14
MARINE BIOLOGY OF THE H1iL)ASEYE RED SEA.
on the MAMIKEBIOLOGY
of the 81-D-4NESE R E D S E A . - l V ,
The
RECEKTHISTOHY
of the C O R A L KEEFS of -the Mid-West Shores of
HEPORTS
the RED SEA. By CYRILCROSSLAXD,
M.A., ELSO.,F.Z.S.
cated by Prof. K. A. HERDRIAX,
F.R.S., P.L.S.)
(Cloinmuiii-
(PLATW1 & 4.)
[Read 2nd May, 1907.1
CONTENTS.
General Description of the Coast .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Evidences of' Elovation ..........................................
Its regularity-Corals and material of reefs remaining i n position.
Raised Atoll of 'I'elh Telln Sorap.
The Living Reefs-Fringing, Scattered, Barrier ....................
The Forination of Coral-Rar ...................................
Conclusionu. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PRp
14
17
19
24
26
GENERAL
DESCRIIWON
OF THE COAST.
d e - of the Eccl Sea " Rift V:iliey '' is
varied on the west by tlirce proniiiient point., Kas Beiias, Ras Elba, and Kar
Etawaya (bee Map, fig. 1, p. 6). Betvecii R:i+ TZawaga and Suakiiii the
coast-line ib, in the iliain, very regiilar, but it bends eastward again at
Trinliitat, aiid is broken by soiric large bays about Agig.
As is well liiiowii, the hides of this trough are composed of r2tnges of
inountainv of granite and other crptalline rocks remarkable for their wild
and jagged shapes and utter b a r r c ~ i e > s ,wliile the actual coast is :I low
plain. Except for it* scnwarcl hortlcr, which ih invariably of elevated coral,
tlie surface of this plain is coinpoarcl of :illmial gravel aiid sand froiii the
hills *. Ti1 certain localities the plain i- di\ided by ranges of low hills,
horntimes capped with coraI, which riw in tlic midst of the plain and run
psrallel, iiiorc or less, to both hea-eoa-t and inountains t.
This is the structure of the whole Red Sea coast at least a s far south as
latitude 18" 10' N., with the exception of the northern part of the Gulf of
Suez, which ic totally different $.
A very >tril;iiig feature of this coad is the large number of canal-like bays
or '' Khor5 " wliich rim into the coa-t-plain. Reference to the i m p 011 p. ti
shows nine large exniiiplcs in tlic slxicie of only 40 miles just north of RawaFi,
THE remarkablc parallelism o l the
*
Artesian borings made by the Public Works Department of the Sudan Government
two miles inland from l'ort Sndsn show that tlie same materials extend downwards as far as
tlie borings went, i. e. to a depth of at least 1000 metres.
i. As these hills are of special interest 1 gite an enumeration of them and description of
those I have climbed in a postscript (p. 27).
1 The crystalline hills are here absent, the eoa9t being formed of high, flat-topped, walllike hills of Ct.etitceoos litnestone which rise alniost directly from the ma.
t.t.O!C
I
1\IAP O F
DONGOLA
AND
ITS T71CINITY.
It;
I
MARINE BIOLOQY OF THE HUDANERE RED S E d .
and the Chart of Suakini Harbour gives tlie featnres of a typical specimen.
The characteristics more or less prominently developed in all are :(I) ( ‘anal-like outline.
(2) Ckossed branches riinniiig N. & S. and E. L% W.
( 3 ) A nearly uniforni depth gently bhoaling as one passes landwards.
(4) A flat bottom of coral-mud.
( 5 ) Precipitous sides of growing reef below sea-level and cliffs above.
The latter are 01 course the height of the neighbohring plain,
generally 2 feet or so in Suakini Harbour, or up to 12 feet in Khor
ShiIYdb.
The late Mr. Barron, who was Geologist to the Sudan Government,
explains thebe formations as resulting from two systems of faults at right
angles to one another. The almost rainless climate, the absence of very
strong tidal Currents, and the protection afforded by growing coral, have
allowed tlie above-mentioned characteristic and almost bizarre features to
remain, whereas in the corresponding structures on the equatorial coasts of
East Africa the erosion of powerful tidal currents and of freshwater streams
has, in most cases, widened creelcs into bays and broken down their vertical
sides into shelving shores.
The Red Sea has an evil reputation with navigators on account of the
number and complexity of its coral-reefs and the presence, in certain areas,
of that especial horror, tlie coral pinnacle which rises out of deep blue water
to?just under the surface. The reefs may be divided into :(u) Fringing rcefs along thc. coast and round islands.
( b ) A Barrier system, which is especially typically developtd between
19’ N. and 24‘ N.
(c) Scattwetl reefs, including sonic of Atoll form.
The climate is noted for an extreme aridity, which is however not so
complete in the southern part of the coast as in the northern. Among the
hills furious rain-storms are frequent in August and winter, which flood
the valleys for a few hours a t a time in tlie way described by travellers in :he
Sinai Peninsula, and by Egyptian Survey Departtnent officials for the desert
bordering the Gulf of Suez *. These conditions are of coursi’ the best
possible for the conveyance of thc large quantities of detritus which liave
formed the great alluvial maritime plain.
The tides have been referred to in the Narrative (p. 3), where it is stated that
the middle part of the Red Sea is pract,ically tideless. There arc>,however,
variations of level of two feet or thereabouts which occur almost daily, but
their occurrence is subject to no fixed rule. The rise or fall inay be fairly
regular for several days, high water being at nearly the same time each
day, then it may remain continuously low or continuously high for :L short
period. Occasionally the water niay sink to a foot below its lowest normal
*
Uarron &L Hume : (‘Topography and Geology of the Eastern Desert of Egypt, Central
portion,” issued by the Survey Department, I’nblic n’orks 31 inistrg, p. 282.
HISTORY O F THE CORAL REEF‘S.
0. CHOSYLAND-RECENT
17
level and remain down for oiie or inore days. Further, the mean I c ~ e li b
about three feet lower in winnier than in winter.
