Geology Reference
In-Depth Information
(streams presently too small to have created the valleys
they occupy),
entrenched meanders
, and
relict fluvial
features
in deserts. Deserts with hyper-arid climates
today contain landforms created by fluvial processes -
alluvial spreads, pediments, and valleys carved out by
streams. Wind erosion does not readily obliterate these
features, and they linger on as vestiges of former moist
episodes. The geomorphic effects of changing land use
are evident in the evolution of some Holocene river
systems. The Romans transformed fluvial landscapes in
Europe and North Africa by building dams, aqueducts,
and terraces (p. 9). A water diversion on the Min River in
Sichuan, China, has been operating ceaselessly for over
2,000 years. In the north-eastern USA, forest clearance
and subsequent urban and industrial activities greatly
altered rivers early in the nineteenth century. To expand
upon these points, this section will look at the effects
of glacial-interglacial cycles on fluvial landscapes, at the
impact of Holocene climatic and vegetation changes in
the USA, and at the complex Holocene history of river
systems in Mediterranean valleys and in Germany.
it affects seasonality, is more pronounced in sediments
deposited at high latitudes.
Quaternary
glacial-interglacial cycles
have caused
distinctive changes in middle- and high-latitudes land-
scapes. At the extremes, cold and dry climates alternated
with warm and moist climates. These changes would
have affected weathering, erosion, transport, and depo-
sition, causing shifts in the type and rate of geomorphic
processes operating. As a rule, during warm and wet inter-
glacials, strong chemical weathering processes (such as
leaching and piping) would have led to deep soil and
regolith formation. During cold and dry glacials, per-
mafrost, ice sheets, and cold deserts developed. The land-
forms and soils produced by glacial and by interglacial
process regimes are generally distinctive, and are normally
separated in time by erosional forms created in the rela-
tively brief transition period from one climatic regime to
another. When the climate is in transition, both glacial
and interglacial processes proceed at levels exceeding
thresholds in the slope and river systems (Figure 14.2).
Leslek Starkel (1987) summarized the changes in a tem-
perate soil landscape during a glacial-interglacial cycle.
During a cold stage, erosion is dominant on the upper
part of valley-side slopes, while in the lower reaches of
valleys abundant sediment supply leads to overloading of
the river, to deposition, and to braiding. During a warm
stage, erosion thresholds are not normally exceeded, most
of the slopes are stable, and soil formation proceeds, at
least once the paraglacial period ends (p. 271). Meander-
ing channels tend to aggrade, and erosion is appreciable
only in the lowest parts of undercut valley-side slopes
and in headwater areas. All these changes create distinct
sequences of sediments in different parts of the fluvial
system. Equivalent changes occurred in arid and semi-
arid environments. For instance, gullying eroded talus
deposits formed during prolonged mildly arid to semi-
arid pluvial climatic modes, leaving talus flatiron relicts
during arid to extremely arid interpluvial climatic modes
(Gerson and Grossman 1987). In north-western Texas
and eastern New Mexico, a vast sheet of Quaternary loess
covering more than 100,000 km
2
and up to 27 m thick,
known as the Blackwater Draw Formation, records more
than 1.4 million years of aeolian sedimentation (Holliday
1988, 1989). Six buried soils in the formation reveal that
stable landscapes obtained under subhumid to semiarid
Glacial-interglacial cycles and fluvial
landscapes
The Earth's
orbital cycles
wield a considerable influence
over climate. They do so by changing seasonal and latitu-
dinal patterns of solar radiation receipt. Orbital variations
in the Croll-Milankovitch frequency band (roughly
19,000 to 400,000 years) appear to have driven climatic
change during the Pleistocene and Holocene epochs.
Orbital forcing has led to climatic change in middle and
high latitudes, where ice sheets have waxed and waned,
and to climatic change in low latitudes, where water
budgets and heat budgets have marched in step with
high-latitude climatic cycles. The 100,000-year cycle of
eccentricity registers in Quaternary loess deposits, sea-
level changes, and oxygen-isotope ratios of marine cores.
The precessional cycle (with 23,000- and 19,000-year
components) and the 41,000-year tilt cycle ride on the
100,000-year cycle. They, too, generate climatic changes
that register in marine and terrestrial sediments. Oxygen
isotope ratios in ocean cores normally contain signatures
of all the Earth's orbital cycles, though the tilt cycle, as
