small, shallow depressions, or they may be large enough to destroy a city block. Regions in which there is
considerable evidence of the dissolution of carbonate rocks are said to exhibit karst topography. Figure 1-12
shows the topography of a karstic region.
An extreme form of karst topography, called tropical karat, is prominent in regions of very high rainfall. The
single most distinctive feature of tropical karst is the haystack, a steep-sided, knobby remnant of a limestone
formation. Figure 1-13, page 124, shows the topographic expression of numerous haystacks. In arid regions,
limestone is just as resistant to erosion as sandstone and, like sandstone, it may form the caprock of ridges or
Drainage Developed on Limestone Rocks. In regions of karst topography, solution cavities
within the limestone cause internal drainage; therefore, few established surface water systems are present in such
regions. Those that are present generally follow the angular alignments of joints, resulting in rectangular
dendritic drainage patterns. Areas of tropical karst display no surface drainage; the highly permeable bedrock
ensures that all drainage is internal. In contrast, surface drainage is well-developed on limestones in arid climates
where karat topography does not exist. In such areas, intermittent streams are arranged in a medium- to fine-
textured, angular dendritic drainage pattern. A few sag and swale gullies are present in the karat topography of
humid regions. Occasionally, short, white-fringed gullies occur around the sinkholes. There are no gullies
formed in tropical regions. In arid regions, there are very few gullies because of the shallow residual soils formed
over limestone terrane.
Vegetation in Areas of Limestone Rock. Very fertile, cohesive, clay-rich residual soils form
on limestones of humid environments. These conditions favor the growth of deciduous trees. In fact, orchards
are frequently planted on limestones in temperate climates. Only a thin, weak soil develops in arid regions;
consequently, vegetation is usually sparse.
(b) Engineering Properties of Limestone. In general, limestone is a tough, hard, durable rock that
serves as a quality construction material for all purposes. However, some impure varieties of limestone contain
abundant amounts of clay that adversely affect the toughness and durability of the rock. These limestones should
be avoided when selecting material for construction.
(2) Coquina. This is a coarse biochemical sedimentary rock composed entirely of poorly cemented
calcareous shells and shell fragments. This sedimentary rock type will readily effervesce when brought into
contact with a 10 percent solution of HCl. Massive accumulations of shell fragments occur only in localized
areas; therefore, coquina is very limited in lateral extent.
(3) Dolomite. Limestones often undergo chemical reactions following their deposition. For example,
magnesium ions (Mg+2) may partially replace the calcium ions (Ca+2) present in the calcium carbonate. The result
is a calcium-magnesium carbonate rock called dolomite (CaMg(CO3)2). The characteristics of dolomite are
similar to the limestone from which it was derived, although dolomites are more resistant to weathering than their
limestone counterparts. This is especially evident in humid or tropical climates where dolomites do not develop
the karst topography associated with pure deposits of calcium carbonate. Whereas limestone readily effervesces
when a drop of HCl is placed on its surface, dolomite will react with HCl only when it is in a powdered form.