It is found that there i- a true tide of a few inches, but th,it this iswamped by the greater changes of level indnced Lp ch:mges of barometric
pressure. The ‘‘tides ” are thereforc qiiite irregular and cannot, be l’redicted.
My own obieriations h a i e betin innde daily for about six imnths a t
Dongola, where, owing to itq po-ition ncar the head of a very long bay, the
effect of the qtrength and direction of the ivinda is very marked, \but even
there the wind is not the onlj- factor in producing c h a n g ~ of
i Irvel.
The difference in the level of the whole of the Red Sea in thc winter and
summer is a result of the i n o n + o o ~in~ ~the Indiaii Ocean. The t o t d
difference between high
mntrr i n winter a i d lowed in suinnier is about
six feet.
C‘AVSES OP THE FORMS O K T H E YRESRXT HEEE’ SYSTEM.
The tlieory advanced is tliat-(i.) A- the region i- one where conderablc
upward niovenient (at least 500 feet) ha+ occurred since modern reefs began
in the Sea, and w h t w elevation i- still in progrestG, Darwin’s ~nl)sicJencc~
theory cminot be calletl into nccoiint for the existence of the barrier reefs
at a distance from l m d aiitl wpratcvl froin it by d e q ) water, or of ihe atoll
forms of: certain reefs. (ii.) The rchtious between elevated and <till living
reefs show that the foundations of hoth are ranges of sandstone hills partly
below wa-level. (iii.) The forinu of the reef- are due to the balance of
aggrading over degrading agmtq, the fornier being the growth of coral,
nullipores, kc., and tlie latter the corro-ive action of the -pa and tlir rotting
cau.etl hy boring orginisin<, &c.
A not,c on the probable cause of tlic change- effected after tlie rrpliea\.al of
a coral-reef, whereby it takes
structure characteristic of
*-
011
the Iiardness, homogeneity, and crvstalliiie
C‘ord-R~g,’’is appended.
EVIDENCES
OF ELEVATION.
The cxistence of‘ the coast-plain :inti its I)readtli, which on tht. Sudan
coast averages five miles, czw again-t a n y conzidcrahle inovenien t of depreshon
during its forination. The r a i d reef- inclicate upward inoveiiients of
1500 feet or more, ant1 more recent cEianges are rccordetl as raihed beacheand oltl erosion-lines 01- cliffs, of wliich esainpl~sare foniid througl~outthe
length of the Sudan coast.
A t Agig. a mile inland, h i t only :I few feet ; L ~ J O W tlie SKI, i h n raiscd beach
in the form of :L long ridge of rollccl pe1)liles of crj-stalline rock correq~onding
to tlyat of coral-fraginent.. which hortlers the present shore.
Both are
surmoniited by :I line of graws, the seashore having been, apparently, :I
favourite place of hepnlture through d l ages. Even the cle-ign of tho. grave5
LISN. J0URN.-ZOOLOGY
VOL. S X S I .
3
15:
XQHINE BIOLOGY OF THE SUDANESE RED SEA.
remains much the same, except that the modern stones are but a fraction of
the size of the monoliths of the old days.
Some particularly conspicuous cases of raised erosion-lines on cliffs are to
be seen a t Tella Tella Suraya Island in the south, and on Haysoit Island in
Khor Dongonab in the north. These are treble lines, the modern one at sealevel, and two others at points 3 and 5 feet above this. A single raised line
may be seen in the harbours of Port Sudan and Suakim. The larger of the
islets of the landlocked North Bay of Khor Dongonab is of limestone, and has a
line of undermined cliffs along its northern, or exposed, side, the base of which
is now sonie feet above sea-level and separated from the shore by a stretch of
a n d about a hundred yards wide, and covered with the xerophytic plants
usually found on these dry sandy islets. One should note that t,he higher cliffs,
seen from a distance, appear to show lines of underinining in some cases, but
that these, on a nearer view, turn out to be the results of subaerial denudation.
The hills are formed of horizontal strata whicli are in some cases alternately
hard and soft, and the latter, standing out from the general level, siinulate
the overhanging part of a marine cliff. But in these selected cases, as in
that described below, the smaller details of wave-action are preserved so that
the former, lower, position of the cliff' is evident beyond doubt.
The horizontal position of the strata shows that the elevation of the limestone from the sea has been effected in a rcgiilar manner ; but evidence of the
steadiness of this movement is given by the occurrence everywhere of elevated
coral colonies, which are in the same position exactly, relatively to the
surrounding rock, as that in which they once grew in the sea. Such colonies
may be of quite delicate branched species, and good examples of such are
exposed in the trench which surrounds one of the blockhouses which once
protected Suakim from the Dervishes, but generally the fact is more easily
made out in the case of massive species. Vary conspicuous examples were
seen on Tella Tella Suraya, an island of which the Admiralty ' Pilot Book '
notes that it is dist'inguished by t,he presence of many cairns of stones
upon its highest parts. The island is about 40 feet high and surrounded by
cliffs, mostly of rather loose coral-fragments and abundant shells &c. of
present-day species. The " cairns on the level summit of these cliffs are
simply large hemispherical coral colonies which survive t,he weathering
that has removed the surrounding softer material.
Corals similarly
situated .are exposed in inany places on these coasts, the sunmiit of Jebel
Tgtiwib in Khor Dongonab bearing a part,icularly fine series. There
would be no difficulty in identifying them as species at present living
in the adjacent sea. The island is a miniature elevated atoll, the present
bottom of the lagoon being just above the sea-level, and consisting of
a flat sheet of mud, which was drying and slightly crusted with salt at the
time of our visit, a t the beginning of March. This depression is surrounded
by high ground, its present coininunication with the sea being probably
subterranean. The lagoon is hordered by I d i c s , of the species which live in
"
C . (‘ROWLAND-RECENT
HISTORY O F THE CORAL lilCEl~-.
1!I
salt s w t n i p , ant1 about 4s f w t a h thein :tr(l sniall cliff5 n1iic.h 41ow very
distinct inarks of having been untlerniined hy waves at ;L tinw ~ v h e nthe
lagoon was a t a lower level a i d contailid water. Above these the ground
dopes steeply to the level of the summit of thr cliff;.
