Geoforum, Vol. 6, pp. 237-246, 1975. Pergamon Press Ltd. Printed in Great Britain

Coastal Morphology; Southwest Great Abaco Island, Bahamas*

C. N. RAPHAEL, Ypsilantit

Summary: An examination of a portion of Great Abaco Island, Bahamas, reveals that the coastal zone may be subdivided

into two morphological units: barrier and lagoon complexes, and rock benches with boulder ramparts. The sand barriers,

colonized by multistoried vegetation, are stratigraphically thin and are characterized by narrowcoralline beaches and beach

rock. The imbricete boulder ramparts adjacent to deeper water, reveal that, on occasion, high wave energy conditions

occur. It is apparent that hurricanes and tropical storms are significant in modifying coastal southwest Great Abaco.

Introduction km’. The Banks are separated by the deep Northwest

Providence Channel into two distinct archipelagos; the

The Bahama Banks are the most extensive shelf seas in the Little Bahama Bank lies to the north and the Grand

tropical Atlantic. Separated from Florida Ily a deep channel, Bahama Bank to the south (Fig. 1). The isalnds occupy

the shallow platforms extend over an area of some 150,000 13,200 km* or about 7% of the area, are generally confined

1 I 79

I 77 75

-% %

Great Aboco z

c

0 26

Eleuthera c5

24

miles

C. Nicholas RAPHAEL, Professor of Geography, Eastern

Michigan University, Ypsilanti, Mich. 46197, USA.

The writer gratefully acknowledges the assistance of Dr. Eugene

JAWORSKI in the field.

237

Fig. 1

l Location of the Bahama

Banks and the study area.

238 GeoforumlVolume G/Number 31411975

to the margins of the Banks, and resemble very large

atolls. The seascape is one of the finest areas of contem-

porary limestone deposition in the world and in recent

years has attracted considerable interest.

Geologists probing for gas and oil, and investigating the

nature of carbonate deposition have concentrated their

studies on the intertidal and adjacent submarine zones

(e.g. MULTER, 1970; SHINN et a/., 1965).

Traditionally, the geologic history of the Bahama platform

has involved classic rigid earth ideas and conventional

concepts of carbonate deposition. One interpretation for

its origin suggests that the calcareous platform was

deposited on the eroded and dissected North American

craton and it therefore overlies a sialic basement (TALWANI.

et al., 1959). Others have inferred that the islands form a

mega-atoll and could conceivably represent carbonates

deposited on the periphery of volcanic islands which have

subsided beneath the sea (NEWELL, 1955). A third

possibility is that carbonates were deposited on a shallow

sea floor. As sedimentation continued, subsidence of the

ocean basin to great depths occurred. More recently

however, the concepts of sea-floor spreading and plate

tectonics have been used to explain the origin of the

banks. As postulated by Dietz and others, a small mediter-

North America seperated and rotated to the north away

from Africa (DIET& eta/. 1970). This rift was then filled

with elastic sediments derived from the adjacent continents

principally through the process of turbidity currents.

With continued separation of the North American and

African plates since Late-JurassicKretaceous time,

ecological conditions improved for incipient carbonate

deposition. Eventually, the Bahama platform was

separated from North America and Africa, and deposition

and subsidence continued.

Coastal geomorphic studies of the Bahama platform are few

and are restricted to the cays of the Great Bahama Bank. A

their origin.

geomorphological reconnaissance by Doran with its large

scale maps is confined to the extreme southeastern portion

of the archipelago, whereas Lind has limited his work to the

landforms of Cat Island (LIND, 1963, DORAN, 1955).

Both writers after identifying and mapping exposed

marine surfaces considered them as indicators of

a higher than present stillstand of the sea in the past 5000

years. Fieldwork undertaken on the southwest coast of

Great Abaco (Fig. 2) suggests that here many coastal

landforms are the product of hurricanes and tropical storms

and hence do not necessarily require higher stillstands for ranean was created during early to mid-Mesozoic time as

2(

77’25’

l

Fig. 2

Location of the investi-

gated sites and the offshore

topography (U.S. Naval

Oceanographic Office, H.O.

Charts 2499 and 5990).

