particles. A soil that is composed mainly of clay will typically exhibit steep-sided, U-shaped gullies, whereas one
composed almost entirely of silt will exhibit box-shaped gullies.
Vegetation in Areas of Shale. In humid areas, shales are either intensely cultivated or heavily
forested. The cohesive nature of the soils that develop in such areas is favorable for the growth of deciduous
trees.
(b) Engineering Properties of Shale. In most cases, blasting is not required for the excavation of shale;
however, shales generally make very poor construction material due to their weakness and lack of durability. In
addition, many shales have a tendency to swell upon prolonged contact with water, making them undesirable as
foundations.
b. Chemical or Biochemical Sedimentary Rocks. Sedimentary rocks with textures composed of
interlocking crystals that formed as minerals precipitated from solution are called chemical rocks. These rocks
are subclassified based on mineral composition rather than on particle size. In addition, there are a few types of
sedimentary rocks largely made up of the remains of plants and animals. These are the biochemical sedimentary
rocks. Chemical and biochemical rocks are closely related. In fact, some rock types display characteristics of
both groups. For this reason, the major rock types of each of the two groups will be discussed here under one
heading.
(1) Limestone. Limestone is an example of a type of sedimentary rock that may occur as either a
chemical or biochemical rock. Those formed by biochemical processes are the most important because they make
up approximately 90 percent of all limestone deposits. Biochemical limestones form as water-dwelling organisms
composed of calcareous shells and skeletons die and subsequently settle to the bottom of shallow lakes and seas.
Chemically derived limestone deposits are formed when the concentration of calcium carbonate in the water
exceeds its solubility level. In these cases, the calcium carbonate (CaCO3) precipitates (separates) from solution
and sinks to the floor of the body of water. Limestones vary in color depending on the amounts and types of
impurities present. Most pure limestones are white or light gray, while impure varieties may range from gray to
black. Limestone is easily identified, since it readily effervesces when placed in contact with a 10 percent
solution of hydrochloric acid (HCl). The overall photo tone of limestone in humid regions is a relatively uniform
light gray, but it may be interrupted by the occurrence of darker spots that indicate sinkholes (see Lesson
1.B.2.b.(1)(a)). This mottled tone may be continuous throughout a limestone formation, or it may be patchy,
depending on the number, size, and location of sinkholes. In arid regions, limestone normally exhibits a uniform
light tone. Limestone is the most abundant chemical/biochemical sedimentary rock, and it represents
approximately 10 percent of all sedimentary rocks.
(a) Relationship of Topography to Limestone Rocks.
Landforms Developed in Areas of Limestone. Because limestone is highly soluble, it is very
susceptible to chemical weathering in both humid and temperate climates. Water, which percolates through joints
and along bedding planes, dissolves the rock, forming subterranean void spaces known as caves and caverns. The
unsupported roof of such void spaces may eventually collapse, forming a series of surface depressions called
sinkholes. These sinkholes may develop over time, or they may be created catastrophically; they may consist of
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