i l t of b o u l d e r s and s h i n g l e , r i s i n g 6-9 f e e t above t h e f l a t . There i s no beach-foot beachrock o r conglomerate.
( f ) Beach c r e s t : a b o u l d e r f i e l d with s c a t t e r e d T o u r n e f o r t i a .
IV. THE SOUTHERN OR SWELLWARD RIM A . General c h a r a c t e r i s t i c s
The s o u t h e r n r i m of t h e a t o l l , f a c i n g t h e Southern Ocean s w e l l s , i s 57 m i l e s long; only o n e - t h i r d of t h i s l e n g t h i s covered by i s l a n d s , t h e s m a l l e s t p r o p o r t i o n of t h e t h r e e a t o l l rims. The s o u t h e r n rim, i n a d d i t i o n , h a s c h a r a c t e r i s t i c s n o t found a t o t h e r p a r t s o f Rangiroa:
t h e s e a r e shown s c h e m a t i c a l l y i n F i g u r e s 9 and 6B, and e l a b o r a t e d i n t h e d e t a i l e d p r o f i l e s 9-11 ( S e c t i o n IVB; Figure 1 0 ) .
(1) Reef r i m w i t h i s l a n d
From o b s e r v a t i o n s a t Porahu, P e a r i , Vaihoa and Utoto, a b a s i c f o u r f o l d d i v i s i o n may b e made i n t h e morphology of r e e f rims with i s l a n d s on t h e s o u t h s i d e of Rangiroa, i n t o (a) t h e seaward r e e f f l a t ;
(b) t h e f e o ; (c) t h e i s l a n d ; and (d) t h e lagoon f l a t and r e e f .
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The s o u t h e r n rim i s a p p r e c i a b l y wider than tile n o r t h e r n one, ranging from 800 t o almost 1200 y a r d s . The seaward r e e f f l a t i t s e l f i s 100-120 y a r d s wide: i t s o u t e r edge, with i t s P o r o l i t h o n and Orange zonemargin, i s s i m i l a r t o t h a t e l s e w h e r e ; b u t t h e r e i s no well-developed r o c k - f l o o r e d r e e f f l a t a t o r n e a r low w a t e r l e v e l , a s on t h e n o r t h and west r i m s . I n s t e a d , t h e i n n e r p a r t o f t h e r e e f f l a t i s occupied by b a r e s w e l l s of e x f o l i a t i n g r e e f r o c k , s p a l l i n g o f f c o n c e n t r i c s h e e t s up t o 1 f o o t i n t h i c k n e s s ( P l a t e s 22 and 2 3 ) . This i s s e e n on a much s m a l l e r s c a l e on conglomerate rock i n some of t h e n o r t h e r n h o a .
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The e x f o l i a t i n g rock r i s e s perhaps 2-3 f e e t above low w a t e r l e v e l , and i s succeeded landward by a narrow and d i s c o n t i n u o u s r o c k - f l o o r e d moat.The f e o , lagoonward o f t h e seaward f l a t , i s a s t r i p of e l e v a t e d r e e f
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rock 80-120 y a r d s wide, which e v i d e n t l y extends along t h e whole of t h e s o u t h e r n r i m o f t h e a t o l l ( P l a t e s 19, 20, 26, 2 7 ) . I t i s deeply and i n t r i c a t e l y eroded, and i n t e r s e c t e d by narrow, deep and winding channels.
The g e n e r a l l e v e l of i t s swnmit i s 6 f e e t above t h e f l a t ; some p i n n a c l e s
