FOLDINGS OF THE CRUST
The oscillations which we have just described leave the strata not far from their original horizontal attitude. Figure 167 represents a region in which movements of a very different nature have taken place. Here, on either side of the valley V, we find outcrops of layers tilted at high angles. Sections along the ridge r show that it is composed of layers which slant inward from either side. In places the outcropping strata stand nearly on edge, and on the right of the valley they are quite overturned; a shale SH has come to overlie a limestone LM although the shale is the older rock, whose original position was beneath the limestone.
It is not reasonable to suppose that these rocks were deposited in the attitude in which we find them now; we must believe that, like other stratified rocks, they were outspread in nearly level sheets upon the ocean floor. Since that time they must have been deformed. Layers of solid rock several miles in thickness have been crumpled and folded like soft wax in the hand, and a vast denudation has worn away the upper portions of the folds, in part represented in our section by dotted lines.
Dip and strike. In districts where the strata have been disturbed it is desirable to record their attitude. This is most easily done by taking the angle at which the strata are inclined and the compass direction in which they slant. It is also convenient to record the direction in which the outcrop of the strata trends across the country.
The inclination of a bed of rocks to the horizon is its dip. The amount of the dip is the angle made with a horizontal plane. The dip of a horizontal layer is zero, and that of a vertical layer is 90 degrees. The direction of the dip is taken with the compass. Thus a geologist’s notebook in describing the attitude of outcropping strata contains many such entries as these: dip 32 degrees north, or dip 8 degrees south 20 degrees west,—meaning in the latter case that the amount of the dip is 8 degrees and the direction of the dip bears 20 degrees west of south.
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.
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. A downfold, where the strata dip from either side toward the axis of the trough, is called a syncline. 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.