Drainage Developed on Flat-Lying Interbedded Sedimentary Rocks. The horizontal
uniformity of flat-lying interbedded rocks allows for the development of dendritic drainage patterns. The
drainage tends to be controlled by the most resistant rock type, which is usually sandstone in humid climates and
may be either sandstone or limestone in arid environments. Flat-lying sedimentary rocks that have been
uniformly uplifted will have a deeply entrenched drainage system with exposed rock cliffs along the major
watercourses (see figure 1-14). This is caused by the fracturing of sedimentary rock layers during the uplift,
creating weak areas susceptible to stream erosion. In humid regions, the shapes of gullies formed in flat-lying
interbedded sedimentary rock vary considerably but are usually of the sag-and-swale type, indicating cohesive
soils. In arid regions, a lack of significant residual soil restricts the development of gullies.
Vegetation in Areas of Flat-Lying Interbedded Sedimentary Rocks. In humid environments,
the major rock type present in a sequence of flat-lying interbedded sedimentary rocks is generally responsible for
the predominant vegetation type. Gentle slopes and valleys composed of shale are cultivated, whereas the steeper
sandstone slopes remain forest-covered. In limestone and shale combinations, cultivated areas are found near
major sources of water, such as sinkholes and drainage systems. If the component layers of the interbedded
sequence are thick, then natural banding of vegetation may be visible along the hillslopes where alternating rock
types are exposed. However, vegetational preferences in areas of thinly bedded layers are not easily
distinguished. Interbedded sedimentary deposits in arid climates are generally rugged and have a thin soil cover.
Therefore, most of these regions are barren except for some grass and scrub growth.
(b) Engineering Properties of Flat-Lying 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 shale is not. For more information regarding the
specific engineering properties of each of these rock types, see Lesson 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.
(2) Tilted or Folded Interbedded Sedimentary Rocks. Flat-lying interbedded sedimentary sequences
are often subjected to processes, such as faulting or folding, that cause the beds to deviate from their original
horizontal positions. When this occurs, the resulting sequences are referred to as tilted or folded interbedded
sedimentary rocks. Figure 1-15, page 1-28, illustrates a sequence of tilted sedimentary rocks, and figure 1-16,
page 1-28, depicts a folded sedimentary rock sequence.
Several structural forms may result from folding. Two of the more important ones, anticlines and synclines, are
shown in figure 1-16, page 1-28. Anticlines are arches in the rock strata. As such, the beds of an anticline dip
away from one another, and the oldest rocks are located in the center of the structure. Synclines, in contrast, are
troughs in the rock strata. The beds of a syncline dip toward one another, and the youngest rocks are located in
the center. Occasionally, anticlines or synclines are tilted so that they "plunge." Figure 1-17, page 1-29, shows an
example of a plunging anticline. Tilted, folded, and deformed rock structures make up some of the world's largest
mountain ranges.
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