For other elevations see the postscript on the randstone coast-hills (11. 27).
Two sets of clcvations have been shown to have occurred since reefs of
modern corals began to be formed in tlic Hctl Sea : (i.) A niajor b:eries up
t o a t least 500 feet (Rawnyn, Mal~nw:r,dic.). (ii.) Minor rlevatioss which
are very recent and are prohnblp still going on. (Recent raised heac.hes and
erosion-lines on the cliff..)
(1
THE LI\ I,CG
fiEEF\.
Pri/t~/i//g
Rc<titx.
Tlicse reefs O C C I I ~ practically w crywherc on the ol)en coast an<l in the
bays ant1 “ khors ” except at tlirir extreme l a n d w r d end.;. Tlicir absence
from pasts of Khor Dongonab has hoen noted in the NarriLtive (p. 5). They
are absent also froin wi:i11 lengths of the coast-linc at Has Bnridi (noted by
Darwin, 011 the east 4de of the sra) and from the ,shore!, of ‘‘ 1)okhana
Bay ” in Khor Dongona\). As in onc or two &dar cases in Briti-11 East
Africa, I can give no reasons for tllesc oniis,cions cscept in one case-the
Iinrl~ourof Dabaclib, on the Sudan toad betwcen Salak and Port Sudan.
Here the coral-border to the coaht-plain is almost entirely covrred by a deposit
of the very fine hand, tlerivetl froin crystalline rocks, which a t times Bsf high
winds causes thick “fogb” in tlic R d Sea. This fouls the water and prew n t s coral-growth in the immediate Iicinity. Probably the case of “ Dokhana
Ray ” is the same ; the water certainly i.: middy there, but I have not determined the origin of the mud, ant1 in sonie cascs, e. g . Suakim Harbour and
wpcially the North B:iy of Khor Dongonab, coral can flourish in very cloudy
uater *.
The shore-reef is broad where tlie coast is low : e. y., at the entrance to
Sualiini Harbour, where it i b but a foot or two above sea-level, the reef is
1; sca-miles broad, while at Port Sudan (Mersa Sh6kh Barixd) the coast
is 6 feet high and the reef only $ mile wide. This difference is obviously
due to the lesscr rate of erosion where fallen rock protects the base of the
<~liffsfor tt time against the sea. It is obvious that the formation of reef-flats
by erosion is going on here just as on the Zanzibar coast, but that here ii is in1poq4i)le to tlrterniine wliat proportion of tlie total breadth attained is due to the
cutting-tlmvii of the land and what to tlie addition due to recent coral-growth.
At a point just north of Mersn Abu Hainhiiia, at Dabadib, Mersa Fijab,
and sonic‘ other points, the coral-liinestone has been nearly completely
renioved, the shore being forined of the rolled grey pebbles of tlle alluvial
* Note added Sept. 1907.--Fre.li
nnd
‘<Dokhana Bay” nearlj
water, flowing underground, enters the
all the J e l l round.
Z ~ a
Rt
2*
Llabadlb
20
MARINE BIOLOGY OF T H E SITDAKF:SE RED SEA.
jiI;iin.
That the greater part of tlic reef-surface was formed by the removal
of elevated coral is conspicuously demonstrated, in the first caqe mentioned,
by the presence of numerous isolated pillars and grotezque rocks of this
material bcattered over its landward portion, in tlw third hy the presenct' of
coral-rock islands ; but at Dabadil), as already mentioned, only a narrow
band of rock surface is visible at water-level.
Only narrow reefs are found below the comparatively high cliffs (3040 feet) of the east side of the North Bay of Khor Dongonab. I a t t r i h t e
this to the reefs being sheltered from the prevailing minds, and consider it
merely as :L further indication of the inq)ortance of erosion as a factor in
reef-formation on coral comts.
Thc surface of the shore-reefs is gcnerallp sandy, but 1m-e rock-patches
occur, ehpecially a* a border to the growing edge. Stones :%rerare or quite
absent except at the slightly raieetl seaward margin, wliere the remains of
dead coral colonies are soinetiines watteretl. Such large and solid stones as
those so plentifully scattered on the reef-edge in Zanzibar are, froin the
nature of the case, altogether ab:,ent from thi:, area, which h a i been formed
directly by growth of coral and nu1liI)ore *.
The raised edge comibtz of a gravel of broken coral well covered with
nullipore. Outside this is a gentle slope of corals, generally stunted and
accompanied by nullipore and a1cjonari:ms (Xeniid=), which extends to a
depth of n fathom or so, after which i:, a precipitous slope of luxuriant
corals reaching as far as we can see with the water-telescope, say to
10 fathoms or beyond.
There is generally a more or less regulw boat-channel or series cif pools of
water a fathom or SO deep, with niinicrou9 outlet:, to the open sea, some of
which provide anchorage- for +a nl li d<~
(or " tlhows " a s they are callrtl in
Zanzibar).
I n shallower water than that found generally off the shore-reefs the
lnaxiiiiuln depth at which coral grow- lu\uri:iiitlj- decreases, for instance on
the Shubuk boundary-reef, to he dtwril,ed later. A slope of inud with scattered corals is visible below the precipice of luxuriant growihs. Siniilarlp
inside the harbours, wliere also the species of corals change, the delicately
brancliecl and fail-like form., which are in the majority outside, giye place to
Inassive P o i d e s , Meantdrizia, and -o on. On this coast it, i, the ma&ve
gener:i which are c1iar:icteriztic of slielteretl water?, not vier wmG.
The fringing-reef on the east &le of Jebel Jfayiti is also wortliy of note.
The island is a pyramidal hill, thti enbt side heing precipitous. From the
su~nniitone looliz down upon :I brown fringing-reef covered hy a foot or two
*
The stones foond on the raised reef-edge of the Zanzibx coasta are the harder portions
of the rock removed diirimg the forinntion or the reef, which lave been preserl ed in the w6y
described in the p n p r (Zoc. czt.). The " nepro-hands" r,f Pacific atolls probabl: have x
siniihr origin.
of' water, pwlia1":
one Iiundi-cd y i r d s wide, antl apparently of iiniforrn
structure throughout.
