the Pleistocene, then intertidal erosion would require 0.5–
1.0 million years to plane a reef 1 km in width, well in
excess of the time available in each glacial–interglacial
cycle. Furthermore, shoreline retreat is an episodic rather
than a continuous process in consolidated reef limestones,
proceeding by alternating phases of intertidal notch cutting
and then collapse. Next, intertidal backwearing is only part
of the process of limestone removal, the other main process
being solutional lowering of the exposed surface. On inland
limestone substrates on Grand Cayman, mean subaerial erosion has been measured at 0.37 mm a
À1
, with subsoil and
submangrove rates of 0.16 mm a
À1 and 0.21 mm a
À1
,
respectively. Thus if reefs were emergent by 125 m at the
glacial maximum, then it would take over 300,000 years
to remove such a column of limestone at the highest of these
mean rates of lowering. Finally, the heterogeneity of lithification in reef limestones is reflected in the fact that erosion
is extremely localized and gives rise to a pitted and cavernous microtopography rather than undifferentiated planation;
thus, erosion is often accompanied by the preservation of
original depositional reefal surfaces.
Alternative explanations for the morphology of
modern reef surfaces
It is clear from modern process studies that once reefs are
formed they are difficult to remove by marine and subaerial
erosion processes. In addition, the refining of the magnitudes and duration of ocean temperature change and sealevel fall following the late twentieth-century advances in
environmental reconstruction do not provide the contexts
within which wholesale reef removal can be realistically
achieved. Given that present sea levels have been at or near
to their present levels for at most the last 6,000 years and in
some cases for only the last 1,000 years (only 0.6–0.1% of
the last glacial–interglacial cycle), these conclusions
suggest that the topography of modern reefs is often
inherited from underlying, older reef surfaces (see also
Chapter Antecedent Platforms). On the basis of bathymetric
charts, Daly argued for the smoothness of these reef basements. Davis (1928) argued that such smoothness was the
product of effective lagoon infilling during the upgrowth
of marginal reefs. However, more recent detailed lagoon
and shelf floor mapping, both of bottom morphology and,
using seismic techniques, of bedrock surfaces below reefal
sediments, has shown many basements to be characterized
by highly irregular bedrock surfaces (Bloom, 1974), often
organized into characteristic limestone erosional, or karstic,
forms. Topographic features range in scale from small
solutional features (e.g., solutional pipes in the Florida
Keys; Dodd and Siemers, 1971) to marginal raised rims
around saucer-shaped reef platforms (MacNeil, 1954; Harvey, 1977), “knoll and basin” topography in atoll lagoons
(e.g., Chuuk lagoon, Federated States of Micronesia; Shepard, 1970), and “blue holes” in back-reef environments
(e.g., Lighthouse Reef blue hole, Belize Barrier Reef;
Stoddart, 1962), to large scale regional karst landscapes.
The most comprehensive exposition of karstic control on
reef landscapes was provided by E. G. Purdy who, on the
basis of seismic profiling and shallow drilling, argued that
the regional morphology of the Belize Barrier Reef reflects
a drowned doline karst in the drier northern barrier reef and
on the central and southern barrier reef and a drowned tower
karst in the lagoon and a flooded basement cockpit karst on
the barrier reef margin itself. These latter two morphologies
have resulted from the submergence of a karst marginal
plain, supplied by solutionally aggressive drainage waters
from the noncarbonate Maya Mountains. The underlying
karstic control is clear on the Belize Barrier Reef because
of the interaction of the depth below present sea level of
the basement and the sea-level history at this location;
Holocene reef development has been dominated by vertical,
“catch-up” behavior and thus postglacial reef growth has
exacerbated the underlying pre-Holocene topography
(Purdy, 1974).
Elsewhere, the relations between basement depth, sealevel history, and postglacial reef growth modes (give-up
vs. catch-up vs. keep-up) have been more complicated
and given rise to a wider range of contemporary reef
forms; in cases where the basement has been close to present sea level, reef growth may have completely filled the
accommodation space and thus obscured any antecedent
topography (Hopley, 1982). In some locations, the underlying antecedent topography is not a karst erosion surface
but a depositional one and thus reef topography reflects
colonization of stabilized carbonate shoal or reef top storm
deposits (e.g., Garrett and Scoffin, 1977) or reef growth on
underlying river levee (Choi and Ginsburg, 1982) or deltaic deposits (e.g., Maxwell, 1970).
Conclusions
R. A. Daly’s “Glacial Control Hypothesis,” developed in
the first quarter of the twentieth century, importantly
engaged coral reef science with the question as to how
reefs had reacted to Pleistocene changes of ocean temperature and sea level. Developments in Quaternary Science
and in process geomorphology have, however, not
supported Daly’s assumptions about the rapidity at which
volcanic materials and coral reef limestones can be
removed by erosion and thus the ease with which reefal
surfaces can be planed to smooth lagoon floors and reefal
shelves during the glacial phases of the Pleistocene.
Rather modern reef topographies often reflect the complex
underlying topography of much older, pre-Holocene surfaces and the erosional and depositional process histories
to which they have been subjected. Antecedent topographic control is, however, variable and relates to the
degree to which subsequent reef growth has either accentuated or obliterated the basement surfaces from which
modern reefs have developed.
Bibliography
Bloom, A. L., 1974. Geomorphology of reef complexes. In Laporte,
L. F. (ed.), Reefs in Time and Space. Tulsa, OK: Society of
Economic Paleontologists and Mineralogists. Special publication, 18, pp. 1–8.
