from beach rock on six cays in Belize range from AD 345
to 1435, although Gischler and Lomando (1997) note that
these may not be reliable ages and should only be taken
as an approximation. On the Great Barrier Reef, Chivas
et al. (1986) extracted nine firmly cemented clam shells
(Tridacna) from low intertidal beach rock at Lady Elliot
Island. The beach rock was known to have formed during
the twentieth century, although two of the three nonmodern
clams had conventional radiocarbon ages of 1830 and
6540 yBP, indicating millennial scale reworking of these
well-worn shells.
While still in its infancy, optically stimulated luminescence (OSL) and thermoluminescence (TL) dating on
feldspar and quartz grains from beach rocks in northeastern Brazil has been carried out (Tatumi et al., 2003). This
technique has limited potential in most coral reef areas
where both beach rock components and cements comprise
calcium carbonate.
Morphodynamics and shoreline change
Beach rock formation alters the nature of the shoreline,
and turns what was once a mobile beach into a rocky
shore. In so doing the permeable character of the beach
is changed to an impermeable barrier that precludes swash
infiltration and inhibits seaward groundwater flows. Two
effects of these changes can be noted. First, the impermeable ramp does not reduce swash uprush and backwash,
and may result in run-up reaching higher levels than previously causing overtopping of the beach rock and scour
behind it. Second, the impeded groundwater outflow is
redirected to the sides and base of the outcrop which
together with wave, current, and tidal actions can result
in lateral erosion and undermining of outcrops. Thus, paradoxically, while beach rock can be an effective natural
beach defense, equivalent to a revetment, like many other
shore protection structures, edge effects can result in basal
undercutting and lateral erosion. Evidence of the former is
often expressed in cracks and fractures, sometimes in
attractive tessellated patterns, as well as subsidence of
beach rock bands and slabs. Evidence of the latter can
include landward offset of the waterline and retreat of
the nearby beach. The presence of beach rock also has
a significant ecological impact, as surficial and interstitial
flora and fauna of the mobile beach is replaced by assemblages of benthic organisms on beach rock substrate.
Beach rock, as a consolidated rock, provides an excellent marker of the actual shore position when it formed.
Persistence of the “frozen beach” (Caldas et al., 2006) is
dependent on the sediment budget for that shoreline sector. A positive budget can result in accretion, burying the
outcrop landward of the new shore. A negative budget
can result in beach erosion, isolating the beach rock outcrop to seaward. This latter scenario seems especially
common on reef coasts and reef islands, where one or
more lines of beach rock extend offshore. While outcrops
parallel to the present shore are most common, some
amazing strandline patterns can be found. On atolls,
lagoonward migration of reef islands is a common feature.
In addition to leaving a trail of beach rock to seaward, continued lagoonward migration may ultimately expose
lagoonward dipping beach rock (from the former lagoon
shore) on the seaward beach. There are several examples
of this on modern reef islands, although the best historical
descriptions are from the Maldives (Gardiner, 1903) and
Funafuti atoll (David and Sweet, 1904).
Beach rock as a sea-level indicator
Not only can fossil beach rock be used to provide evidence
of planimetric shoreline change, but because its vertical
range is restricted to between tide marks, its value for
paleo-sea level studies has long been recognized. Hopley
(1986) has provided the most definitive study of beach
rock as a sea-level marker, indicating its pros and cons.
He concludes that it is not particularly reliable because
the exact upper limit of formation is poorly constrained.
Indeed, Kelletat (2006) has argued that the large vertical
extent in some beach rock occurrences in microtidal locations may be ascribed to cementation in the supratidal
zone, although this has been vigorously disputed (Knight,
2007).
Notwithstanding these reservations, relic beach rock is
still seen as an important paleo-sea level marker, although
rarely as the sole indicator. In reefal areas emphasis has
been on detecting mid-late Holocene changes in sea level
and specifically to identify whether or not there has been
a sea-level high stand. Elevations of relic beach rock
have contributed to confirming a sea level higher than present on Cocos (Keeling) Islands, the northern Great Barrier Reef, Cook Islands, French Polynesia, and elsewhere
in the Pacific. Beyond the major reef areas, beach rock
has also been used to develop more continuous sea-level
histories, such as in northeast Brazil (Brazil, Coral Reefs)
where beach rock elevations and AMS dating of mollusk
fragments from 12 samples suggest that sea level was
at –3 m 7000 cal yBP, reached þ1.3 m about 5900 cal
yBP, after which it fell in linear fashion to its present position (Caldas et al., 2006). And, on the Sardinia–Corsica
coast in the Mediterranean numerous beach rock outcrops
have been preserved along the shorelines and at different
depths on the continental shelf down to –29 m. These have
been dated from 9705 to 180 (cal yBP) enabling Lambeck
et al. (2004) to derive a detailed local sea-level history
over the last 10,000 years.
Surface features and beach rock erosion
Beach rock is a striking feature of coral reef coasts and reef
islands. The contrast between fresh light-colored biogenic
sands and darker beach rock outcrops is often quite stark.
Most observers note the inclined, banded nature of intertidal outcrops, their low-relative relief and bare surfaces.
