The groundwater flow is usually radial from the core of
the cay outward toward the beach (Vacher and Quinn,
1997). In the intertidal beach zone, interstitial seawater
evaporates and calcium carbonate in the form of aragonite
is deposited in the interparticulate pores of the beach sediments to form beachrock (Stoddart and Cann, 1965).
Also, where large colonies of sea birds are resident over
long periods their guano can cement and/or replace the
cay sediments to produce phosphate rock or cay rock.
Serial changes in cay vegetation have been reported
(Flood and Heatwole, 1986) as well as changes in their
shape related to climatic fluctuations (Flood, 1986) and
the impact of tropical cyclones (Flood and Jell, 1977;
Scoffin, 1993; Verstappen, 1954; Woodroffe, 1993).
There is considerable anxiety being expressed by those
Indian Ocean and Pacific Island nation people who live
on the low lying coral cays. They are concerned about
the predicted climate changes and sea level rise
(Woodroffe et al., 1990). Any sea level change will impact
on the sediment source and the supply of sediment to and
from the cay (Kench et al., 2005). Predicting the present
and future stability is a challenging task for geomorphologists and engineers (Dickinson, 2001; 2009). Examples
have been reported where the residual beach rock outcrops
indicate the earlier presence of cays which were destroyed
during hurricane/cyclone events.
The dates of Holocene sea level maximum for selected
oceanic islands have been summarized by Nunn (1994)
and Dickinson (2001) who found that in the millennia
since 5,000 BP, no single scenario prevails and the
observed patterns of sea level behavior vary depending
on just where on the Earth the observations are made. This
variability is related to the different properties and behaviors of the lithosphere.
Summary
A cay is a supratidal feature developed on the reef top. It
represents a stage in the evolutionary accumulation of reef
derived detritus commencing as a subtidal back, developing further as an intertidal bank, and then a supratidal
island (unvegetated) to vegetated island (several steps in
the complexity of vegetation).
A cay is the product of the interaction of the geological
processes of sediment production, erosion, and transportation and the hydrodynamic process related to tidal processes and wave action (and cyclonic/typhoon/hurricane
activities). Usually on oval or elongate platform reefs,
the nodal point where the current/wave action energy
decreases and sediment accumulates is situated on the reef
top and not beyond the reef top in the area of the leeward
sediment wedge.
The action of oceanic birds is of paramount importance
in transporting seeds to colonize the surface of the cay; the
dead vegetation adds to the humic deposits thereby
enhancing the possibility of vegetation further colonizing
the surface of the cay. Guano material also enhances the
fertility of the soil profile, and around the beaches of the
cays beach rock, may form in the subsurface. On the larger
cays, a freshwater/brackish water lens can develop and
enhance the vegetative growth from shrubs to extensive
forests of Casuarina and Pisonia trees.
Bibliography
Chivas, A., Chappel, J., Polack, H., Pillans, B., and Flood, P. G., 1986.
Radiocarbon evidence for the timing and rate of island development, beach-rock formation and phosphatization at Lady Elliott
Island, Queensland, Australia. Marine Geology, 69, 273–287.
Dickinson, W. R., 2001. Paleoshoreline record of relative Holocene
sea levels on Pacific Islands. Earth-Science Reviews, 55, 191–234.
Dickinson, W. R., 2009. Pacific atoll living: how long already and
until when? Geological Society of America Today, 19(3), 4–10.
Flood, P. G., 1977. Coral cays of the Capricorn and Bunker groups,
Great Barrier Reef Province, Australia. Atoll Research Bulletin,
195, 1–24.
Flood, P. G., 1980. Cyclone “Simon” changes cays. Reflections
Newsletter of the Great Barrier Reef Marine Park Authority, 6, 4.
Flood, P. G., 1981. Coral cays and cyclones. Beach Conservation,
42, 6.
Flood, P. G., 1986. Sensitivity of coral cays to climate variations,
Southern Great Barrier Reef, Australia. Coral Reefs, 5, 13–18.
Flood, P. G., 1988. Shoreline changes on coral cays, Capricorn Section, Great Barrier Reef Marine Park, Australia. Proceedings
Sixth International Coral Reef Symposium, Australia, Vol. 2,
pp. 219–224.
Flood, P. G., and Heatwole, H., 1986. Coral cay instability and species turnover of plants at Swain Reefs, Southern Great Barrier
Reef, Australia. Journal Coastal Research, 2, 479–496.
