Geology Reference
In-Depth Information
A Initial data set
B Initial correlation
continuous terrace
surfaces
correlation of terraces
based on
initial survey
marine terrace
platforms
survey point
on terrace
modern channel bed
Evolving Terrace Correlations
C Expanded data set
D Revised correlation
additional survey
points on terrace
revised terrace
correlations
E Differentiation of terrace types
F Final correlation
final terrace
correlations
continuous strath
surfaces
strath surface
tread of
fill terrace
continuous fill
terrace surfaces
Fig. 2.17 Correlation of remnants of river terraces based on an example from the Mattole River, northern
California.
A. Traditional technique of surveying the altitude of terraces in local reaches provides isolated points that must be
correlated. B. If it is assumed that the past and present river gradients were parallel, correlations are then based on
the height of a terrace above the modern river bed. C. More spatially continuous surveying of the upstream extent
of terraces can define their gradients more reliably than can be done solely with correlated spot heights. D. When
compared with the modern gradient, the reconstructed terraces are clearly not parallel to the modern profile. E. When
the surfaces of aggradational terraces are distinguished from those of strath terraces, contrasts in gradients between
the two terrace types become apparent. F. Terrace reconstruction based on survey, dating, and geomorphic field
evidence shows strath terraces with steep gradients merging into the present river profile. Aggradational terraces with
gentle gradients result from deposition forced by rising sea level (rising base level). Modified after Merritts et al.
(1994).
dollars per square kilometer. But, if the problem
is sufficiently important, then an investment in
very high-resolution lidar topography can be
well worth it.
Other problems can arise when a regional
chronology of geomorphic markers has been
developed and is subsequently applied to the
deformational analysis of local sites. Suites of
aggradational terraces or of glacial moraines are
commonly dated by assembling ages from
individual outcrops spread over a broad area.
Such amalgamations can be reasonable and may
be the only practical approach. Nonetheless, it is
important to be aware of the possible presence
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