The line of intersection of a layer with the horizontal plane is the strike. The strike always runs at right angles to the dip.
Dip and strike may be illustrated by a book set aslant on a shelf. The dip is the acute angle made with the shelf by the side of the book, while the strike is represented by a line running along the book’s upper edge. If the dip is north or south, the strike runs east and west.
Fig. 170. Folded Strata, Coast of England
A syncline in the center, with an anticline on either side
Folded structures. An upfold, in which the strata dip away from a line drawn along the crest and called the axis of the fold, is known as an anticline ([Fig. 169]). A downfold, where the strata dip from either side toward the axis of the trough, is called a syncline ([Fig. 170]). There is sometimes seen a downward bend in horizontal or gently inclined strata, by which they descend to a lower level. Such a single flexure is a monocline ([Fig. 171]).
Fig. 171. A Monocline
Degrees of folding. Folds vary in degree from broad, low swells, which can hardly be detected, to the most highly contorted and complicated structures. In symmetric folds (Figs. [169] and [180]) the dips of the rocks on each side the axis of the fold are equal. In unsymmetrical folds one limb is steeper than the other, as in the anticline in [Figure 167]. In overturned folds (Figs. [167] and [172]) one limb is inclined beyond the perpendicular. Fan folds have been so pinched that the original anticlines are left broader at the top than at the bottom ([Fig. 173]).