I th clear-cut, ~ t l g emeets tlie hliie-l~laclr tleep sea
(lirwtly, without t l i ~
intervention of :I 1):itcIiof lighter colour anywhere, giving
the inost forcihlc inipr(b-ion of tlic c d p of~ ;L f:itlionilcss ahysq.
Scutteretl lierjs trrctl the Sl~itlitdArea.
It will be conveiiicnt in considering the rernaining reefs to divide the
coast into two section+- tlic first from Has Bcnns to Su:Lkini, the wcond from
Siiakitn to tht. bounclary with the 1t:ili:ui territory, Ras Kas:ir. T11e foriiier
is characterized I)y tho p r w w c e of well-deiclopetl harrier systems antl deep
water close to all reef<; the latter by inniinier:il)lc>small scattered reefs and
sand arid coral ihlaiiils (tlic Su:tkini :irehipelago) in comparatively shallow
water and ii ,ihelving Imttom, and, i n thc hoiith, :I coniparatiiely plight tlevrlopment of recent coral-growth. The contrast is thus n very strongly
in:irkecl one ant1 very .ignificant al\o *. One of thehe wattered islaiids, Tella
Tella Kuraya, lias :ilre:icly bccn tlrtcribctl.
The only other feattire of interest in thi+ neiglibourhood is thci Shuhuk
area, a hundred scliiarc miles of tlie moht intricate p i w ~ g wamong reefb,
surrounded on tlie N.E. and E. sitlcs by an uiibrolreii reef +iniil:ir to the
ortlinarp fringing-rwf of the shore, ant1 on the S. :inti W.hy the ~n:iiiilantl.
Most of the rcefs euclosetl arc iiirwly ~and1)aiiksc:ip~wlwith a fatliom or so
of growing coral, but on tht. in or(^ c i l ) o w l N.and W. sides thc coral goes
d e q m xiitl, whcrc not growing c w x l , the hottorn i+ 1:irgely composed of cleat1
fragiiir.iits of coral, 41clls, k c .
The land hercabout.; is $0 lo\\ :UHI *o cut into by great shalloil lagoons,
that it i h clifficult to clcfinc a coast-line :it all. It is easy to
rc~niarknhlcarea Iia. bec.11 formed, the diorr-reef spreading srawartls rapidly
in the hallow water, where its t1t:hri.: would be :it once :~~il:1l)lt3
as n substratum on which the growing zone of coral roultl aclvance seawards. Within
the g r o i!ig
~ edge thc accumulated mass of corals &c. wonld tindergo decay
ant1 bolution into tlic niazc of reefs now found there; while others wrre being
carled out of the low coast, :mtl tlicw, when their siirfaces were covtvxl with
coral, living ant1 dcatl, wonlcl liccoine indistiiiguiP1ia~)l~
from thow which hat1
grown up in sittr.
T h e llurriei. System.
But little is to be added to the account given by Darwin in his 'Coral
Heefs,' and for a description I refer to his work p. 102 & pp. 143--146 (the
latter refrrencc including x special section on the reefs brtwecii Siiakirn ant1
lialaib), :ind to tlic niap on 1). 6 (scipiu). But the facts I h a w given
abolc show that it is impossible to suppose ;L secondary subsitlenor of the
* Note added Sept. 1907.-On land tlie contiast consists in the absence of sandstonr
coast-hille, the significance of which will be ~ e e nafter readirip the following paps and the
postscript.
22
MARINE BIOLOGY OF THE SUDANESE BED SEA.
middle part of the Red Sea in orclcr to explain the occurrence of barrier-reefs
on this coast. The absence of the regular barriers in the southern section is
not it consequence of elevation ; for tlie evidence is distinct that the recent
movements have been the sitme here as in the north, but is rather correlated
with the different slope of the sea-bottoni, and probably, as will be explained
later, with the structure of the side. of this part of the rift-valley.
That the latest moveinelit ha. been one of elevation is shown by the
character of the erosion-marlis left on the cliffs, which include details which
~vouldbe coniparatively soon weathered away froin so soft a rock, especially
when one c o n d e r s the important part played by mid-erosion 011 this desert
coast. The very recent c1iar:icter of the fossils contained in tlie cliff:. iy
evidence in the sanie direction.
The present form of most of tlic3c reef- is the result of the saiiie procesres
as have foriiied those of Zanzibar :ind East Africa *, where the forceb of
erosion, with tlie aid of organic growth insufficient to add to the bulk of the
material, ha\ e c a n ed ont a11 the cliaractcristic forms of coral reef,. I n the
Red Sea, coral-growth is still adding greatly to the buIk of the reefs, but that
the foundations of the present reef3 : i i ~due to erosion the exainination of tile
physical geography of this cw;lst 1)ro\es conclusively.
Ehrenberg, over seventy years ago, stated that the foundation of the Iled
Sea reefs originated in this way, but here the problem i b more complicated
than on the Equatorial coasts by the iinposddity of distiiiguisliing between
that area of a reef wliich i:. due to the erosion of eleaated coral-rocb and that
which has been added by recent growth, except in the ease of some of the
Khor Dongola (Dongonab) reefq, u here :.ncli additions have not completely
concealed the original found a t’ionq.
A t first sight it seems m0.t prol)nl)le that these last-mentioned reefs are
examples of thc mode of formation mggested by Darwint in the caws of the
Farsail and Dahlali archipelagoes, iiz. by the collection of hediineiit on an
uneven bottom froin which coral-growth took its ri,se. The Dongola Barrier
(map, fig. 3, p. 15), indeed, seem:. an obvious ca*e of this occurrence, the
bank continning awash for a long tli*tance to the west of the sand islands,
and the bottom in the neighbourhootl being sandy with n o rery consideralile
masses of coral or iiullipore growth. But a closer examination shows that
this explanation, wemingly ,SO plau:.ible, i b not correct, and that, in fact, the
bank has a rock foundation merely overlaid Iiy the sandbank and such coral-
* C. Crossland, ‘<
The Coral Reefs of Zanzibar,” Proc. Camb. Phil. SOC.xi. p. 493 ; and
The Beefs of Pemba tmd the East African mainlaad,” 1. c. xii. 1904,p. 35.
f I confine my quotation to I)arwin, as he is the only author who has described the reefs
of this part of the Red Sea in general terms. The wider problem has been attacked by
numerous worliers.
