Lesson 3/Learning Event 2
Maps provide valuable information, especially when planning the soil survey. In some cases, maps
showing the suitability of terrain for various military purposes, prepared by friendly foreign or enemy
agencies, may be of considerable value in planning. There are several kinds of maps which provide
different types of information about an area under investigation.
Geological Maps. Obviously a close relationship exists between geology and soil conditions.
Geological maps and brief descriptions of regions and quadrangles have been published in the folios of
the U.S. Geological Survey. Generally the smallest rock unit mapped is a formation, and geological
maps indicate the area extent of these formations by means of letter symbols, color, or symbolic
patterns. Letter symbols on the map also indicate the location of sand and gravel pits, and the rear of the
map sheet sometimes has a brief discussion entitled "Mineral Resources," describing the location of
Topographic Maps. Ordinary topographical maps may be of some use in estimating soil conditions,
particularly when used with geologic maps. Topographic maps, especially when the contour interval is
20 feet or more, tend to give only a generalized view of the land surface. Inspection of the drainage
pattern and slopes can provide clues of the nature of rocks, depth of weathering, soil, and drainage. For
example, sinkholes may indicate limestone or glacial topography; hills and mountains with gently
rounded slopes usually indicate deeply weathered rocks; and parallel ridges are commonly related to
steeply folded, bedded rock with hard rock along the ridges. Features such as levees, sand dunes, beach
ridges, and alluvial fans can be recognized by their characteristic shapes and geographic location.
Agricultural Soil Maps. Agricultural soil maps and reports are available for many of the developed
agricultural areas of the world. These studies are concerned primarily with surface soils, generally to a
depth of about six feet. Their value as aids in the engineering study of surface soils is apparent. For
example, if the same soil is shown to occur in two different areas, it can be sampled and evaluated for
engineering purposes in one area, and the amount of sampling and testing can then be sharply reduced in
the second area. Information on topography, drainage, vegetation, temperature, rainfall, water sources,
and rock location may be found in an agricultural report. Soils usually are classified according to their
texture, color, structure, chemical and physical composition, and morphology.
Aerial Photographs. The use of aerial photographs in delineating and identifying soils is based upon
the recognition of typical patterns formed under similar conditions of soil profile and weathering.
Principal elements which can be identified on a photograph, and which provide clues to the
identification of soils to a trained observer, are landforms, slopes, drainage patterns,