Lesson 3/Learning Event 2
erosional characteristics, soil color or "tone", vegetation, and land use. Each of the following brief
descriptions serve only as an example of information which may be derived from the examination of
aerial photographs.
Landform. The "form" or configuration of the land in different types of deposits is definitely
characteristic and can be identified on aerial photographs. For example, glacial forms such as moraines,
kames, eskers, and terraces have readily identified forms. In desert areas, characteristic dune shapes
indicate areas covered by sands subject to movement by wind. In areas underlain by flat-lying, soluble
limestone, the aerial photo typically shows sinkholes.
Slope. Prevailing ground slopes generally represent the texture of the soil. Steep slopes are
characteristic of granular materials, while relatively flat and smoothly rounded slopes may indicate more
plastic soils.
Drainage Patterns. A very simple drainage pattern is frequently indicative of pervious soils. A
highly integrated drainage pattern is frequently indicative of impervious soils, which in turn are plastic
and lose strength when wet. Drainage patterns also reflect underlying rock structure. For example,
alternately hard and soft layers of rock cause major streams to flow in valleys cut in the softer rock.
Erosional Patterns. Considerable information may be gained from the careful study of gullies.
The cross-section or shape of a gully is controlled primarily by the cohesiveness of the soil. Each abrupt
change in grade, direction, or cross-section indicates a change in the soil profile or rock layers. Short,
V-shaped gullies with steep gradients are typical of cohesionless soils; U-shaped gullies with steep
gradients indicate deep, uniform silt deposits such as loess. Cohesive soils generally develop round,
saucer-shaped gullies.
Soil Color. The color of the soil is shown on the aerial photograph by shades of gray, ranging
from white to black. Soft, light colors or tones generally indicate pervious, well-drained soils. Large
flat areas of sand are frequently marked by uniform light gray color tones, a very flat appearance, and no
natural surface drainage. Clays and organic soils frequently appear as dark gray to black areas. In
general, sharp changes in the color tone represent changes in soil texture. These interpretations should
be used with care.
Vegetation. Vegetation may reflect surface soil types, although its significance frequently is
difficult to interpret because of the effects of climate and other factors. To interpreters with local
experience, both cultivated and natural vegetation may cover may be reliable indicators of soil type.
Land Use. Ready identification of soils is frequently facilitated by observing agricultural land
use. For example, orchards require well-drained soils, and the presence of an orchard on level ground
would imply a sandy soil. Wheat is frequently grown on loess-type soils. Rice usually is found in
poorly draining soils underlain by impervious soils, such as clay. Tea grows in well draining soils.
SUBSURFACE INVESTIGATION
The third phase of the field investigation is to obtain data on the subsurface soils and formations. You
may do this directly with Samples or indirectly with geophysical surveys and seismic probes. Table 6
shows the range of possible methods of subsurface exploration.
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