GeoforumIVolume G/Number 31411975 239

Physical Setting

Tectonically, the Bahama Banks are considered stable; no

major displacements have yet been reported. The absence

of volcanic activity and submarine trenches would seem

to confirm crustal stability. However, as in most active

carbonate areas, subsidence is active. A bore hole on

Andros revealed that the carbonates have a thickness of at

least 5000 m and that these shallow water deposits date

from early Cretaceous time (GOODELL & GARMEN, 1969).

This and other borings clearly imply downwarping since

the upper Mesozoic. Although the dominant geological

theme has been shallow water carbonate accumulation and

subsidence, other slight changes in elevation have been

detected. Doran suggests positive displacements of 8m on

Little lnagua (DORAN, 1955). This conclusion is based on

the presence of coral in growth position at that level.

In southwest Great Abaco a rock bench paralleling the

coast varies vertically as much as o.6m. At Rocky Point

the bedrock is inundated at normal high tide whereas

Fig. 3

A rock surface exposed during

low tide near Rocky Point

Fig. 4

The bench, terminated by a boulder rampart, dips north- westerly towards Rocky Point where it is at about mean sea level. The wave-cut notch is exposed during low tide.

240

southeastward this level calcarenite surface is above high

spring tides (Figs. 3 and 4). Also tension cracks, some of

which have been welded, tend to substantiate some slight

regional unrest. Local uplifts of similarly ‘stable’ areas

of the Pacific have also been reported by NEWELL and

BLOOM (1970). Thus, slight eustatic sea level changes on

this coast may be difficult to determine because of

slight tectonic modifications.

Southwest Great Abaco was selected for this study

because of contrasting submarine and subaerial conditions

Banks in the vicinity of Sandy Point are shallow; many

portions are intertidal exposing elongated sand flats

twice per day. The mean depths here are less than 3m and

thus a favorable harvesting area for conch fishermen who

use 4m poles to harvest these bottom dwellers. From

Rocky Point southeastward the coast is adjacent to the

Northwest Providence Channel where depths of 10m are

rapidly encountered in the rocky offshore area. Swell

waves are normally high (0.6-lm) and break directly on

the rocky scarp which is less than 1 m in elevation. Due

to the character of the submarine bottom, low waves

are normally generated and are associated with

depositional landforms such as at Sandy Point.

Exposed rock coasts adjacent to deeper water such as

Rocky Point, are being attacked by higher waves.

Great Abaco is frequently struck by tropical storms and

hurricanes. Accurate data indicate that between 1871

and 1973 forty-nine tropical cyclones of hurricane

intensity passed within 150 km of its southwest coast

(CRY, 1965). Within this period of about a century, 6

hurricanes and numerous tropical cyclones passed over

southern Great Abaco. Tannehill reports 7 hurricanes

within 150 km of Sandy Point between 1804 and 1871

(TANNEHILL’, 1956). Although pre-nineteenth century

data are sketchy, between 1500 and 1796, 7 storms

of at least tropical cyclone intensity have been

documented for the Bahamas (MILLAS, 1968). The

combined data of Cry and Tannehill and others reveal

that between 1804 and the present, 58 hurricanes passed

near the study area. More than one-half of these

tropical depressions were located between Florida and

Great Abaco for at least 24h.

Although rainfall is characterized by distinct seasonality

(Aw, Koppen), the coastal vegetation is luxuriant and

l The upper story includes sea grape (Coccoloba uvifera), coconut

palm ~CCJCOS sp.l, palmetto LSabalpalmettol and occasionally Australian pine (Casuarina equisetifolia). Near ground level the thickets are dominated by sea oxeye (Borricbia arborescents),

low bushy senna (Cada sp.), the shrub cosmopolitan espancil (Soriana maritima) and dense vines (Smilax sp.).