GLACIAL CONTROL HYPOTHESIS
489
1.0 million years to plane a reef 1 km in width, well in
excess of the time available in each glacial–interglacial
cycle. Furthermore, shoreline retreat is an episodic rather
than a continuous process in consolidated reef limestones,
proceeding by alternating phases of intertidal notch cutting
and then collapse. Next, intertidal backwearing is only part
of the process of limestone removal, the other main process
being solutional lowering of the exposed surface. On inland
limestone substrates on Grand Cayman, mean subaerial erosion has been measured at 0.37 mm a
À1
, with subsoil and
submangrove rates of 0.16 mm a
À1 and 0.21 mm a
À1
,
respectively. Thus if reefs were emergent by 125 m at the
glacial maximum, then it would take over 300,000 years
to remove such a column of limestone at the highest of these
mean rates of lowering. Finally, the heterogeneity of lithification in reef limestones is reflected in the fact that erosion
is extremely localized and gives rise to a pitted and cavernous microtopography rather than undifferentiated planation;
thus, erosion is often accompanied by the preservation of
original depositional reefal surfaces.
Alternative explanations for the morphology of
modern reef surfaces
It is clear from modern process studies that once reefs are
formed they are difficult to remove by marine and subaerial
erosion processes. In addition, the refining of the magnitudes and duration of ocean temperature change and sealevel fall following the late twentieth-century advances in
environmental reconstruction do not provide the contexts
within which wholesale reef removal can be realistically
achieved. Given that present sea levels have been at or near
to their present levels for at most the last 6,000 years and in
some cases for only the last 1,000 years (only 0.6–0.1% of
the last glacial–interglacial cycle), these conclusions
suggest that the topography of modern reefs is often
inherited from underlying, older reef surfaces (see also
Chapter Antecedent Platforms). On the basis of bathymetric
charts, Daly argued for the smoothness of these reef basements. Davis (1928) argued that such smoothness was the
product of effective lagoon infilling during the upgrowth
of marginal reefs. However, more recent detailed lagoon
and shelf floor mapping, both of bottom morphology and,
using seismic techniques, of bedrock surfaces below reefal
sediments, has shown many basements to be characterized
by highly irregular bedrock surfaces (Bloom, 1974), often
organized into characteristic limestone erosional, or karstic,
forms. Topographic features range in scale from small
solutional features (e.g., solutional pipes in the Florida
Keys; Dodd and Siemers, 1971) to marginal raised rims
around saucer-shaped reef platforms (MacNeil, 1954; Harvey, 1977), “knoll and basin” topography in atoll lagoons
(e.g., Chuuk lagoon, Federated States of Micronesia; Shepard, 1970), and “blue holes” in back-reef environments
(e.g., Lighthouse Reef blue hole, Belize Barrier Reef;
Stoddart, 1962), to large scale regional karst landscapes.
The most comprehensive exposition of karstic control on
reef landscapes was provided by E. G. Purdy who, on the
basis of seismic profiling and shallow drilling, argued that
the regional morphology of the Belize Barrier Reef reflects
a drowned doline karst in the drier northern barrier reef and
on the central and southern barrier reef and a drowned tower
karst in the lagoon and a flooded basement cockpit karst on
the barrier reef margin itself. These latter two morphologies
have resulted from the submergence of a karst marginal
plain, supplied by solutionally aggressive drainage waters
from the noncarbonate Maya Mountains. The underlying
karstic control is clear on the Belize Barrier Reef because
of the interaction of the depth below present sea level of
the basement and the sea-level history at this location;
Holocene reef development has been dominated by vertical,
“catch-up” behavior and thus postglacial reef growth has
exacerbated the underlying pre-Holocene topography
(Purdy, 1974).
Elsewhere, the relations between basement depth, sealevel history, and postglacial reef growth modes (give-up
vs. catch-up vs. keep-up) have been more complicated
and given rise to a wider range of contemporary reef
forms; in cases where the basement has been close to present sea level, reef growth may have completely filled the
accommodation space and thus obscured any antecedent
topography (Hopley, 1982). In some locations, the underlying antecedent topography is not a karst erosion surface
but a depositional one and thus reef topography reflects
colonization of stabilized carbonate shoal or reef top storm
deposits (e.g., Garrett and Scoffin, 1977) or reef growth on
underlying river levee (Choi and Ginsburg, 1982) or deltaic deposits (e.g., Maxwell, 1970).
Conclusions
R. A. Daly’s “Glacial Control Hypothesis,” developed in
the first quarter of the twentieth century, importantly
engaged coral reef science with the question as to how
reefs had reacted to Pleistocene changes of ocean temperature and sea level. Developments in Quaternary Science
and in process geomorphology have, however, not
supported Daly’s assumptions about the rapidity at which
volcanic materials and coral reef limestones can be
removed by erosion and thus the ease with which reefal
surfaces can be planed to smooth lagoon floors and reefal
shelves during the glacial phases of the Pleistocene.
Rather modern reef topographies often reflect the complex
underlying topography of much older, pre-Holocene surfaces and the erosional and depositional process histories
to which they have been subjected. Antecedent topographic control is, however, variable and relates to the
degree to which subsequent reef growth has either accentuated or obliterated the basement surfaces from which
modern reefs have developed.
Bibliography
Bloom, A. L., 1974. Geomorphology of reef complexes. In Laporte,
L. F. (ed.), Reefs in Time and Space. Tulsa, OK: Society of
Economic Paleontologists and Mineralogists. Special publication, 18, pp. 1–8.
GLACIAL CONTROL HYPOTHESIS
489