sea levels modify reef flat hydrodynamics. In the atolls of
the Caribbean, these mixed islands typically comprise
a windward shingle ridge and leeward sand area. Lime
and Hunting Cays are examples (Stoddart et al., 1982).
Reef islands deposited on atoll rims generally fall
within this class, although the proportions of sand and
shingle vary. On open-ocean atolls (see Atolls) in very
low energy areas, e.g., close to the equator, islands may
be predominantly sand but at latitudes where cyclones
are more regularly experienced they tend to be dominated
by shingle and boulders (see Tropical Cyclone/
Hurricane). These linear shingle islands are generally
larger than those found for example on barrier reef systems and are thus able to maintain a freshwater lens and
diverse vegetation. Many of these islands have been home
to Polynesian peoples for centuries or millennia and are
commonly referred to as motu [Figure 3d; see Atoll
Islands (Motu)].
Vegetated shingle cays
Vegetated shingle cays are uncommon. One Tree Island,
Lady Elliot Island and East Hoskyn and Fairfax Islands
(Figures 2d and 3e), all located at the southern end of the
GBR, are among the few cays of this type to have been scientifically analyzed. However, Tomascik et al. (1997) suggest that they are probably common on the exposed parts
of the Great Sunda Barrier in Indonesia. These cays typically develop near the windward margins of larger reef
flats or centrally on smaller ones exposed to high energy.
Most vegetated shingle cays are compact in form as linear
shingle cays are usually either too narrow to retain adequate freshwater or too mobile for vegetation to endure.
Where vegetation survives, the greater stability of the
shingle cay allows vegetation succession to proceed until
cyclonic disturbance occurs (Hopley, 1982). Observations
from the atoll of Ontong-Java suggest that in areas of episodic high-energy storms and sufficient sediment supply,
these cays may go through cycles of destruction, reformation, and revegetation (Bayliss-Smith, 1988). Stoddart
et al. (1982) noted the mobility of these cays in the Caribbean, where substantial changes were recorded at North
Spot and Rugged Cays between 1960 and 1972.
Hopley (1982) suggested that shingle cays developed
from shingle ramparts, with the “tongues” of shingle and
rubble that commonly trail leeward from ramparts also
possibly involved; One Tree Island in the southern GBR
has formed this way. The near concentric shingle ridges
at Lady Elliot Island similarly document its formation
over several millennia by the progradation of shingle
ridges deposited during episodic storms (Chivas et al.,
1986). However, as noted above, evidence from various
locations suggests that storms may also destroy these
features.
Mangrove islands
In low energy areas, mangroves may colonize reef flats
(Figure 2e) and encourage sediment accumulation and
island formation. Sediments deposited beneath the
mangroves vary from peats to carbonate mud. Mangrove
islands are rare on the GBR, but are more common in
Torres Strait (Hopley, 1997) where despite a relatively
high-tide range mangroves more frequently develop in
sheltered areas behind shingle ramparts. Similarly, reef
top mangrove islands are reported from various locations
in Indonesia without rampart protection but where lowenergy conditions prevail due to proximity to the equator
and associated benign wind regime [e.g., Pulu Panjang
in the Berau Islands (Tomascik et al., 1997)]. They are
common on reefs lacking windward ramparts in the Caribbean (Stoddart and Steers, 1977), but are generally
restricted to high reef tops, areas of low energy, and areas
of relatively low tidal range. Mangroves are common on
the reefs of Belize and are associated with different types
of island (Stoddart et al., 1982). “Mangrove cays” are
mud mounds with a simple covering of mangroves: “Mangrove cays with dry sand areas” have featureless low lying
sand areas within the mangroves; “Moat Islands resemble
the low wooded islands of the GBR; and “Mangrove
range” are complex and extensive arrays of mangroves
with intermittent sand ridges especially on the windward
side (see also Woodroffe, 1995). Mangroves are also common in sheltered locations within the three offshore atolls
(e.g., Murray et al., 1999). In Florida Bay, mangrove
islands began to accrete as the shallow Miami Limestone
was flooded in the late transgression, with various models
proposed to explain the location and processes of formation. Recent geochronological and lithological investigations suggest net accumulation punctuated by erosion
episodes over the past few thousand years as the rate of
relative sea-level rise slowed to its present position (Oches
et al., 2009) (see Mangrove Islands).
Multiple islands
Multiple vegetated islands on a single reef are rare on the
GBR; there are just two long-standing examples – Fairfax
(Figure 3e) and Hoskyn Islands in the Bunker Group.
Each includes both a vegetated shingle and vegetated sand
cay on a single reef platform, shingle cay to windward,
and sand cay to leeward (Figure 2d). On large reefs subject
to seasonal reversal of winds but only rare storms, sand
cays may develop at opposite ends of the platform; this
appears to be the case at Masig-Kodall (Yorke Island) in
Torres Strait (9
45
0 S) where seasonally reversed waves
interacting with an unusually shaped reef produces two
depositional nodes and islands. The same process is probably responsible for the occurrence of multiple islands in
the Indonesian Archipelago, where according to Tomascik
et al. (1997, p. 819) they are “not a rare occurrence.”
Low wooded islands
The basic pattern of a low wooded island comprises
a windward shingle cay, a leeward sand cay, and significant mangrove development over the intervening reef
top (Hopley, 1997; Figures 2f and 3f; see Low Wooded
Islands). This combination of features may occupy
CORAL CAY CLASSIFICATION AND EVOLUTION
243
the Caribbean, these mixed islands typically comprise
a windward shingle ridge and leeward sand area. Lime
and Hunting Cays are examples (Stoddart et al., 1982).