Geoforum/Volume G/Number 3/4/l 975

diverse. Sand barriers, separating shallow lagoons from

the sea, are colonized by grasses, especially sea oats

(Uniola paniculatal and saltmarsh grass (Distichlis

spicata), and prostrate pioneer plants such as beach

morning glory (lpomeapes-caprae) and sea-purslane

(Sesuvium portulacastrum). Landward, where slash-

burn or plantation activity has occurred, the vegetation is

two-storied. * Distinct vegetation communities similar to

those noted on Great Abaco colonize other tropical coasts

(SAUER, 1962). Even with the passage of hurricanes the

low vegetation adjacent to the coast has a low mortality

rate and little difficulty reestablishing itself in spite

of heavy seas and washover deposits. This is most evident

in the coastal areas where the three fathom contour is

displaced seaward such as between Blackwood and George

Points, and Sandy and Rocky Points. Here a shallow shelf normally protects the beach to within a few meters of

the high water mark.

Coastal Types

The coasts, which reflect the submarine environment, may

be divided into two morphological units: barrier and

lagoon complexes, and rock benches with boulder

ramparts.

The barriers of southwest Abaco are for the most part

composed of biogenetic sand and have maximum elevations

of 3 m (Fig. 5a). Test pits indicated that sedimentary structures were absent, although buried soil profiles were

common. A representative pit on the Sandy Point barrier

revealed 25 cm of medium to coarse sand overlying 40 cm

of fine organic sand. At a depth of approximately 0.6 m eolianite was encountered which became more consolida-

ted with depth. The surficial stands are composed of coral

and shell fragments. However, entire conch shells

(Strombussp. ) do occur suggesting that waves as well as

wind are agents responsible for deposition and modifica-

tion of the barriers.

Where severe coastal erosion has occurred such as at

Sandy Point, exposed eolianite 0.6 m thick outcrops above

the level of spring tides. Coastal distribution of eolianite

is patchy. However, more prominent exposures occur

where coastal erosion of the barrier is evident. Outcrops

which dip 7-9 degrees seaward are bored by terrestrial

organisms, and commonly contain root fragments and

shells, especially Callista eucymata (Dall). Petrographic

and hand lens inspection revealed that the eolianite

fragments are finer, more rounded, and more tightly

packed than beach rock sediments. Cementation of eolianite

is possibly caused by the seepage of rain water charged

GeoforumlVolume G/Number 3/4/1975 241

a.Sandy Point Fresh Sand

b. George Point

I

1

0 60 120 270 330

12 c. Hole in the Wall

g E’6 z

0 1 0 15 30

d. Rocky Point

b 15 30 I5 60 75

e. Blackwood Point

Boulder Rampart

Carbonate Bedrock

Fig. 5

l Selected coastal orofiles

with calcium carbonate through porous dunes (RUSSELL,

1962). Its development is assisted by occasional wetting

of the deposit and is associated with seasonal rainfall. On

Great Abaco water tables fluctuate about 0.3 m with the

oscillating tides.t Such a fluctuation will also cause

periodic wetting of the barrier and probably accounts for

t Dr. Daniel TURNER (Eastern Michigan University) has carefully

recorded the oscillation of water levels in four open wells in

the settlement of Sandy Point. He reports that water level

fluctuations in wells, due to the rise and fall of the tide, average

around 0.3 m. During a heavy rain in February, 1969, the

water levels rose to the surface of the ground causing some

flooding of the village.

242 GeoforumlVolume G/Number 3/4/1975

Fig. 6

. Parallel bands of beach rock exposed during low spring tide at Sandy Point.

the more indurated eolianite with depth. Thus, the alter-

nate wetting and drying may be caused by seasonal

precipitation from above or fluctuation of the water table

below; quite likely both are significant as they are inter-

related.

Beach material which has been indurated by calcareous

cement is referred to as beach rock. Its occurrence is

thought to be controlled by location and temperature of

ground water (RUSSELL & McINTIRE, 1965). Beach rock

observed on Great Abaco is regionally associated with

eolianite and therefore is localized. The outcrops are

composed of coarse coral and shell fragments giving the

appearance of a cemented shell hash. Beach rock is

intertidal ranging from low spring tide to approximately

high spring tide (Fig. 6). The individual bands dip 10-l 1

degrees seaward and look cuesta-like. Between the parallel

bands, intertidal pools are colonized by coral (Parities sp. ),

urchins (Echinoiderms sp. ), rock-boring marine worms

and other organisms.

