a. Clastic Sedimentary Rocks. Clastic rocks are sedimentary rocks composed of broken fragments of
preexisting igneous, sedimentary, and metamorphic rocks. Due to differences in source material and weathering
processes, the size of the broken rock fragments varies greatly. This wide range of particle size serves as the basis
for further subdivision of clastic rocks. Although there are numerous specific rock types in this category, only the
four most important ones will be discussed in the following paragraphs.
(1) Conglomerate. Conglomerates are elastic rocks composed primarily of well-rounded, gravel-sized
particles of any rock type with finer-grained materials filling the pore spaces between the granule, pebbles,
cobbles, and boulders. These types of sedimentary rocks result from the cementation of unsorted material that
may have been deposited by glaciers, rivers, lakes, or oceans. Conglomeritic rocks are generally limited in their
lateral extent, but when present in sufficient quantity, they exhibit a light photo tone on aerial photographs. Wide
variations in the composition, degree of cementation, and degree of weathering of the component particles make
the engineering properties of conglomerate highly unpredictable; therefore, they should be avoided in
construction.
(2) Breccia. Breccia is very similar to conglomerate in that it also consists mainly of gravel-sized
particles with fine-grained material filling the interstitial spaces; however, in this case, the large fragments are
angular instead of rounded. These types of localized deposits result from landslides, fault movement, and talus
accumulations. When breccias are of sufficient extent to be identifiable on aerial photographs, they appear light-
colored. Like conglomerates, breccias make poor construction materials due to their unpredictable engineering
properties.
(3) Sandstone. Sandstones consist of cemented sand grains that may be either rounded or angular,
depending on the distance over which the sand traveled before deposition. Sandstone deposited near the source
area tends to be made up of angular, poorly sorted particles of relatively nonresistant material, such as feldspar
and mica. On the other hand, sandstones that have been deposited a considerable distance from the source area
are commonly composed of well-rounded, well-sorted particles of fairly resistant material, such as quartz. The
photographic tones of sandstone are normally light due to their well-drained structure and light-colored
component minerals; however, jointing may occasionally cause them to appear "scrabbled."
(a) Relationship of Topography to Sandstone Rocks.
Landforms Developed in Areas of Sandstone. The resistivity of sandstone formations to
weathering and erosion depends largely on the type and amount of cementing material present as well as on the
environment in which the sandstone is located. Poorly cemented sandstones readily disintegrate, whereas well-
cemented, or indurated, sandstones are very resistant. In fact, most sandstones are fairly resistant; therefore, in
most cases, the main differences in landforms produced are due to difference in climate. In temperate or humid
regions, sandstones form a bold topography with massive, steep-sided hills. Sandstone may also serve as the
overlying caprock in these areas because of their high relative resistance to erosion as compared to other types of
sedimentary rocks. In arid or semiarid environments, sandstones from the caprock of numerous plateaus and
ridges separated by rugged, angular terrain. Sharp, vertical cliffs are also common. Joints are prevalent on
sandstones of all regions, although they may not be distinguishable in temperate or humid regions where soil
cover is thick.
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EN5341
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