The flattening of the crests is evident for both linguoid and
asymmetric wave ripples. The troughs are very narrow
compared to the width of the crests. Because of the greater
flattening of the ripple crests, the crestal lines often get
obliterated; the crestal width increases with
a corresponding decrease of trough width. The mechanism
of formation of flat-topped ripples is attributed to scouring
by tidal currents.
Double-crested ripples
These ripples generally contain double crestal trains with
identical spacing of 5–6 mm. The ripples are typically
asymmetric with almost straight crests which often terminate laterally against the linguoid ripples. These form as
a result of changes in water depth with changing tidal
level. The double-crested ripples are often supposed to
be diagnostic of intertidal flats (Reineck and Singh,
1980; Terwindt, 1988). Klein (1970) explained the mechanism of formation of secondary currents over the primary
ones from estuarine environments.
Ladder-back ripples
These are interference ripples in which two sets of ripples
maintain an oblique to perpendicular relationship with
their crestal trains. Reduction of water depth particularly
during the ebb phase controls the size and orientation of
the current ripples. With the decline of water level, the size
of the current ripples decreases resulting in superimposition of the small ripples over the larger sets. Many different configurations exist for the ladder-back type to
complex network of ripple trains.
Backwash ripples
The backwash ripples occupy the highest topographic
areas and concentrate in the regions of maximum advance
of wave swash in the swash platform of the estuarine environments. These are gentle undulations parallel to river
banks and formed away from the river channel margin.
The bedform is prominent due to variations of color from
the crests to the troughs of the ripples. They are sinuous in
plan and are generally asymmetrical in profile with lee
slope direction toward the river channel. Dark minerals,
mostly biotite, concentrate along the troughs of the ripples, whereas light-colored quartzo-feldspathic minerals
mark the ripple crests. Thus, instead of being marked by
their relief, these ripples are characterized by sinuous,
alternate light and dark color bands.
Swash marks
These are tiny, curved ridges or markings on a sandy
swash platform. The curved ridges with their convexity
landward mark the maximum advance of wave swash.
The ridges are generally of insignificant heights and
exhibit strike-wise continuation for several meters
although with minor breaks at places. The ridges are generally comprised of very fine sand grains. Swash marks
result from the lobate fronts of dying waves during backwash and mark the line of maximum advance of wave
swash. In the estuarine areas, the swash marks document
the limit of the outer bank of the estuaries, and as
a result their alignment is at right angles to the shoreline
alignment. Hence, the swash marks in an estuary occur
almost at right angles to their counterparts on the sea
beach.
Rill marks
Rill marks are dendritic erosional structures on a sandy
swash platform made by a system of small rivulets originating from the flow of a thin layer of water during
a falling water stage. Rill marks are of various forms and
dimensions, and their morphological variations are primarily controlled by local topography, slope of sediment
surface, grain size, and water flow. Rill marks are quite
abundant in sandy platforms of estuarine environments.
Partially conical rill marks
These small bedforms appear in the form of partially
developed conical depressions whose walls are sculptured
by fine rills. The cones are about 15 cm across. Water
drained from the conical rills unites to form larger rill
marks (70–80 cm long) with accumulation tongues downslope. A sudden change in slope of the platform is marked
by a change in the morphological variety of rill marks.
Bifurcating rill marks
These rill marks often exhibit downslope bifurcation and
a sinuous or meandering pattern. The bifurcation is often
quite overt with the last bifurcations opening in the downslope and downcurrent direction. These rill marks are confined to a slope angle ranging from 2
to 3
and extend for
a distance of 3–8 m on sandy platforms.
Branching rill marks
Branching rill marks are composed of small rill systems
bundled together to form a broad channel. They have very
prominent bifurcations that yield a dendritic pattern. The
finer rills unite together downslope and are often confined
to an eroded broad channel whose walls stand 3–4 cm
high from the rill floor, which is also characterized
by coarser lag materials of fragmental shells and mud
pellets. Branching rills occur on a slope angle ranging
from 3
to 6
.
Rhomboid marks
Rhomboid marks are formed on swash platforms in estuarine regions. They are diamond-shaped structures with
their long diagonals aligned at right angles to the longitudinal profile of a river. There are two different sets of
rhomboid marks which appear as superimposed
large-scale and small-scale reticulate patterns on the sandy
surface. The smaller set ranges from 2 to 3 cm along their
longer diagonals and 0.8–1.2 cm along their shorter diagonals. The larger set has longer diagonals about 1 m and
shorter diagonals about 45 cm. Rhomboid marks have
positive relief of a few mm to less than 1 cm from the normal sediment surface. Rhomboid marks originate from
570
SEDIMENTARY STRUCTURES
asymmetric wave ripples. The troughs are very narrow
compared to the width of the crests. Because of the greater
flattening of the ripple crests, the crestal lines often get
obliterated; the crestal width increases with
a corresponding decrease of trough width. The mechanism
of formation of flat-topped ripples is attributed to scouring
by tidal currents.