The geomorphological significance of beach rock is that it

is exposed only on eroding coastlines and the Sandy Point

barrier is no exception (RUSSELL, 1962, p.205; RUSSELL &

MCINTIRE. 1965, p.20, 23). The beaches are narrow and

with high tide and moderately strong onshore winds the

low barrier is undercut. According to local informants at

Sandy Point, 25 m of shoreline has been lost since 1965.

This is substantiated by the erosion of roads leading to the

beaches and dead Casuarina trunks which litter the shore.

Stratigraphically the barrier-lagoon complexes are thin.

Sections in several auger holes and pits indicate that the

barrier is about 3 m thick south of Sandy Point and

overlies a carbonate bedrock (Fig. 5a). The lagoon margins

are fringed with red mangrove (Rhizophora mangle) which

attain heights up to about 6 m. Landward of the red

mangrove to approximately high tide, the lagoon shore is

colonized by black mangrove (Avicennia nitida) 5-6 m

high. The open waters of the lagoon, with the exception

of localized sand, are free from sediment. Drowned sinks

or blue holes and subaerial sinkholes suggest that the hard

carbonate beneath the lagoon and barrier is pre-Recent in

age. Considering the dense vegetation canopy fringing the

lagoon, peat deposits are relatively thin and pinch out at

the mean high tide level. The peat forms a dense reddish

organic mat with occasional living Rhizophora roots.

Cores, 1 m long, revealed 0.6 m of alternating peat and

shelly sand above the bedrock. The barrier and lagoon

occupied by low mangrove (R. mangle) is veneered with

40 cm of dense peat. Beneath the peat the flat carbonate

surface was again encountered. One other estuarine

environment visited northeast of Sandy Point was cored

to a depth of 1 m before the hard carbonate surface was

encountered.

The auger holes and cores indicate that the carbonate

surface located at approximately low tide is the base upon

GeoforumlVolume G/Number 3/4/1975 243

Fig. 7

l An imbricate boulder rampart between Blackwood and Rocky Points. The bench exhibits a whitish-gray zone and is not inundated during high tide.

which the barrier and shallow lagoons are resting. The

elevation, widespread occurrence, and lateral seaward

extent of the carbonate surface suggest that the modern

patch reefs are growing on this surface. The barrier and

the relatively thin peat deposits overlying carbonate rock

suggest these coastal features are probably Recent in age.

On southwest Abaco, a rock bench extends from Rocky

Point to the vicinity of Blackwood Point and has an

average elevation of 0.6 m above mean sea level (Fig. 5d).

At Hole in the Wall another bench surface is encountered

at an elevation of 10 m (Fig. 5~). Inland, both benchesare

terminated by a spectacular boulder rampart composed of

limestone blocks up to 1.5 m across (Fig. 7)*. The bench in

the vicinity of Rocky Point averages 30 m in width and

displays color zonations noted on other benches in the

Bahamas and Florida (MULTER. 1970). An intertidal yellow

zone is characterized by rock pinnacles and intensive

weathering; a black zone inundated by spring tides

illustrates some effects of intertidal erosion; and a smooth

grayish-white zone covered during storms extends to the

base of the boulder rampart. The only unconsolidated

materials are the large boulders. Smaller gravels are welded

onto the flat surface particularly on the intermediate black

zone. A prominent feature are tension cracks which have

been filled by such fragments and cemented. Planation and

solution weathering have left these tension cracks 12-15 cm

above the bench surface.

The erosion occurring on tropical limestone coasts is

complex and involves chemical, biological, and mechanical

processes. The general consensus is that solution weather-

* In the literature it is also referred to as a storm ridge, boulder

ridge, or typhoon rampart.

ing in some form is responsible for notch development (NEWELL, 1964). Notches in the bench observed near Rocky

Point are sporadically distributed and are best developed

in sheltered embayments. At Hole in the Wall, a well-

defined, continuous notch and visor parallels the lee of

that narrow calcareous peninsula suggesting that chemical

and biological agents may indeed be more significant than.

mechanical activity. The level at which active notches occur

also varies. Fairbridge states that the maximum undercut

below the visor occurs at mean sea level whereas Russell

believes notches are cut “between sea level and an

elevation of 1.5 m or more, with visors to about 3 m

higher” (RUSSELL, 1963; FAIRBRIDGE, 1947). In the

investigated areas the notch floor is just below spring tide,

and the visor is partly inundated during high tide (Fig. 4).

