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.
Fig. 168. Dip and Strike
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.
Fig. 169. An Anticline, Maryland
The inclination of a bed of rocks to the horizon is its dip ([Fig. 168]). 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°. 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° north, or dip 8° south 20° west,—meaning in the latter case that the amount of the dip is 8° and the direction of the dip bears 20° west of south.