For an epitome of recent views see J. Stanley Gardiner’s paper “ The Formation of (!oral
Reefs,” iu ‘ Katiii-e,’veb. 18th, 1904.
C. CHOSSLANU-KECEXT
HISTORY OF THE CORAL REEFS.
2 ;3
growths as occur *. I n places, a strip of hard coral-rock showing. sections of
the contained corals and shells is left k J W P about water-level, below the sand
of which the islets are composed, aiid this is true of most of the other sand
islets in Kltor Dongola.
I have seen no reefs of any importance, on the Sudan coast at least, which
could have had banks of beditmiit, tr~nsportedand deposited by currents,
for their foundations. On a smaller scale, however, p o d d e examples are
those bounding the fine harbour of Mohamed Ghul, several square miles of
reefs in the neighbourhood of the village of Dongola, and many cd those
described in the Sliuhulr area in the south. Such banlis are only to t e found
in comparatively sheltered situations, never in the open sea around the Barrier
spstern,
Below water-level n careful examination of the bottoin show<, in inany
places, bare rock-flats usually thinly coated with tnud. It is thus shown th:tt
the Barrier aiid other reefs of Khor L)ongola h a r e been niercl~cai 1 ed ont
of the rock by the action of the sea.
Tlie ma1 Barrier system is even more easily qeen to he formed lty c rosion.
Tlie fact that the reefs Itetween Maltawa I+land and the pniiisula of
&a\\ aya are the rernains of a former land-connection between the two i5
ahnost obvious, especially a G a rrinnant of this lo-t latic3 remains as an i+olatcrl
pillar of coral-limestone, 8 feet high, on “ St. Fillan’s Reef,” nearly lnidwaj
between the two. Although Maliawa 1.lantl i+ high, about 500 feet mill not
be far from tlir mark, it* southern end i+ little above bea-level. Si iiilarly,
Rawaya, thong11 hill. up to 200 fcct in height are present, has cons derablt.
:ireas which mu-t 111 course of time IN> reduced to reefs, S O low are they. The
iiccli jorning it to the mainland i+ Itarticu1:irly low and narrow, SO that
Raw:t):i inuzt soon 1)ecornea chaiu of limestolie irland+ connected by a coniplicated reef zy-teni iiidi,stingiiiihnl,Ir from that now connecting it to Idakan :t
Island. Similarly the Barrier bystern to the ,iouth of Maliawa, p e r h a p a s far
as the Tiflah Idands, iz a continuation of the Hawaya-Makawa Itill+, either as
eroded reiriiiants of the range o r a3 hubmarine hills not yet elernteti above
sea-level. The latter must be true of the southern reefs, in whicli case the
most likely poe.tnlate to account hoth for tlieir growth and for tlie tAiistcnce of
other rangee. of coral-capped hill5 tt+/iic/t are jm-allel to t h e sides of ~ I L?.jftP
valley, is that the faulting which produced it rebultcd in the side.; bein,; in the
form of a series of s t e p or parallel ridges, upon which the reinaine. of or,ganisms
~ o u l daccumulate f ar more rapidl?- than on the intervening trough- or flats,
SO raising the ridge. until corals, fin:tllp of reef-building ipecieh, coil d carry
the ridge up to the surface coinparati\ ely rapidly t.
‘
* From. Rlr. Stanley Gardiner’s last iepoit (in Nature ’) on the Percy Sladen Expedition
in H.M.S. Sealarlr ’ (January 1906) it, appears that the fringing-reefs ot the Seyr idles art:
another Ciwe where recent coral merelv coats over older rock.
t J . S’tnnlrp G‘aidiner, “ The I3uilding of Atolls,” Proc. Camb. Phil. Soc. 1902.
24
MARINE BIOLOGP OF T H E SUDANESE RED YEA.
Further north is another 11rominent point, Ras Benas, which shelters a
large bay and has a large island to the south, exactly in the same way as
Ras Rawaya. So like, indeed, are the appearances of the two points that
the same name, Makawa *, has been given to the islands in both cases.
Tlie Jezira Ridge on Ras Benas corresponds to the high ground on
Rawaya, and the Makawa of latitude 23’ 50‘ N. is obviously the continuation
of this range. Similarly the southern continuation of tlie range is represented by the “Horseshoe Reefs,” which still retains a fragment of the
ancient island in the forin of ‘‘ a white rock” which “looks like a boat,”
below which this range ends in another set of reefs.
The third prominent point of this coast is at the mouth of the Straits of Jubal
(leading into the Gulf of Suez), where the Zet Hills run southwards into
the sea, where they are broken into a chain of islands ending in that of Shadwan. The process of denudation has here made comparatively little progress.
Few reefs exist except those fringing islands-and the islands are high on
all their areus, e.g. Shadwall 990 feet, Jubal 410. But imagine denudation
to have progressed to a far greater extent and to have cut down the smaller
islands to sea-level, their foundations being at tlle same time preserved or
added to by the growth of coral, and we see that the conversion of a range of
hills to a peninsula, island, and c h i n of barrier-reefs becomes complete, as in
the case of Rawaya, Maltawa, and Tiflah. The repetition of this arrangement
on this coast is striking, but just as some ranges are entirely inland (e. 9.the
Abu Hamhma range), so otliers arc completely transformed to barrier-reefs,
or perhaps were never elevated so high a s to be continuous with the land, or
even to project above the sea. For instance, the long reef, SIiab Snadi,
seems to be a continuation of the promontory of Salalra, but there are no hills in
the vicinity that could be part of its range. Tlie Barrier system inside Foul
Bay, south of Ras Benas, is not connected with that point or its “ Jexira ridge.”
To the south this reef system bears the Mirear (lat. 24’ 40‘) and S i p 1
(lat. 23’) Islands and some small rocks, the islands heing low and flat, only
ti to 12 feet above sea-level, and composed of elevated coral.
