Fig. 105.—Diagram to illustrate the various relations a stream may sustain to the outcrop of vertical layers of rock.
When the strata are vertical, three distinct cases may arise ([Fig. 105]). The stream may flow (1) with the strike (aa); (2) at right angles to the strike (bb); or (3) oblique to it (cc) at any angle whatsoever. It is perhaps not possible to say which of these positions is most favorable for erosion, for the character of the rock, the thickness of its layers, its ability to stand with steep slopes, and the strength of the currents concerned, would influence the result. A stream which flows at right angles to the strike (bb, [Fig. 105]) would cross more cleavage-planes in a given distance than a stream flowing in any other direction, and would strike the outcropping edges of layers at the angle of greatest advantage. A stream flowing along the strike (aa), on the other hand, has better opportunity to sink its channel on cleavage-planes, and the current oblique to the strike (cc), has some of the advantages of each of the others.
Fig. 106.—Diagram to illustrate the various relations a stream may sustain to the outcrops of inclined layers of rock.
When the strata are inclined five cases may arise. (1) The stream may be parallel to the strike (aa, [Fig. 106]), when it makes no difference which way the current flows; it may be at right angles to the strike (bb′), and (2) flowing with the dip (toward b′), or (3) against it (toward b); it may be oblique to the strike, and flowing (4) in the general direction of dip (toward c′); or (5) in the opposite direction (toward c). As before, the stream flowing at right angles to the strike would cross the largest number of layers in a given distance, and so have an opportunity to take advantage of more cleavage-planes than a stream in any other position. But in the case of inclined strata a new element enters into the problem. When the stream flows parallel to the strike, the valley which is in process of deepening is not sunk vertically, but is shifted more or less in the direction of the dip ([Fig. 107]). This is called monoclinal shifting. The result is that there is a constant tendency to undermine (sap) the valley bluff on the down-dip side, and this process of sapping will, according to its rate, accelerate the growth of the valley, especially in width. Monoclinal shifting is favored by the presence of a hard layer (H), as shown in [Fig. 107], if this stratum is the bed of the stream.
Fig. 107.—Diagram to illustrate monoclinal shifting. The valley abc, as seen in cross-section, becomes deb, as the stream lowers its channel.
In the second and third cases mentioned above, the only difference is in the angle at which the current strikes the outcropping edges of layers and laminæ. The mechanical advantage is with the stream which flows with the dip. In the fourth and fifth cases something will depend on the angle which the stream’s course makes with the strike. In all these cases, as in those where the strata are vertical, much will depend on the thickness and resistance of the layers and on the strength of the currents concerned.