within the area. Sandstones normally support heavy forest cover, while limestones are typically cultivated.
Shales may be either forest-covered or cultivated. Vegetation in arid regions consist of a few scattered grasses
and some sparse scrub growth in areas where water-bearing rocks outcrop at the surface.
(b) Engineering Properties of Tilted and Folded Interbedded Sedimentary Rocks. The
engineering properties of interbedded rock sequences depend on the individual rock types present. In general,
sandstones and limestones are suitable for most construction purposes, whereas shales are not. For more
information regarding the specific engineering properties of each of these rock types, see Lessons 1.B.2.a.(3)(b),
page 1-20; Lesson 1.B.2.b.(1)(b), page 1-22; and Lesson 1.B.2.a.(4)(b), page 1-21, respectively.
3. Metamorphic Rocks. Metamorphic rocks are formed from changes in the mineral composition and/or the
physical character of preexisting igneous, sedimentary, or metamorphic rocks. These changes that are brought
about by the application of heat, pressure, or chemically active fluids may occur in several situations. For
example, during mountain-building processes, large rock formations are subjected to intense stress and high
temperatures associated with large-scale deformation. This results in regional metamorphism, which is the mode
of formation of most metamorphic rocks. Metamorphism may also take place when molten magma comes in
contact with country rock. The solutions escaping from the magma may chemically alter the surrounding rock
material. In addition, the country rock will most likely be "baked" under the extremely high temperatures. This
type of metamorphism is known as contact metamorphism. Finally, metamorphism may occur along fault zones
where the rocks are broken and distorted by pressure created from fault movement.
The metamorphic rock group is a small one; it makes up only about seven percent of the rocks exposed on the
earth's surface. Metamorphic rocks are classified either foliated or nonfoliated (see figure 1-18). Foliated
metamorphic rocks exhibit a series of parallel planar structures along which the rock flakes or splits into thin
sheets. Nonfoliated rocks are massive and exhibit no structural features.
a. Foliated Metamorphic Rocks. The platy structure of foliated metamorphic rocks is the result of
exposure to very high pressures, usually those encountered in regional metamorphism. Because regional
metamorphism is the most widespread form of metamorphism, foliated rocks are the predominant metamorphic
rock type. Within this category, three basic individual rock types are further classified based on their grain size
(see figure 1-18).
(1) Slate. This is a very fine-grained, homogeneous rock formed from shale that has been subjected to
low-grade regional metamorphism. Slate is highly foliated; therefore, it readily splits into thin plates. The color
of slate may range from light gray to red or black, but the tone on aerial photography is normally a uniform light
(a) Relationship of Topography to Slate.
Landforms Developed in Areas of Slate. Slate weathers rather quickly, producing a very
rugged, highly dissected topography with sharp parallel ridges and steep side slopes. Hilltops are relatively low
but of approximately equal elevation.