(4) Chert. Although the origin of chert is not well understood, it is believed to form from the
precipitation of silicon dioide (SiO2) from seawater. There are many impurities associated with silicon dioide,
and their presence is reflected by the various colors of chert. Chert may occur as localized nodular concentrations
or as massive beds, in which case they exhibit a light to medium gray homogeneous photo tone. The ability to
recognize chert is important because of its very poor engineering characteristics. Due to the adverse reaction of
chert with the alkalies of portland cement, this rock type should not be used as an aggregate for such cement.
(5) Coal. This is an organic sedimentary rock composed mainly of carbonaceous plant material. The
first step in the formation of coal is the accumulation of vegetation at the bottom of a stagnant body of water. A
lack of oxygen in this environment allows for only a partial decomposition of organic mater. The partially
decomposed accumulation is known as peat, the lowest grade of coal. Eventually, the peat may become covered
by sediments, such as sand, silt, and clay. As more and more sediments are deposited, the increase in pressure
due to overlying weight forces water and organic gases out of the peat. In addition, the carbon content of the peat
increases. As the process continues, the coal is progressively transformed from light brown, low-grade peat
through mid-grade lignite and bituminous coal to jet-black, high-grade anthracite. Mid to high-grade varieties of
coal are used extensively as fossil fuels.
c. Interbedded Sedimentary Rocks. As previously mentioned, sedimentary rocks are initially deposited in
more or less horizontal layers called beds. Any disruption in the depositional process, such as a change in the
velocity of the transporting medium or a change in the sediment source area, may result in interbedded
sedimentary deposits. These deposits consist of a series of alternating layers of different rock types. For
example, shales may be interbedded with sandstones and limestones. In fact, all sedimentary rocks are
interbedded to various degrees, depending on their mode of formation. In many areas, interbedded sedimentary
rocks still exist as they were deposited--in horizontal layers. These sedimentary sequences are called flat-lying
interbedded sedimentary rocks. On the other hand, when the horizontal layers of interbedded sedimentary rocks
are deformed by faulting or folding, the resulting assemblage is referred to as tilted or folded interbedded
sedimentary rocks.
(1) Flat-Lying Interbedded Sedimentary Rocks. These rocks consist of alternating horizontal layers of
two or more rock types. If the individual component layers are less than 25 feet thick, they are considered to be
thin beds, whereas those that are greater than 25 feet thick are considered to be thick beds. Flat-lying interbedded
sedimentary rocks are likely to show banding on aerial photographs. The banding, if present, will coincide with
the contours of the land surface. In humid regions, thin-bedded sedimentary rocks normally develop deep
residual soils that mask any banding that may exist. Therefore, an overall medium gray tone is predominant in
these areas.
(a) Relationship of Topography to Flat-Lying Interbedded Sedimentary Rocks.
Landforms Developed in Areas of Flat-Lying Interbedded Sedimentary Rocks. Interbedded
sedimentary rocks composed of horizontal layers form hills of equal elevation in all environments. The variations
in rock type exposed along the hillsides are responsible for the banded appearance of flat-lying interbedded
sedimentary rocks on aerial photographs. If the interbedded sequences are composed of thick layers of rock, the
side slopes of the hills will
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