Drainage Developed on Sandstone Rock. Because sandstones are highly permeable,
homogeneous rock types and medium- to coarse-textured dendritic drainage patterns are normally developed,
except in areas where extensive jointing modifies the drainage pattern to that of rectangular or rectangular
dendritic. Deep valleys form along the eroded joint patterns. There are generally very few gullies developed on
sandstone terrain, again because of the high permeability and low surface runoff. Where present, gullies are V-
shaped because of the granular, noncohesive nature of the particles.
Vegetation in Areas of Sandstone Rock. In humid climates, sandstones heavily support the
growth of natural vegetation. The vegetation tends to be uniform, with coniferous trees predominating. In arid
climates, sandstone will not support large amounts of vegetation due to a shallow soil cover and extensive
caprock exposure.
(b) Engineering Properties of Sandstone. The toughness, hardness, and durability of sandstones
vary widely, depending on their composition and degree of cementation. Clean, compact, quartz-rich sandstones
that are held together by silica cement are generally good material for all types of construction. Impure, poorly
cemented varieties, on the other hand, should be avoided in most cases. For this reason, it is important to
carefully select sandstones that are to be used in construction.
(4) Shale. Shale is a general term applied to all sedimentary rocks composed of consolidated silt and
clay-sized particles. Whereas sandstones are normally lithified by the cementation of individual grains,
cementing agents are usually unable to penetrate deposits of finer-grained material, such as silts and clays.
Therefore, compaction of these materials under the weight of overlying sediment is the primary process
responsible for the formation of shale. A distinctive characteristic of shales is their tendency to break along
planes of weakness that lie parallel to the original bedding plane. This property is called fissility. Photographic
tones of shales in humid areas are dull gray with some mottling due to variations in moisture and organic content.
In arid regions, the tone is uniformly light and dull except for some occasional parallel banding. Shales are very
extensive rock formations, covering over 50 percent of the earth's exposed land surface.
(a) Relationship of Topography to Shale.
Landforms Developed in Areas of Shale. Because of its mode of formation (compaction
rather than cementation), shale tends to crumble more easily than other types of sedimentary rocks in almost all
environments. Consequently, these weak rocks are very susceptible to weathering and erosion in both humid and
arid climates. In humid regions, low, rounded hills and valleys (sometimes referred to as "sag and swale"
topography) are common. "Badlands", a type of highly dissected topography characterized by rounded ridges and
steep side slopes, are often formed on shale in arid climates.
Drainage Developed on Shales. Because homogeneous shale are relatively impervious, large
amounts of surface runoff are likely. This results in the formation of a medium- to fine-textured dendritic
drainage pattern in areas underlain by such rocks. The thick layers of cohesive soil that rapidly develop over
shales are conducive to the development of U-shaped or box-shaped gullies, depending on the relative percentages
of clay and silt-sized
EN5341
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