In geology, a terrace is a step-like landform.
A terrace consists of a flat or gently sloping
geomorphic surface, called a tread, that is
typically bounded one side by a steeper ascending
slope, which is called a "riser" or "scarp."
The tread and the steeper descending slope
together constitute the terrace.
Terraces can also consist of a tread bounded
on all sides by a descending riser or scarp.
A narrow terrace is often called a bench.
The sediments underlying the tread and riser
of a terrace are also commonly, but incorrectly,
called terraces, leading to confusion.
Terraces are formed in various ways.
Fluvial terraces
Fluvial terraces are remnants of the former
floodplain of a stream of river.
They are formed by the downcutting of a river
or stream channel into and the abandonment
and lateral erosion of its former floodplain.
The downcutting, abandonment, and lateral
erosion of a former floodplain can be the
result of either changes in sea level, local
or regional tectonic uplift; changes in local
or regional climate; changes the amount of
sediment being carried by the river or stream;
change in discharge of the river; or a complex
mixture of these and other factors.
The most common sources of the variations
in rivers and streams that create fluvial
terraces are vegetative, geomorphic, and hydrologic
responses to climate.
More recently, the direct modification of
rivers and streams and their watersheds by
cultural processes have result in the development
of terraces along many rivers and streams.
Kame terraces
Kame terraces are formed on the side of a
glacial valley and are the deposits of meltwater
streams flowing between the ice and the adjacent
valley side.
Marine terraces
A marine terrace represents the former shoreline
of a sea or ocean.
It can be formed by marine abrasion or erosion
of materials comprising the shoreline; the
accumulations of sediments in the shallow-water
to slightly emerged coastal environments;
or the bioconstruction by coral reefs and
accumulation of reef materials in intertropical
regions.
The formation of a marine terrace follows
this general process: A wave cut platform
must be carved into bedrock.
Although this is the first step to the process
for the formation of a marine terrace, not
all wave cut platforms will become a marine
terrace.
After the wave cut platform is formed it must
be removed from interaction with the high
wave energy.
This process happens by either change in sea
level due to glacial-interglacial cycles or
tectonically rising landmasses.
When the wave cut has been raised above sea
level it is preserved.
The terraces are most commonly preserved in
flights along the coastline.
Lacustrine terraces
A lake terrace represents the former shoreline
of either a nonglacial, glacial, or proglacial
lake.
As in case of marine terraces, a lake terrace
can be formed by either the abrasion or erosion
of materials comprising the shoreline, the
accumulations of sediments in the shallow-water
to slightly emerged environments, or some
combination of these.
Given the smaller size of lakes relative to
the size of typical marine water bodies, lake
terraces are overall significantly narrower
and less well developed than marine terraces.
However not all lake terraces are relict shorelines.
In case of the lake terraces of ancient ice-walled
lakes, some proglacial lakes, and alluvium-dammed
lakes, they often represent the relict bottom
of these lakes.
Finally, glaciolacustrine kame terraces are
either the relict deltas or bottoms of ancient
ice marginal lakes.
Structural terraces
In geomorphology, a structural terrace is
a terrace created by the differential erosion
of flat-lying or nearly flat-lying layered
strata.
The terrace results from preferential stripping
by erosion of a layer of softer strata from
an underling layer of harder strata.
The preferential removal of softer material
exposes the flat surface of the underlying
harder layer creating the tread of a structural
terrace.
Structural terraces are commonly paired and
not always associated with river valleys.
Travertine terraces
A travertine terrace is formed when geothermally
heated supersaturated alkaline waters emerge
to the surface and form waterfalls of precipitated
carbonates.
References
See also
Parallel Roads of Glen Roy
Landform
Machair
Raised beach
Raised shoreline
Terrace
Wave-cut platform
External links
Here is a good example of a river terrace:
http:www.geographie.uni-erlangen.degalleryasiariver_terraces_near_kasbeki.jpg
