(b) Engineering Properties of Gneiss. Gneiss is a reasonably hard, tough, and durable rock that
serves as a good foundation and building stone. In addition, it breaks into bulky, irregular pieces, providing an
excellent aggregate for most types of construction.
b. Nonfoliated Metamorphic Rocks. Nonfoliated metamorphic rocks are massive and show no evidence of
planar foliation. They usually result from localized alteration processes, such as contact metamorphism or
grinding action along fault planes; therefore, nonfoliated metamorphic rocks do not compose major landforms.
Instead, they occur as small, scattered masses. Photographic interpretation of these rocks is difficult because of
the small areal extent of single deposits. The two most common types of nonfoliated metamorphic rocks are
quartzite and marble.
This is a course-grained, light-colored metamorphic rock formed from the
recrystallization, or cementation, of sandstones. High temperatures and pressures cause individual sand grains
within the sandstone to become fused together, forming a homogeneous, nonfoliated structure. For this reason,
quartzites break across the original sand grains, exposing a smooth, glassy surface. This hard, tough, nonfoliated
metamorphic rock is the most resistant of all rock types; therefore, it forms topographically sharp ridges with
shallow soil profiles. The physical properties of quartzite make it an excellent construction material, but the
expense and difficulty of excavation and crushing make it costly to use.
(2) Marble. This is a compact, crystalline rock formed from the metamorphism of limestones or
dolomites. The original calcite or dolomite grains are made to interlock with one another upon the addition of
calcite into the structure during metamorphosis. Both the texture and color of marble vary considerably. The size
of individual component grains may range from fine to coarse, and white bandings and mottles caused by
impurities, such as iron oxides and organic matter, are common. Like limestone and dolomite, marble is easily
recognized in the field by its characteristic effervescence when exposed to a 10 percent solution of HCl. The
weathering characteristics, as well as the engineering properties of marble, are also similar to those of other
carbonate rocks. Because of it softness, however, marble is rarely used as an aggregate for pavements on
highways and airfields.
Once the types and characteristic of the rocks of a specific area have been identified, a geologic overlay may then
be created. A geologic overlay is a transparent record or map of the spatial relationships and characteristics of
rock types. The overlay may be superimposed on another graphic representation of the area, such as a map or
aerial photograph, which serves as a frame of reference for the geology.
1. Steps Involved in the Creation of a Geologic Overlay.
There are several steps to be followed in creating a geologic overlay. The general procedures are as follows:
a. Assemble the available source materials pertaining to geology for the area of concern.