Page 56 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
Special Problems with Sedimentary Rocks 51

Case History
Drilling for oil into a salt dome turned out rather badly when drilling
encountered a working salt mine and caused it to suddenly become flooded
with water. The miners worked a hasty retreat and a fishing boat on a
nearby lake nearly went down the drain.

3.3.5 Geomorphology of Tilted Sedimentary Rock Layers

The influence of geomorphology becomes particularly significant where rock
layers are pushed up, folded, and tilted. This is common in or near mountainous
areas due to plate movements. High-angle tilting results in parallel ridges of more
resistant rock layers called hogbacks (Fig. 3.7). Tilting at a lower angle results
in lower hills called cuestas that are steep on one side and flat on the other.
Cuestas in turn are transitional to relatively flat-lying mesas.

Because hogbacks and cuestas create resistant ridges, rocks between the ridges
obviously must be softer, more readily eroded, and have gentler slopes. Streams
follow elongated valleys between the ridges with periodic crossings that developed
while the land was being tilted. The result is a characteristic trellis drainage
pattern. Such a pattern on maps or airphotos is a sure sign that there are tilted
rock layers of varying hardness.

3.4 METAMORPHIC ROCKS

Definition of Metamorphism
Metamorphism literally means ‘‘change in form.’’ Metamorphic rocks are igneous
or sedimentary rocks that have been transformed by heat, pressure, and/or
chemical solutions. A guiding principle of metamorphism, as in the case of
weathering, is that the farther a material is removed from its nascent environment,
the more vulnerable it is to change. Thus, igneous rocks formed at high temper-
atures may be only slightly modified by metamorphism at a high temperature,
whereas shale subjected to the same temperature will be completely transformed.
Moderate metamorphism transforms shale into slate, used for roofs and

Figure 3.7
Hogbacks form the
hills and soft rocks
such as shale form
the valleys in a
tilted sedimentary
rock area.

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