The conspicuous absence of well-developed notches in

many localities and the presence of tension cracks indicate

that the visors collapse rapidly. Immediately seaward, in

5-6 m of water, several large tabular boulders were

observed suggesting that this is the case.

Landward of the cliff face, a rampart approximately 2.5 m

high is normally encountered (Fig. 5d). Although occasion-

ally composed of sand or a mixture of sand and blocks,

the principal constituents are blocks of varying size. The

larger blocks are tabular and exhibit an imbricate structure

leaning inland. The average length along the long axis is

aboutl.2 m, however blocks with lengths of 2.5-4 m are

frequently encountered. Boulders composing the higher

elevations of the barrier generally are more spherical and

have diameters of about 0.6 m. The barriers composed of

blocks have contrasting vegetative covers (Fig. 8). Generally

the boulders are covered with a dense coppice of woody

vegetation, especially sea grape (C. uvifera). Others are

244 Geoforum/Volume G/Number 3/4/1975

Fig. 8

The leeward side of a boulder rampart which has washed over and exhibits a tongue- like morphology. To the right and left .of center the ramparts have been vegetated.

Fig. 9

A gravel ridge at Blackwood Point consisting of coral fragments. The coral is derived from patch reefs 3 km offshore.

virtually devoid of vegetation and the blocks are loose and

not compacted. Morphologically, the boulder ramparts

resemble washover fans commonly noted on sand barriers

which have been breached by storm waves. One such

washover was lobate in shape with the tongue of the ram-

part extending 60 m back onto a sand ridge. The rampart

was so fresh that no pitting, staining, or any other type of

weathering was evident. Isolated blocks from this and the

other ramparts were noted 70 m leeward of the barrier

crest. Occasionally large coral heads are incorporated in

the ramparts indicating that patch reefs as well as the cliff

face is a source area for the ramparts.

A barrier profiled at Blackwood Point has characteristics

of the Sandy Point area as well as the Rocky Point area

(Fig. 5e). The offshore zone slopes seaward and is

colonized by living coral approximately 1-2 km out to sea.

A low sand barrier separates a shallow mangrove lagoon

from the platform. Mangrove (R. mangle) is‘attempting to

gain a foothold on the rocky offshore zone as evidenced

by a few scattered prop roots. Sporadic peat deposits occur

suggesting that the mangrove may have been more

extensive in the past. However, occasional high energy

waves discourage extensive development. Between the

high tide strandline and the established sand barrier a

GeoforumlVoturne G/Number 3/4/l 975

rubble ridge composed of coral averaging 25 cm in

diameter and unharvested conch (Strombussp. ) shells

has been deposited (Fig. 9).

Sea Level, Tropical Storms, and the Coast

With the retreat of Pleistocene glaciers at higher latitudes

meltwaters returned to the ocean basins and sea levels

rose. The eustatic rise, which began about 20,000 years

ago, continued until 4000 years ago when a standstill was

reached (BLOOM, 1970, REDFIELD, 1967, COLEMAN &

SMITH 1964). Therefore the present level of the sea is at

its highest since the last interglacial. Some writers, however,

suggest that in the past 4000 years sea level eustatically

deviated 2-3 m from the present level (FAIRBRIDGE, 1961,

BLOCH, 1965). Lind concluded that many Holocene

coastal landforms on Cat Island, including boulder

ramparts and sand barriers are a response to late-

Recent higher than present stand of the sea (LIND,

1969). A seaward decrease in spacing and width of

accretionary ridges in the southeast Bahamas has been cited

as probable evidence of a eustatic sea level drop by Doran

(WRAN, 1955, p.32). The only investigated coastal area

clearly suggesting higher stillstands of the sea is at Hole in

the Wall (Fig. 5~). Rather than uplift or Recent eustatic

change, this bench probably represents a pre-Recent level.

