Geology Reference
In-Depth Information
A
Forgetting a Marine Terrace
A
A'
Inner edge of
terrace platform
L
B
terrace surface
young
A
A'
H
α
H'
terrace flat
removed
H
C
H'
sealevel
Fig. 11.11
How marine terraces are forgotten in the landscape.
With increasing age, marine terraces are progressively removed or forgotten from the landscape. A. DEM of a portion
of the terraced landscape in Santa Cruz, California, showing progressive loss of well-preserved terraces with elevation.
Two of the more prominent, younger terraces are delineated by white lines at approximately the back edge of each
terrace. B. Cartoon of a cross-section across two channels and intervening terrace tread (e.g., along the line AA
′
in the
DEM), at early (solid) and late (dashed) times. C. Cartoon of a long profile of terrace and incising channel, the latter
shown at early (solid) and later (dashed) times. Deepening of the stream channel below the terrace surfaces, from H
to H
′
, incites broadening of the adjacent valley walls, which in turn bite into the remnants of the flat marine terrace
treads. The resulting time scale for removal of the terrace flat is dependent on the distance between adjacent
channels,
L
, and is inversely dependent upon the rock uplift rate, which in turn scales the stream incision rate.
Modified after Anderson
et al.
(1999) and Anderson and Anderson (2010).
the tributaries etch rapidly into the terraced flats.
In this landscape, terraces older than the fifth
terrace are lost from the record. In this fashion,
the number of identifiable terraces and the
pristinity of the terrace features - as reflected in
the distinctness of the paleo-sea cliff edges and
in the fraction of the original terrace flat that
remains - are governed by the efficiency of
fluvial and hillslope processes as the landscape
responds to relative base-level fall.
Modeling river incision
Models of river incision have evolved over the last
decade to include how rivers should respond to
patterns of rock uplift or base-level fall (e.g.,
Whipple and Tucker, 1999), how this response is
modified by orographic precipitation patterns
(see review in Roe, 2005), and how knickpoints
propagate in fluvial networks. Whipple (2004) has