Flood, P. G., and Jell, J. S., 1977. The effect of cyclone “David”
(January, 1976) on the sediment distribution patterns on Heron
Reef, Great Barrier Reef, Australia. Proceedings Third International Coral Reef Symposium, Miami, Vol. 2, pp. 119–125.
Gourlay, M. R., 1988. Coral cays: products of wave action and geological processes in a biogenic environment. Proceedings
Sixth International Coral Reef Symposium, Townsville, Vol. 2,
pp. 491–496.
Hopley, D., 1981. Sediment movement around a coral cay, Great
Barrier Reef, Australia. Pacific Geology, 15, 17–36.
Hopley, D., 1982. The Geomorphology of the Great Barrier Reef:
Quaternary Development of Coral Reefs. New York: Wiley.
Hopley, D., 1997. Geology of reef islands of the Great Barrier Reef,
Australia. Developments in Sedimentology, 54, 835–866.
Hopley, D., Smithers, S. G., and Parnell, K., 2007. The Geomorphology of the Great Barrier Reef Development Diversity and
Change. Cambridge: Cambridge University Press.
Kench, P. S., McLean, R. F., and Nichol, S. L., 2005. A new model
of reef island evolution: Maldives, Indian Ocean. Geology, 33,
145–148.
McLean, R. F., and Stoddart, D. R., 1978. Reef island sediments of
the northern Great Barrier Reef. Philosophical Transactions
Royal Society of London, A, 291, 101–117.
Nunn, P. D., 1994. Oceanic Islands. Oxford: Blackwell.
Scoffin, T. P., 1993. The geological effects of hurricanes on coral
reefs and the interpretation of storm deposits. Coral Reefs, 12,
203–221.
Stoddart, D. R., 1965. British Honduras cays and the low wooded
island problem. Transactions of the Institute of British Geographers, 36, 131–147.
Stoddart, D. R., 1969. Post-hurricane changes on the British Honduras reefs and cays: re-survey of 1965. Atoll Research Bulletin,
131, 1–25.
Stoddart, D. R., and Cann, J. R., 1965. Nature and origin of beach
rock. Journal of Sedimentary Research, 35(1), 243–247.
192
CAY FORMATION
the cay outward toward the beach (Vacher and Quinn,
1997). In the intertidal beach zone, interstitial seawater
evaporates and calcium carbonate in the form of aragonite
is deposited in the interparticulate pores of the beach sediments to form beachrock (Stoddart and Cann, 1965).
Also, where large colonies of sea birds are resident over
long periods their guano can cement and/or replace the
cay sediments to produce phosphate rock or cay rock.
Serial changes in cay vegetation have been reported
(Flood and Heatwole, 1986) as well as changes in their
shape related to climatic fluctuations (Flood, 1986) and
the impact of tropical cyclones (Flood and Jell, 1977;
Scoffin, 1993; Verstappen, 1954; Woodroffe, 1993).
There is considerable anxiety being expressed by those
Indian Ocean and Pacific Island nation people who live
on the low lying coral cays. They are concerned about
the predicted climate changes and sea level rise
(Woodroffe et al., 1990). Any sea level change will impact
on the sediment source and the supply of sediment to and
from the cay (Kench et al., 2005). Predicting the present
and future stability is a challenging task for geomorphologists and engineers (Dickinson, 2001; 2009). Examples
have been reported where the residual beach rock outcrops
indicate the earlier presence of cays which were destroyed
during hurricane/cyclone events.
The dates of Holocene sea level maximum for selected
oceanic islands have been summarized by Nunn (1994)
and Dickinson (2001) who found that in the millennia
since 5,000 BP, no single scenario prevails and the
observed patterns of sea level behavior vary depending
on just where on the Earth the observations are made. This
variability is related to the different properties and behaviors of the lithosphere.
Summary
A cay is a supratidal feature developed on the reef top. It
represents a stage in the evolutionary accumulation of reef
derived detritus commencing as a subtidal back, developing further as an intertidal bank, and then a supratidal
island (unvegetated) to vegetated island (several steps in
the complexity of vegetation).
A cay is the product of the interaction of the geological
processes of sediment production, erosion, and transportation and the hydrodynamic process related to tidal processes and wave action (and cyclonic/typhoon/hurricane
activities). Usually on oval or elongate platform reefs,
the nodal point where the current/wave action energy
decreases and sediment accumulates is situated on the reef
top and not beyond the reef top in the area of the leeward
sediment wedge.
