The line of intersection formed by the dipping bed with the plane of the horizon is called the line of strike and is necessarily at right angles to the line of dip. (See Fig. 154.) If a piece of slate be held in an inclined position and lowered into a vessel of water, the wet line will represent the strike. As long as the direction of the dip remains constant, the line of strike is straight, but as the direction of the dip changes, the strike changes also, always keeping at right angles to the dip, and in such cases as the Appalachian Mountains the lines of strike are sweeping curves.
Fig. 154. - Model of anticline. P, axis pitching to the left; S S, line of strike; D, line of dip. The dotted line is the plane of the axis. (Willis).
Outcrop is the line along which a dipping bed cuts the surface of the ground, and is, of course, due to erosion, which has truncated the folds of strata. Except in the case of fractured beds, which will be considered in the following section, if there were no erosion, there could be no outcrop. When the surface of the ground is level, outcrop and strike become coincident, because the surface then is practically a horizontal plane. With the dip remaining constant, the more rugged and broken the surface becomes, the more widely do strike and outcrop diverge. For a given form of surface, outcrop and strike differ more when the beds dip at a low angle than when the dip is steep, for when the strata are vertical, outcrop and strike.again coincide, and the more nearly the strata approach verticality, the more closely do the two lines come together.
Having digressed to make these necessary definitions, we may now return to the subject of folds.