Anatomy of Folds

There are two important directions associated with inclined strata, namely, dip and strike. True dip gives the maximum angle at which a bed of rock is inclined and should always be distinguished from apparent dip (Fig. 2.1). The latter is a dip of lesser magnitude whose direction can run anywhere between that of true dip and strike. Strike is the trend of inclined strata and is orientated at right angles to the true dip, it has no inclination (Fig. 2.1).

Figure 2.1: Illustration of dip and strike: orientation of cross-hatched plane can be expressed as strike 330 , dip 60 .

Folds are wave-like in shape and vary enormously in size. Simple folds are divided into two types, that is, anticlines and synclines (Fig. 2.2a and b). In the former, the beds are convex upwards, whereas in the latter, they are concave upwards. The crestal line of an anticline is the line that joins the highest parts of the fold, whereas the trough line runs through the lowest parts of a syncline (Fig. 2.2a). The amplitude of...