The action of oceanic birds is of paramount importance
in transporting seeds to colonize the surface of the cay; the
dead vegetation adds to the humic deposits thereby
enhancing the possibility of vegetation further colonizing
the surface of the cay. Guano material also enhances the
fertility of the soil profile, and around the beaches of the
cays beach rock, may form in the subsurface. On the larger
cays, a freshwater/brackish water lens can develop and
enhance the vegetative growth from shrubs to extensive
forests of Casuarina and Pisonia trees.
Bibliography
Chivas, A., Chappel, J., Polack, H., Pillans, B., and Flood, P. G., 1986.
Radiocarbon evidence for the timing and rate of island development, beach-rock formation and phosphatization at Lady Elliott
Island, Queensland, Australia. Marine Geology, 69, 273–287.
Dickinson, W. R., 2001. Paleoshoreline record of relative Holocene
sea levels on Pacific Islands. Earth-Science Reviews, 55, 191–234.
Dickinson, W. R., 2009. Pacific atoll living: how long already and
until when? Geological Society of America Today, 19(3), 4–10.
Flood, P. G., 1977. Coral cays of the Capricorn and Bunker groups,
Great Barrier Reef Province, Australia. Atoll Research Bulletin,
195, 1–24.
Flood, P. G., 1980. Cyclone “Simon” changes cays. Reflections
Newsletter of the Great Barrier Reef Marine Park Authority, 6, 4.
Flood, P. G., 1981. Coral cays and cyclones. Beach Conservation,
42, 6.
Flood, P. G., 1986. Sensitivity of coral cays to climate variations,
Southern Great Barrier Reef, Australia. Coral Reefs, 5, 13–18.
Flood, P. G., 1988. Shoreline changes on coral cays, Capricorn Section, Great Barrier Reef Marine Park, Australia. Proceedings
Sixth International Coral Reef Symposium, Australia, Vol. 2,
pp. 219–224.
Flood, P. G., and Heatwole, H., 1986. Coral cay instability and species turnover of plants at Swain Reefs, Southern Great Barrier
Reef, Australia. Journal Coastal Research, 2, 479–496.
Flood, P. G., and Jell, J. S., 1977. The effect of cyclone “David”
(January, 1976) on the sediment distribution patterns on Heron
Reef, Great Barrier Reef, Australia. Proceedings Third International Coral Reef Symposium, Miami, Vol. 2, pp. 119–125.
Gourlay, M. R., 1988. Coral cays: products of wave action and geological processes in a biogenic environment. Proceedings
Sixth International Coral Reef Symposium, Townsville, Vol. 2,
pp. 491–496.
Hopley, D., 1981. Sediment movement around a coral cay, Great
Barrier Reef, Australia. Pacific Geology, 15, 17–36.
Hopley, D., 1982. The Geomorphology of the Great Barrier Reef:
Quaternary Development of Coral Reefs. New York: Wiley.
Hopley, D., 1997. Geology of reef islands of the Great Barrier Reef,
Australia. Developments in Sedimentology, 54, 835–866.
Hopley, D., Smithers, S. G., and Parnell, K., 2007. The Geomorphology of the Great Barrier Reef Development Diversity and
Change. Cambridge: Cambridge University Press.
Kench, P. S., McLean, R. F., and Nichol, S. L., 2005. A new model
of reef island evolution: Maldives, Indian Ocean. Geology, 33,
145–148.
McLean, R. F., and Stoddart, D. R., 1978. Reef island sediments of
the northern Great Barrier Reef. Philosophical Transactions
Royal Society of London, A, 291, 101–117.
Nunn, P. D., 1994. Oceanic Islands. Oxford: Blackwell.
Scoffin, T. P., 1993. The geological effects of hurricanes on coral
reefs and the interpretation of storm deposits. Coral Reefs, 12,
203–221.
Stoddart, D. R., 1965. British Honduras cays and the low wooded
island problem. Transactions of the Institute of British Geographers, 36, 131–147.
Stoddart, D. R., 1969. Post-hurricane changes on the British Honduras reefs and cays: re-survey of 1965. Atoll Research Bulletin,
131, 1–25.
Stoddart, D. R., and Cann, J. R., 1965. Nature and origin of beach
rock. Journal of Sedimentary Research, 35(1), 243–247.
192
CAY FORMATION