Reef islands deposited on atoll rims generally fall
within this class, although the proportions of sand and
shingle vary. On open-ocean atolls (see Atolls) in very
low energy areas, e.g., close to the equator, islands may
be predominantly sand but at latitudes where cyclones
are more regularly experienced they tend to be dominated
by shingle and boulders (see Tropical Cyclone/
Hurricane). These linear shingle islands are generally
larger than those found for example on barrier reef systems and are thus able to maintain a freshwater lens and
diverse vegetation. Many of these islands have been home
to Polynesian peoples for centuries or millennia and are
commonly referred to as motu [Figure 3d; see Atoll
Islands (Motu)].
Vegetated shingle cays
Vegetated shingle cays are uncommon. One Tree Island,
Lady Elliot Island and East Hoskyn and Fairfax Islands
(Figures 2d and 3e), all located at the southern end of the
GBR, are among the few cays of this type to have been scientifically analyzed. However, Tomascik et al. (1997) suggest that they are probably common on the exposed parts
of the Great Sunda Barrier in Indonesia. These cays typically develop near the windward margins of larger reef
flats or centrally on smaller ones exposed to high energy.
Most vegetated shingle cays are compact in form as linear
shingle cays are usually either too narrow to retain adequate freshwater or too mobile for vegetation to endure.
Where vegetation survives, the greater stability of the
shingle cay allows vegetation succession to proceed until
cyclonic disturbance occurs (Hopley, 1982). Observations
from the atoll of Ontong-Java suggest that in areas of episodic high-energy storms and sufficient sediment supply,
these cays may go through cycles of destruction, reformation, and revegetation (Bayliss-Smith, 1988). Stoddart
et al. (1982) noted the mobility of these cays in the Caribbean, where substantial changes were recorded at North
Spot and Rugged Cays between 1960 and 1972.
Hopley (1982) suggested that shingle cays developed
from shingle ramparts, with the “tongues” of shingle and
rubble that commonly trail leeward from ramparts also
possibly involved; One Tree Island in the southern GBR
has formed this way. The near concentric shingle ridges
at Lady Elliot Island similarly document its formation
over several millennia by the progradation of shingle
ridges deposited during episodic storms (Chivas et al.,
1986). However, as noted above, evidence from various
locations suggests that storms may also destroy these
features.
Mangrove islands
In low energy areas, mangroves may colonize reef flats
(Figure 2e) and encourage sediment accumulation and
island formation. Sediments deposited beneath the
mangroves vary from peats to carbonate mud. Mangrove
islands are rare on the GBR, but are more common in
Torres Strait (Hopley, 1997) where despite a relatively
high-tide range mangroves more frequently develop in
sheltered areas behind shingle ramparts. Similarly, reef
top mangrove islands are reported from various locations
in Indonesia without rampart protection but where lowenergy conditions prevail due to proximity to the equator
and associated benign wind regime [e.g., Pulu Panjang
in the Berau Islands (Tomascik et al., 1997)]. They are
common on reefs lacking windward ramparts in the Caribbean (Stoddart and Steers, 1977), but are generally
restricted to high reef tops, areas of low energy, and areas
of relatively low tidal range. Mangroves are common on
the reefs of Belize and are associated with different types
of island (Stoddart et al., 1982). “Mangrove cays” are
mud mounds with a simple covering of mangroves: “Mangrove cays with dry sand areas” have featureless low lying
sand areas within the mangroves; “Moat Islands resemble
the low wooded islands of the GBR; and “Mangrove
range” are complex and extensive arrays of mangroves
with intermittent sand ridges especially on the windward
side (see also Woodroffe, 1995). Mangroves are also common in sheltered locations within the three offshore atolls
(e.g., Murray et al., 1999). In Florida Bay, mangrove
islands began to accrete as the shallow Miami Limestone
was flooded in the late transgression, with various models
proposed to explain the location and processes of formation. Recent geochronological and lithological investigations suggest net accumulation punctuated by erosion
episodes over the past few thousand years as the rate of
relative sea-level rise slowed to its present position (Oches
et al., 2009) (see Mangrove Islands).
Multiple islands
Multiple vegetated islands on a single reef are rare on the
GBR; there are just two long-standing examples – Fairfax
(Figure 3e) and Hoskyn Islands in the Bunker Group.
Each includes both a vegetated shingle and vegetated sand
cay on a single reef platform, shingle cay to windward,
and sand cay to leeward (Figure 2d). On large reefs subject
to seasonal reversal of winds but only rare storms, sand
cays may develop at opposite ends of the platform; this
appears to be the case at Masig-Kodall (Yorke Island) in
Torres Strait (9
45
0 S) where seasonally reversed waves
interacting with an unusually shaped reef produces two
depositional nodes and islands. The same process is probably responsible for the occurrence of multiple islands in
the Indonesian Archipelago, where according to Tomascik
et al. (1997, p. 819) they are “not a rare occurrence.”
Low wooded islands
The basic pattern of a low wooded island comprises
a windward shingle cay, a leeward sand cay, and significant mangrove development over the intervening reef
top (Hopley, 1997; Figures 2f and 3f; see Low Wooded
Islands). This combination of features may occupy
CORAL CAY CLASSIFICATION AND EVOLUTION
243