THE FORMATION
OF CORAL-RAG.
An instructive comparison can be made between the elevated coral of the
Red Sea coast and that of Zanzibar. Both are low, and contain fossilized
specimens of the species now living in the adjacent sea, and are consequently
among the latest of geological formations. I n both cases the alteration of
the relative level of land and water has been effected in so regular a manner
that the corals of the land retain relatively the position in which they grew
when submerged. But in spite of these fundamental resemblances the physical
properties of the rocks are as widely different as possible.
* This Arabic name, meaning
than its donors supposed,
that which is resistant,” has therefore a deeper significance
C. CHOSSLtW1)-RECENT
HISTORY O F THE CORAL REEFY.
25
In one’s mental photograph+ of tlic t v o coast<, thr most prominent feature
in either case is the black colour and fantastic forim of the Zanzibar coast
rocks and the light yellow and usiially forinlebb c1i:iracters of thobe of the
Sudan. These are the outward Iisiblr signs of cqnnlly striking tlifferencrs
in their physical constitutions.
In Zanzibar and B r i t i h East Africa the low cliffh arc of an exceedingly
hard crystalline rock, which is so homogeneous and tenaciou.; that the overhanging shelf left by the iinderniining of the 5ea may, under favourable
circumstances, project for more than four feet witliont its breaking down.
The surface of the rock, both on the coa<t and inland, i., as already mentioned,
alniost black, though when broken the rock itself is seen to bc white or nearly
so. The raised coral of the Red Sen coast, on the other hand, is genernlly soft,
often indeed like loose sandy gravel of h e l l s and coral fragineiits with the
larger coral colonies a h embedded stones. The bases of‘ the cliffs are somewhat
harder ; and though their underniining results in form5 which distinctly recall
tlie cliffs of East Africa, the lack of homogeneity and cohesion of their material
rrsults either in a monotonous formlessncss or, wlierr somewhat harder rock
occurh, the substitution of picturc~squeinasses of fallen rock for the bizarre
pillars, brackets and arches of the Equatorial coasts.
Although fossilr a1-e T cry abuntl:int in the Zanzibar rock antl :ire visible in
section in the cliffs, yet the greater part of the rock near the -urface is SO
homogeneous and shows so little trace of its origin from a loose conglomerate,
that there would be great tlifflculty in hepirating wcli fossils a s o c m r from
the surrounding matrix.
It is important to notice, honmer, that in places at n little depth below thr
surface, the rock becomes much IPSS crystalline and its fossil5 niore easily
sep:il-able ; in short, it :ipproachcs the condition of the Red Sea rock From
quarries made for road nietalling, MY.T. J. Last, Slavery ~‘omniissionerin
Zanzibar, made large collectious of fo+il* and their casts, nnd a wholr serirs
of stages in the crptallizing process could he easily obtained.
On tlir Sudan toad again the rocli hah the characters of that of :!anaibar
in certilin localitieb, hut only i ~ l i e w the corditions to icl~ic~hi t is e.cposetE
appmcirnute to t l m e rtormu1 011 t h e F 5 p t o r ; that is, to being “ ’twist u ind antl
water ” at the sea-level in places where it is under wa\ e-action *.
The whole surface of Zanzibar i b nndrr h i i c l i conditions owing to ii s heavy
rainfall, but rainfall heing practically nil on the Sudan coast we iind the
hardened crystalline rock only I)? thr sea at \vater-level. Further, owing
to the illappreciable tide, the recfs giw constant protection to the cliff 5’ bases,
so that only in exceptional places does one find coral-rock freely exposed to
wahei and spray. For Some reason the rerf i b abhent froin the eastern cliffb
of the islet of Tella Tella Kebira, which are thereforr alniost constantly bathed
in spray, and h a w taken on a11 tlic characteribtics of thr Zanziknr rocks, in
*
See also the preceding paper
011
the coast 31ear hlexmiclris.
inarked contrast to the other islet> in the yicinity, which have only the usual
partial hardening of tlie undermined portionb of their cliff?.
I n Khor Dongonab, where protection of the coast by coral is 1 ~ complete,
s
the foundations of the sand islets is, as already mentioned, of coral-rock, and
:it yen-level, where it is alternately wetted and dried, it has talreii on the
typical yualitie3 of " Coral-Rag." In the same way, beach sandstone has in
seFcra1 cabes been formed by the cementation of the sand of which the islets
are coniposed, and this is as hard as the coral-rag where it overlies it, becoming
softer and softer a3 it rises above the water-level till, at a height of 3 fect
or so, it becomes of a crumbling consistence and passes into tlie loose sand
from which it is derired. (See PI. 1.fig. 3 and it, explanation.)
The factq cited certainly warrant the conclusion that the alteration of tlie
Zanzibar rock from the ma53 of more or 1e.s loosc material, in which conditioii
it was elerated from the sea, to it3 present cryatalline state, and that tlic
roclr of quite similar origin in the Hed Sea has rcniainecl nearly in it.;
pritnitive condition, is due to the absence of' rain and generally of wave-action
and spray in the latter region. Thc~details of this action of water have not
bern worked out, but probably a recrystallization of the calcimn carbonate anti
a partial substitution of magnesium, or dolomitixation, occurs, both of thehe
proce$ses tending to harden the reni:iiiiin,a product.
C'OKCLrSIONS.
The conclusions to
follows :-
lie clrawii
froin the :ibo\-e
III:IJ-
be suminarized as
I. That thc coast of: the Sudiiii, bebides its major elevations amounting
to more than 1100 feet, ha- recently undergone cereral sinall elelations, the inoveiiients having been uniform in their action and not
recently reversed.
2. That the diff'erences between this elerated coral and roclr of piinilar
origin elsewhere, e . y. on the coast of Equatorial East Africa, is due
inainly to the abseiicc of tick and rain in the Red Sea.
3. That the present forin of the reefb is due as much to the eroding
action of the bea upon t1ii.j elevated rock as to the growth of corals.
