Geology Reference
In-Depth Information
of deformation. It is commonly assumed that,
within any area experiencing a regionally
consistent climate, most of the glaciers would be
expected to advance and retreat approximately
synchronously, but there are few well-dated
tests of this contention (Gillespie and Molnar,
1995). In the absence of dates, it is, therefore,
not uncommon to try to correlate with the
global climatic record (Fig. 2.3), but such
correlations should be made with considerable
skepticism.
innovative approach to the problem of defining
geomorphic ages is a boon to anyone attempting
to obtain reliable age constraints and hence
deformation rates.
Correlation of partially preserved geomorphic
features presents another challenge. As older
and older markers are examined, their
preservation typically becomes increasingly
fragmentary. Unless distinctive characteristics
permit discrimination among features of
differing ages, the correct correlation among the
remnants may be difficult to achieve. A traditional
field technique for terrace remnants, for
example, has been to survey the height of
terraces along local reaches and to correlate the
terraces from one site to another based on their
relative height above the river. This approach
can be misleading, because it assumes a
consistency in the longitudinal profile of the
terrace through time. A better technique is to
employ continuous, geodetic surveying along
terrace surfaces (Merritts
et al.
, 1994) and to
document the connection from one reach to the
next whenever possible (Fig. 2.17). Even with
such data, reconstruction of the long profiles
of multiple terraces can be ambiguous. Dis-
tinctions need to be drawn between strath
and aggradational terraces, because, when
aggradational terraces primarily result from a
downstream rise in local base level, their
gradients are likely to be gentler than those of
most upstream strath terraces. Recognition of
different terrace types, continuous geodetic
surveying, dating of terraces, and analysis of
how and why the terrace sequence developed
all assist in creating a reliable reconstruction.
New tools continue to emerge that can assist
geomorphologists in reconstructing land-
scapes. Even where features, such as terraces,
are quite continuous, dense vegetation can
often obscure the correlations among them.
With the availability of lidar bare-Earth DEMs,
the vegetation can be removed and a much
clearer view of the landscape typically emerges
(Fig. 2.18). Especially in heavily forested
landscapes, lidar can revealed the detailed
geomorphic structure of the landscape.
Unfortunately, lidar acquisition is not cheap:
costs for surveys are commonly 200-400 US
Other linear features
Numerous other linear features, both naturally
occurring and man-made, can be used as mark-
ers for gauging displacement. Fairly viscous
debris flows can create raised levees of coarse
debris along their channel margins (Beanland
and Clark, 1994). Fence lines, railroad tracks,
curbs, sidewalks, lines painted on streets, and
even tracks created by cars, motorcycles, or
bikes, can provide linear markers that are readily
measured and are useful for documenting either
coseismic offsets of recent earthquakes (e.g.,
the scarp of the 1992 Landers earthquake in
California displaced alluvial fans that sported
hundreds of motorcycle paths) or slower rates
of creep. These man-made markers are not
geomorphic features in a strict sense, but when
trying to generate a catalog of displacements
along a recently ruptured fault zone, any
displaced linear feature with a known pre-faulting
geometry should be evaluated.
Commonly encountered problems
with markers
The most typical difficulty in extracting the
maximum information from a displaced geo-
morphic surface is the absence of a well-
documented age for the surface. Considerable
effort often is warranted to try to uncover datable
material that can constrain the age of the feature.
A knowledge of the array of available dating
tools, experience with the appropriate field
procedures for collecting suitable material for
a specific dating method, and a thorough and
