Geoscience Reference
In-Depth Information
4. Relief Recall that topography refers to the configu-
ration of the Earth's surface—in other words, the
position, shape, and orientation of hills, valleys, and
mountains. In a related vein, the term relief refers
to the differences between the highs and lows of a
landscape (Figure 11.12a). The Rocky Mountains, for
example, is a region of very high relief because tall
peaks are separated by lower valleys. In contrast, much
of the Midwest has low relief because the landscape is
relatively flat.
Area of high relief
The most important variable associated with relief and soil
development is slope. The slope of the landscape is loosely
defined as the ground surface that connects the higher areas
with lower areas. Slope is significant in the context of erosion
because soils are generally thinner and less well developed
where slopes are steeper. Figure 11.12b shows a hypothetical
landscape with a steep slope next to a relatively flat surface.
Note that the soil on the steep slope is thinner; that is, the bed-
rock is closer to the surface. This pattern occurs because more
erosion takes place on the steeper slope, predominantly due to
water flowing downhill under the force of gravity. As a result,
the rate of erosion on the steeper hillslope may be equal to, or
even greater than, the rate of soil formation (Figure 11.12c). In
contrast, on the more level surface, the soil can develop more
fully because it lies essentially undisturbed from erosion. In
addition, the soil is thicker because sediments eroded from the
steep slope are added where the slope is more level.
Area of low relief
(a)
Steep slope,
thinner soil
Flatter surface,
thicker soil
5. Time Time is a critical variable because soils need it to
develop. If it is assumed that all the other soil-forming
factors (parent material, climate, organisms, relief) are
equal, then the best-developed soils occur on surfaces
that have been stable for the longest period of time.
For example, where would you expect to find the better-
developed soil: in a 3000-year-old sand dune or in a
100,000-year-old stream deposit? The answer is that the
older stream deposit most likely has a better-developed
soil because it had more time to develop. The signifi-
cance of this greater amount of time is that collectively
more additions, translocations, depletions, and trans-
formations have taken place in the older soil than in the
younger soil.
(b)
KEY CONCEPTS TO REMEMBER
ABOUT PEDOGENIC PROCESSES
AND SOIL-FORMING FACTORS
(c)
Figure 11.12 Relief and soil formation. (a) The term relief refers
to the differences between the high and low parts of a landscape. In
this scene, the area in the foreground is a zone of low relief, where-
as the mountain in the background is an area of high relief. (b) Soils
are generally thinner on steeper slopes than on flatter surfaces
because steep slopes are more prone to erode. (c) Relief affects soil
thickness on slopes, such as the exposed bedrock (arrow) on the
steep slope shown here. The soil is very thin on this slope because
of erosion; soil is thicker on top of the ridge and in the valley.
1.
Four pedogenic processes—additions, translocations,
deletions, and transformations—collectively promote
soil development.
The difference between the high and low elevation of
Relief
an area.
 
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