In detail, however, beach rock surfaces are rarely bare,
except where the substrate is being constantly scrubbed
110
BEACH ROCK
to 1435, although Gischler and Lomando (1997) note that
these may not be reliable ages and should only be taken
as an approximation. On the Great Barrier Reef, Chivas
et al. (1986) extracted nine firmly cemented clam shells
(Tridacna) from low intertidal beach rock at Lady Elliot
Island. The beach rock was known to have formed during
the twentieth century, although two of the three nonmodern
clams had conventional radiocarbon ages of 1830 and
6540 yBP, indicating millennial scale reworking of these
well-worn shells.
While still in its infancy, optically stimulated luminescence (OSL) and thermoluminescence (TL) dating on
feldspar and quartz grains from beach rocks in northeastern Brazil has been carried out (Tatumi et al., 2003). This
technique has limited potential in most coral reef areas
where both beach rock components and cements comprise
calcium carbonate.
Morphodynamics and shoreline change
Beach rock formation alters the nature of the shoreline,
and turns what was once a mobile beach into a rocky
shore. In so doing the permeable character of the beach
is changed to an impermeable barrier that precludes swash
infiltration and inhibits seaward groundwater flows. Two
effects of these changes can be noted. First, the impermeable ramp does not reduce swash uprush and backwash,
and may result in run-up reaching higher levels than previously causing overtopping of the beach rock and scour
behind it. Second, the impeded groundwater outflow is
redirected to the sides and base of the outcrop which
together with wave, current, and tidal actions can result
in lateral erosion and undermining of outcrops. Thus, paradoxically, while beach rock can be an effective natural
beach defense, equivalent to a revetment, like many other
shore protection structures, edge effects can result in basal
undercutting and lateral erosion. Evidence of the former is
often expressed in cracks and fractures, sometimes in
attractive tessellated patterns, as well as subsidence of
beach rock bands and slabs. Evidence of the latter can
include landward offset of the waterline and retreat of
the nearby beach. The presence of beach rock also has
a significant ecological impact, as surficial and interstitial
flora and fauna of the mobile beach is replaced by assemblages of benthic organisms on beach rock substrate.
Beach rock, as a consolidated rock, provides an excellent marker of the actual shore position when it formed.
Persistence of the “frozen beach” (Caldas et al., 2006) is
dependent on the sediment budget for that shoreline sector. A positive budget can result in accretion, burying the
outcrop landward of the new shore. A negative budget
can result in beach erosion, isolating the beach rock outcrop to seaward. This latter scenario seems especially
common on reef coasts and reef islands, where one or
more lines of beach rock extend offshore. While outcrops
parallel to the present shore are most common, some
amazing strandline patterns can be found. On atolls,
lagoonward migration of reef islands is a common feature.
In addition to leaving a trail of beach rock to seaward, continued lagoonward migration may ultimately expose
lagoonward dipping beach rock (from the former lagoon
shore) on the seaward beach. There are several examples
of this on modern reef islands, although the best historical
descriptions are from the Maldives (Gardiner, 1903) and
Funafuti atoll (David and Sweet, 1904).
Beach rock as a sea-level indicator
Not only can fossil beach rock be used to provide evidence
of planimetric shoreline change, but because its vertical
range is restricted to between tide marks, its value for
paleo-sea level studies has long been recognized. Hopley
(1986) has provided the most definitive study of beach
rock as a sea-level marker, indicating its pros and cons.
He concludes that it is not particularly reliable because
the exact upper limit of formation is poorly constrained.
Indeed, Kelletat (2006) has argued that the large vertical
extent in some beach rock occurrences in microtidal locations may be ascribed to cementation in the supratidal
zone, although this has been vigorously disputed (Knight,
2007).
Notwithstanding these reservations, relic beach rock is
still seen as an important paleo-sea level marker, although
rarely as the sole indicator. In reefal areas emphasis has
been on detecting mid-late Holocene changes in sea level
and specifically to identify whether or not there has been
a sea-level high stand. Elevations of relic beach rock
have contributed to confirming a sea level higher than present on Cocos (Keeling) Islands, the northern Great Barrier Reef, Cook Islands, French Polynesia, and elsewhere
in the Pacific. Beyond the major reef areas, beach rock
has also been used to develop more continuous sea-level
histories, such as in northeast Brazil (Brazil, Coral Reefs)
where beach rock elevations and AMS dating of mollusk
fragments from 12 samples suggest that sea level was
at –3 m 7000 cal yBP, reached þ1.3 m about 5900 cal
yBP, after which it fell in linear fashion to its present position (Caldas et al., 2006). And, on the Sardinia–Corsica
coast in the Mediterranean numerous beach rock outcrops
have been preserved along the shorelines and at different
depths on the continental shelf down to –29 m. These have
been dated from 9705 to 180 (cal yBP) enabling Lambeck
et al. (2004) to derive a detailed local sea-level history
over the last 10,000 years.
Surface features and beach rock erosion
Beach rock is a striking feature of coral reef coasts and reef
islands. The contrast between fresh light-colored biogenic
sands and darker beach rock outcrops is often quite stark.
Most observers note the inclined, banded nature of intertidal outcrops, their low-relative relief and bare surfaces.
In detail, however, beach rock surfaces are rarely bare,
except where the substrate is being constantly scrubbed
110
BEACH ROCK