In the case of F&igiiy-Keyfs (including the Shubuli area) the laid
is cut down to sea-level behind the rim of' growing coral. Burrie~f h f s are formed: (1) By the direct growth of coral upon subinarine
hill ranges ; tlie northern ends of these have been elevated,
and are now ranges of coral-capped hills, the middle parts remain as
pcninbulas and islands, the southern as Barrier-reefs. (2) By the
ciitting down hy marine crosion of proniontories and islands, a i d of
Cora 1- reefs previously rlcvated.
27
HISTORY UF T H E CORAL 1tEEEh.
C . CKOYYI,ANI)--REC'EK'I'
POSTSCRIPT.
As the theory of the Barrier Borniations lierc advanced turns on the rtrnctnrc
:md topography of the coast hill*, I give the inforinntion I have beea able to
collect.
The hills are iiot distinguishable on any map, indeed part of the country i,,
a < yet, unsurveyed, and I have bud few opportunities of travelling inland.
My account is therefore incomplete, but enough has been done to justify the
CollclUsiOlls clrawn.
Froin beawards these hills are easily distinguidhed from the jagged hills of
igneous origin by their cliffs being of R light yellow colour and by their flat
tops ; generally also by their position being nearer the hea than the lowest of
the igneous hills, and e l e n tliau the mounds of gravel wbioh occur in places
a t the foot of the latter.
PasAig froin South to North, the first range i h met with a few miles
nortli of Mersa Dumr (see nial) p. 6) a h a chain of low butts rising from
the alluvial plain four or five mile- inland. These become higher and inor('
continuous as one passes northwards till they end in two considerahle hills,
of about the saiiie height and area of h e , tlic northern of wliicli i, niarketl
on the charts, whew it is called Table Mountain and given a Iic ight of
1100 feet.
At about five miles inland from L)ongonab are one or two wiall hill5 standing
alone, but further north is a range of higher hills lying inland from the
middle of the North Bay and extending towards the Hamiima range, from
which it ih only separated hy a n interval of a fcw inileb.
The A h Haiulmn * range (which T +o named from its most 1m11iiinent
though iiot highe*t ~ ~ e a lai ,famous lantliriark for sailors) extends from around
the inner branches of Rhor Sliimil) to come distance beyond Khct Abu
Hamiima, lying much nearer the sea than tlo tlir above. Its height is clstiniated
by the Government S u r ~ e y o rat from 500 to 700 feet.
These range- are wliolly inlantl, ri-iiig : i h p t I y froill the maritimc~plain,
which they divide Ioiigitntlinally. J hat c iiot Iieen able to bee whether they
are coral-capped, but IIOJ seen that their h:ises, at least, have the cti.uctur(1
of the range next described.
The hill+ of the Kawaya Peninsula tind cJebel Makawa form an outer range
parallel to the above, and are capped with a stratum of modern coral3 They
were once barrier reefs, growing upon sandstone foundation-.
The highest poilit* of the Rawiya. peninsula are near its north end, and
eastwards froin the *alt-worlih in the south, the northcrii Iieing mi extensive
platean of a height of about 100 f w t . the 1:itter a hill of about twiw this
(1
.
* This may \be translated
its sniumit.
'' Pigeon
IIiIl,'' and is so called from the curiouL shape of
28
MARISE BIOLOaY OF THE YIrDANESE RED SEA.
height. The rest of the surfacc. i+ low or untlulating, generally about twelve
feet above wa-level but in inaiiy place- w r y little above the sea.
The outline of the> peninsula as given on the chart is only approximately
correct ; i t is broken u p by many intricate “ Kliors ” or inlets of the sea, of
which “ Khor Hawaya ” is but on(’.
A section of the Northern plateau is given by a cliff about 60 feet high
which borders a fault, now full of sea-water, known as Khor AtGf. The
thawing given (Pl. 4) shows its structure, which is typical of tlie whole of
this peninsula. Above is a mass about six feet thick of recent corals (A), all
in the position of growth. The large numbers of‘ shells which accompany
these arc still more easily identified as being of living species.
Next is a thicker stratum apparently of con+olidated coral-mud (B & U),
containing two characteristic f o d s , nullipore nodules and a clypeastrid
echinoderin. The latter occurs in large nmnbers and is easily recognized as
a species still living in Khor AtGf.
This is all the reef formation present *. Below it is a bed of gypsum (D)
of varying thicknesses up to 18 feet, this resting on n finely laminated and
very soft sandstone (E), genemll? yellow in colour but here often dark green
or red.
The coral foriiiationi :irp in horizontal strata, and w e uiicor!forntcthZe to the
strafu O I L icliic*li they w s t , which are c onderably folded and contorted.
I n the lower parts of the peiiin+iila, tlie coral stratuin ic thicker, e . y. at
Haysoit Island up to 15 feet are exposed, the coral-mud and other strata not
appearing above the sea-level. The hill in the south (Jebel Abu Shagara)
is similar, forming a great cliff hounding the fault in which lie two inlets of
the sea, “Khor Rawaya ” opening northwards, and Khor Abu Shagara which
r u m in froiii the open sea, i. P . froin the south. The T-iew eastwards from
tlie summit is striking, thp peninbula liere consisting of a series of parallel
ridges and troughs, the result of einallrr fault*, some of which contain seawater inlets. These, like all the Khors above named, rim approximately north
great north and south
and south. Jebel Abu Shagara is itself cleft by
gash i n its southern part, which may he taken as one of the coast inlets
elevated above the sea for oiir examination. It shows the same structure as
the cliff 011 the east sitle, viz., a shnllow cap of coral underlaid by gypsum
and sand.;tone as in the north, but here the gypsuni band is narrow and may
be absent, the sandatone in any ca3e foriiiiiig the great bulk of the hill. This
rock forins also the bulk of the islaiicla of Makawa and Mayiti, which are
about 500 and 300 feet high respectively. 1have not clinibed Makawa, but
Mayiti is coral-capped. The east side of all these hills is precipitous, the
west sloping.
I have found no elevated reefs thicker than 16 feet on the Sudan coast. This agrees
with the accounts given of those of both shores of the Gulf of Suer and the Sinai Peninsula.
