The beds of stratified rock may be horizontal, vertical, or inclined, and inclined strata may stand at any angle between horizontality and verticality. In indurated formations the rate of erosion is influenced both by the position of the strata and by the relation of the direction of the flowing water to their dip and strike. On the whole the strata which are horizontal, or but slightly inclined, are probably less favorable for rapid erosion than those which are vertical or inclined at considerable angles. This is at least true where the layers are of uniform hardness and the joints infrequent. Horizontal strata expose fewer cleavage planes to the water flowing over them than strata in any other position. In Fig. 103 the stream which has the profile ad crosses bedding-planes at _b_ and _c_. In Fig. 104, where the beds dip up-stream, many more division-planes are crossed in the same distance. Since bedding-planes are planes of weakness, it follows that horizontal and nearly horizontal strata are not, under ordinary conditions of erosion, in a position favorable for most rapid wear. When strata are horizontal, it makes no difference which way the stream runs, for the current sustains the same relation to the cleavage-planes whatever its course. In the case of incoherent material the position of the beds, or even their existence, has little influence on the rate of erosion. Such formations are weak in all directions, not simply along bedding-planes. [Illustration: +Fig.+ 105.—Diagram to illustrate the various relations a stream may sustain to the outcrop of vertical layers of rock.] [Illustration: +Fig.+ 106.—Diagram to illustrate the various relations a stream may sustain to the outcrops of inclined 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.