Geology Reference
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from the employment of this technique have
been summarized by Bierman (1994). Studies in
alpine areas ranging from the Rockies to
Australia have shown that rates of the order of
5-15 μ m/yr (5-15 m/Myr) are very common on
exposed bedrock surfaces (Small et  al. , 1999;
Quigley et al. , 2007).
In regolith-mantled landscapes, regolith shields
the bedrock from bombardment by cosmic rays,
such that nuclide production rates are lower at
the bedrock interface. If the landscape is in a
steady state, so that the thickness of regolith ( H )
on a site is constant through time, then the
concentrations of nuclides at the regolith-bed-
rock interface can similarly be used to define the
rock-to-regolith conversion rate (Fig. 7.12B):
Bedrock incision rates
Rivers, along with glaciers, are the most important
geomorphic agents for “setting” the local base
level. Incision by rivers determines adjacent
hillslope gradients, and the success of a river in
removing debris supplied from adjacent slopes
influences its ability to incise underlying bedrock.
If more material comes off the slopes than the
river can transport, it will not incise the underlying
bedrock or lower the local base level, and adjacent
slope angles may tend to decrease (Fig. 7.1). Thus,
a suite of self-regulating feedbacks can develop in
which the long-term rate of base-level lowering by
river erosion, the sediment transporting power of
the river, and the rate of sediment supply from
adjacent slopes are in rough equilibrium.
Bedrock incision rates can be calculated directly
whenever the geometry and age of a former land
surface can be reconstructed. Because of their
predictable longitudinal and cross-sectional geom-
etries, river terraces often provide reliable datums
for such calculations. One has simply to measure
the elevation difference from the terrace tread to
the present river level. Owing to their unconsoli-
dated nature, alluvial river terraces are highly sus-
ceptible to erosion. Consequently, the history of
cut and fill represented by alluvial terraces reveals
repeated crossings of the threshold of critical
power (Bull, 1991), but tells little about long-term
rates of regional denudation or bedrock incision
(Box 7.2). In contrast, fluvial terraces etched into
bedrock (strath terraces) record previous positions
of an actively incising river (Fig. 7.13). Using the
height of the strath above the modern stream, if it
is assumed that the strath resulted from erosion
at  or near the low point of the river's bedrock
channel, and if the elapsed time since the
abandonment of the strath by the incising river is
known, then long-term rates of bedrock incision
can be calculated (Fig. 7.13).
A recently proposed, alternative model for strath
formation (Hancock and Anderson, 2002) posits
that straths are created when the bedrock channel
is covered with alluvium. This “sediment loading
model” (Figs. 2.12 and 7.13) suggests that the
alluvium prohibits vertical incision, but provides
abundant tools (clasts) that can help the channel
etch laterally into the bedrock valley walls. This
/*
(e
Hz
P
)*
z
(7.2)
[CRN]
=
E
The rates so obtained will be average rates over
the time it takes to convert about 0.6 m of rock to
regolith under steady-state conditions. This
conversion rate is important because landscape
denudation by soil-related processes (creep, rain
splash, solifluction, shallow landslides) cannot
occur faster than the rate at which regolith is pro-
duced. Until present, the most rapid known rates
of regolith production are 0.3 mm/yr (Heimsath,
1999). Wherever landscape rates exceed soil
production rates, other processes such as glacial
erosion or bedrock landslides have to contribute
significantly to overall denudation.
At steady state, the rate of regolith production
(Heimsath et al. , 1997) has been argued to vary
as a function of regolith thickness (Fig. 7.12B).
The thickness of regolith, in turn, varies as a
function of slope ( δ z / δ x ), such that thinner soils
prevail on steeper slopes. In some relatively
slowly eroding landscapes, the thickness varia-
bility is a predictable function of the downslope
flux of regolith, which can itself be related to the
curvature ( C = δ
z / δ x 2 ) of the slope (Fig.  7.12A
and C) through the landscape diffusivity constant
( k ). In the resultant steady-state diffusive hillslope
model, downslope rates of soil movement are
fastest where positive curvature is greatest. High-
curvature sites are also where soils are thinnest
and soil production rates are the highest.
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