'l'liib chain of bill-, it i a e\ i h t , H wc mice islands ant1 s i i l ~ i i i , i r i i i (b~inh~
lying parallel to the cotid, tlieir formation aiicl po.ition being a mechaiiic:il
result of the opening of the rift alley. By iiiariiie erosion and the g r o tli
~
of Coral+, thew became levelled dowii ant1 built ul) to a line of barrier reef-,
])art of which ha- n o w been elevntetl :ibo\ c the -ea-lc\ el as the co:ist-r:iiigt+
;ind a- i+land., other part> being oltw tetl to +ncli height$ t h t t rapid coralgrowth has heeii p d l e 011 their suiiiinitr. The deeljer portion- have 1)robntd \r
bccii raised to within tlie limits of growth of reef corals hy the accuniulation
upon theiii of the reinailis of niarine org:inimi+, which flourish e+pecidly on
any elevation of the sra-bottom in the ~ r n ydeacaribeil by J. Stanley Gardiner".
Thus, by degradation of the part- above sea-level and aggradatioii be ow', arr
formed the lwei lines of rcrfs of tlio h r r i c r \ysteni of the present daj Thimode of forin:itioii is probably the origin of the level h:ind of coral-rock which
lies between thc alluvial p v e l of tlie ni:iritiinc plain and the deep mi, and
to the presence of l\liieli tlii. plain, con-i-tiiig ah it docs of large grakc.1 and
-:iiicl to great dclkhs, i. due. ( S e e 1). 14, note at hottoni of page.)
.
CjTKIT, ('Rl IbbLASI).
Sept. 20, 190i.
EXlJLA4XATI0KOF THE PLATES.
PLATIC
1.
Fig. 1. A pearling canoe and its occupants.
2. Diver returning to the canoe with a captured pearl-oyster.
3. Bewh sandstone 011 the island of Sararat, in Ilhor Dongonab.
( a ) . Sea horizon.
( 6 ) . IIard coral-rock, eroded domn to sea-level, whici forins the foundatic,n oi +he
sand island and of the ad,jacent living reefs.
(c) to ( e ) . Band of beach sandstone, smooth and hard a t ( c ) ,rough and nioilerately
hard in the dark band, very soft at ((J).
(f)is the loose sand of which tlie island was origiually composed entirely.
4. Nouth of one of the shallow wlleys wliicli drain Rawaya and open into the, North
Bay. The opening lies between two low digs of coral-rock, one of which it shown
in the background of t h s photograph. During the winter, fresh water l'ercolates
tlirongh the sand of the valley, which it consolidates into bench sandstone vdiere it
meets the sea-water. At I the sandstoue has a smooth surface, a t B Irolten and
apparently undergoing reduction by the sen. This sandstone is found in all tlie
valley moofhs in this bay.
Figs. 6 & 6. Part of a large heap of olil pearl-shells which forms a considerable part of the
island of Uiiiin es Shelih, the Holy Island of Iihor Dongouab. Presuinably these
are the reninins of fisheries made in the days when this species was fished only for
its pearls, not as now maiiily for the mother-of-pearl shell. Keediess t o say their
d u e , which would be very p e a t if the hells were fresh, is now nil.
*
Lb
The Building of ;ItolL,'' Proc. Canib. Phil. S O ~190?.
.
I’IATIS2.
Fig. 1. General view of the clitEs eastwards of Alexandria. The rocks in the foreground are
made of rubble conso1id;tted by the sea, the similarly situated reef in the niiddle
distance is natural sandstone.
2 . Hill of rubble at Chatby, Alexandria, remaining after the surrounding land has been
excavated to its original level.
3. Portion of cliff face situated between 2 and 1 abnve. The upper part of looserubble
containing pottery, &c. : the lower of rery soft sandstone.
]’LATE
3.
Fig. 1. Catacomb passages exposed by the sea, in the eanie locality as above. The cliff is
coherent sand below, rubble &c. above, m in P1. 2. fig. 3. Eight niches for bodies
can be made out i n the foreground, to the left of the main passage.
2 . The ‘‘ Spouting Roclr ” beyond Pnlais station, showing the regular shelf surrounding
d
laid bare during the retreat of a
the rock just below water-level ~ n partially
waye.
1’LATI<
4.
Jebel T&tlwib,in Khor Dongonab, a butt near its southern extremity, seen from its south side.
The letter B is a t a level about 25 feet above the foreground.
A. Coral colonies, in position of growth, bedded in a mass of loose coral ddbris,
shells, Szc.
13. Corals bedded in consolidated coral-mud, forming n harder layer than that formed by
either constituent done and 80 projecting shelfwise a t B. This layer passes
gradually into C.
C. Hardened coral-mud.
The weathered surface forms rounded masses in low
relief.
D. Gypsum strata, here steeply tilted, and upturned a t their ends in the piece shown
in the foreground. They are much folded in other parts of the cliff.
12. Green and red shaly rock underlying and sometimes interstratified with the
gypsum. It is here broken down into sand. This rock contains sheets of glasslike recrystallized gypsum.
F. A fallen piece of B. Contrast its roughness mith the smooth rounded surfaces of C
upon which it lies.
(+.Scree material from A 8 B, consisting of coral and shells.
II. A dark line of reddish broken shell-material, containing many Cidaris-spines. This
is more prominent in some other parts of the hill.
~'RCl~.l.\\l!
JOURN.LINN.
SOC.,Z O O L . VoL. XXXl.
(rn,iit,
S C E N E S I N THE RED SEA.
PL.1
l<nzr.
CE0ssl.aXI ) .
JOURN.
LINN.
S o c . , ZOOL.VOL. >(xXI. P L . 2
1
I
I
2
i
I
I
I
i
3
I
1
<I.
Grout, E n s
c'. Photo.
S H O R E CLIFFS NEARtTALEXANDRIA.
C’ROS21.4Sll.
-
JOURN. LINN.SOC.,ZOOL.VOL. XXXI. PL. 3.
I
FIG. I .
C’. C’. Pllottl.
CLIFFS N E A R ALEXANDRIA.
CROSSLAXD.
JOURN.
LINN.
Soc.. ZOOL.VOL. XXXI. PL. 4.
/