Double-crested ripples
These ripples generally contain double crestal trains with
identical spacing of 5–6 mm. The ripples are typically
asymmetric with almost straight crests which often terminate laterally against the linguoid ripples. These form as
a result of changes in water depth with changing tidal
level. The double-crested ripples are often supposed to
be diagnostic of intertidal flats (Reineck and Singh,
1980; Terwindt, 1988). Klein (1970) explained the mechanism of formation of secondary currents over the primary
ones from estuarine environments.
Ladder-back ripples
These are interference ripples in which two sets of ripples
maintain an oblique to perpendicular relationship with
their crestal trains. Reduction of water depth particularly
during the ebb phase controls the size and orientation of
the current ripples. With the decline of water level, the size
of the current ripples decreases resulting in superimposition of the small ripples over the larger sets. Many different configurations exist for the ladder-back type to
complex network of ripple trains.
Backwash ripples
The backwash ripples occupy the highest topographic
areas and concentrate in the regions of maximum advance
of wave swash in the swash platform of the estuarine environments. These are gentle undulations parallel to river
banks and formed away from the river channel margin.
The bedform is prominent due to variations of color from
the crests to the troughs of the ripples. They are sinuous in
plan and are generally asymmetrical in profile with lee
slope direction toward the river channel. Dark minerals,
mostly biotite, concentrate along the troughs of the ripples, whereas light-colored quartzo-feldspathic minerals
mark the ripple crests. Thus, instead of being marked by
their relief, these ripples are characterized by sinuous,
alternate light and dark color bands.
Swash marks
These are tiny, curved ridges or markings on a sandy
swash platform. The curved ridges with their convexity
landward mark the maximum advance of wave swash.
The ridges are generally of insignificant heights and
exhibit strike-wise continuation for several meters
although with minor breaks at places. The ridges are generally comprised of very fine sand grains. Swash marks
result from the lobate fronts of dying waves during backwash and mark the line of maximum advance of wave
swash. In the estuarine areas, the swash marks document
the limit of the outer bank of the estuaries, and as
a result their alignment is at right angles to the shoreline
alignment. Hence, the swash marks in an estuary occur
almost at right angles to their counterparts on the sea
beach.
Rill marks
Rill marks are dendritic erosional structures on a sandy
swash platform made by a system of small rivulets originating from the flow of a thin layer of water during
a falling water stage. Rill marks are of various forms and
dimensions, and their morphological variations are primarily controlled by local topography, slope of sediment
surface, grain size, and water flow. Rill marks are quite
abundant in sandy platforms of estuarine environments.
Partially conical rill marks
These small bedforms appear in the form of partially
developed conical depressions whose walls are sculptured
by fine rills. The cones are about 15 cm across. Water
drained from the conical rills unites to form larger rill
marks (70–80 cm long) with accumulation tongues downslope. A sudden change in slope of the platform is marked
by a change in the morphological variety of rill marks.
Bifurcating rill marks
These rill marks often exhibit downslope bifurcation and
a sinuous or meandering pattern. The bifurcation is often
quite overt with the last bifurcations opening in the downslope and downcurrent direction. These rill marks are confined to a slope angle ranging from 2
to 3
and extend for
a distance of 3–8 m on sandy platforms.
Branching rill marks
Branching rill marks are composed of small rill systems
bundled together to form a broad channel. They have very
prominent bifurcations that yield a dendritic pattern. The
finer rills unite together downslope and are often confined
to an eroded broad channel whose walls stand 3–4 cm
high from the rill floor, which is also characterized
by coarser lag materials of fragmental shells and mud
pellets. Branching rills occur on a slope angle ranging
from 3
to 6
.
Rhomboid marks
Rhomboid marks are formed on swash platforms in estuarine regions. They are diamond-shaped structures with
their long diagonals aligned at right angles to the longitudinal profile of a river. There are two different sets of
rhomboid marks which appear as superimposed
large-scale and small-scale reticulate patterns on the sandy
surface. The smaller set ranges from 2 to 3 cm along their
longer diagonals and 0.8–1.2 cm along their shorter diagonals. The larger set has longer diagonals about 1 m and
shorter diagonals about 45 cm. Rhomboid marks have
positive relief of a few mm to less than 1 cm from the normal sediment surface. Rhomboid marks originate from
570
SEDIMENTARY STRUCTURES