The investigated areas of Great Abaco suggest that the

accretionary landforms are products of present environ-

mental conditions. When the eye of Hurricane Betsy

passed 65 km south of Sandy Point in 1965 the settlement,

which stands 2-3 m above sea level, was inundated with

1 m of water according to local informants. Thus, it is

obvious that the low barrier can be overtopped. Although

not topographically well developed, washover fans do

occur on the Sandy Point barrier. As noted, this barrier

has shells incorporated in it testifying to the fact that high

wave energies do occasionally occur. Washover deposits are

thin and are generally distributed over a short horizontal

distance (Fig. 5a). Test pits and auger holes reveal that the

maximum accumulation occurs on the crest of the ridge

adjacent to the sea. A comparison with post-hurricane

profiles on Mauritius generally reveals similar deposition

(McINTIRE & WALKER, 1964). In fact, beach-ridge

development in Tabasco, Mexico, is attributed to storm

activity (PSUTY, 1967). The surface morphology of the

barriers is related to overtopping by storm waves.

Variation in height between the two beach ridges south of

Sandy Point and contrasting sediment size at Blackwood

Point are possibly due to storm and do not necessarily

indicate sea level changes (Fig. 5a, b). On the average, the

width of washover deposits is about 10 m. However,

245

stringers of fresh sand have penetrated 3-12 m inland and

on occasion reached the lagoon. The interfingering peat

and sand deposits on the lee of a barrier such as at Sandy

Point probably indicate occasional periods of intensive

storm activity followed by calm conditions and normal

mangrove development.

The amount of sedimentation associated with hurricanes

is not always predictable. A comparison of two hurricanes

of comparable size and intensity passing over the

Florida-Bahama area illustrated that supratidal sedimenta-

tion can not be associated with every hurricane (PERKINS

& ENOS, 1968). The washover fan examined at Sandy

Point may have accumulated during one tropical

storm and may not necessarily represent a deposit

increasing in thickness with every storm. Similarly the

thick upper peat on the fringe of the lagoon suggests that

washover sands seldom reach the lagoon. Because no sand

was introduced into the lagoon fringe, does not mean no

storms occurred during the accumulation of the dense peat.

Although the shallow platforms at Blackwood Point are

normally indicative of low energy coastal environment, the

very coarse sediment of the low barrier clearly reveals that

on occasion storms are capable of breaking up the reef

offshore, traversing the platform, and depositing the

rubble l-1.5 m above the high tide line and onto the

backslope of the barrier. These storm-accumulated gravels,

except for their uncemented nature, are similar to those

noted on Pacific atolls where they have been deposited by tropical storms (cuRR~Yeta/. 1970; NEWELL & BLOOM,

1970, PP. 1874-1877; MARAGOS etal., 1973).

Boulder ramparts also favor a storm hypothesis. The

boulders are stacked and display an imbricate structure

indicating that they were shoved rather than tumbled onto

the platform. They are dispersed randomly over the entire

bench and therefore not entirely concentrated at the barrier

front. If Recent sea levels were significantly higher, it

would seem likely that a pre-modern shoreline would have

survived and consisted of reworked or wave-washed sedi-

ments. No evidence of pebbles or small cobbles similar to

the size fraction noted at Blackwood Point were observed.

Tabular boulders noted at the base of the boulder rampart

at different coastal localities exhibited varying degrees of

weathering suggesting that they were deposited at different

times. Perhaps the most striking evidence are boulder

ramparts which have the configuration of washover fans.

The possibility of moving such large boulders can only be

associated with storm conditions.

Although there may have been eustatic sea level changes

in the past few thousand years, many of the landforms of southwest Great Abaco appear to be closely related to

tropical storm activity. Sands in the Sandy and George

246 Geoforum/Volume G/Number 31411975

Point lagoons are either confined to the lee of the barrier

or are absent. The gently seaward slope of the bench and

widespread occurrence of the offshore bedrock strongly

suggest that this feature is a nip-coastal form. The lack of

broad accretionary topography and the presence of thin

washoever fans, narrow beaches, and beach rock at the

investigated sites reveal that coastal erosion is now

occurring. Tropical storms and hurricanes are not normal

events, however, they are a significant phenomena contri-

buting to the coastal morphology of southwest Great

Abaco.

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COLEMAN J.M. and W.G. SMITH (1964): Late recent rise of sea

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