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. [Illustration: 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. _The Influence of Climate._ Climate has both a direct and an indirect effect on erosion. Its direct influence is through precipitation, evaporation, changes of temperature, and wind; its indirect, through vegetation. Like declivity and rock structure, climate does not affect all elements of erosion equally. The chief elements of climate are temperature, moisture, and atmospheric movements; the principal factors which influence it are latitude, altitude, distance from the sea, direction of prevailing winds, and topographic relations. The effects of variations in temperature on rock weathering have already been discussed (p. 43). They are chiefly mechanical, and are seen at their best where the daily range is great.