Environmental Engineering Reference
In-Depth Information
FIGURE 6.26
Limestone landform developing in a tropical climate. Note numerous “haystack” hills (Manati, Puerto Rico
quadrangle; scale 1:24,000; advanced tropical karst). (Courtesy of USGS.)
Nomenclature
Common fold structures include monoclines, synclines, anticlines, isoclines, overturned
anticlines, and overthrusts. There are also recumbent folds, drag folds, and plunging
folds. Their characteristics are summarized in Table 6.5 and most are illustrated in
Figure 6.30.
The nomenclature of plunging folds is given in Figure 6.31 . The crest of an anticline is
illustrated in Figure 6.32, and a recumbent fold is illustrated in Figure 6.33.
6.3.2
Fracture Cleavage
Cause
Fracture cleavage results from the development of shear forces in the folding of weak beds
with schistosity such as shales and slates.
Formation
Considered as a form of jointing, fracture cleavage is independent of the arrangement of
mineral constituents in the rock; therefore, it is often referred to as false cleavage to dif-
ferentiate it from mineral cleavage.
Since fracture cleavage results from a force couple (Figure 6.34), its lineation is often
parallel to the fold axis; therefore, when it is exposed in outcrops, it is useful in deter-
mining the orientation of the overall structure. Fracture cleavage in a slate is illustrated
in Figure 6.35 .
Flow Cleavage
The recrystallization of minerals that occurs during folding is referred to as flow cleavage.
 
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