=Adjustment of streams to rock structures.=—Valleys (gullies) locate themselves at the outset without immediate regard to the hardness and softness of their beds. It is primarily the slope about the head of a gully which determines its line of growth, though relative hardness often determines the details of slope, even in the early stages of an erosion cycle. Once established, streams tend to hold their courses, even if this involves the crossing of resistant layers. While a region where more and less resistant layers of rock come to the surface is in a youthful stage of erosion, some of the valleys (and therefore the streams) are likely to be located on the less resistant rock, some on the more resistant, and some partly on the one and partly on the other. The streams on the weaker rock will deepen their valleys more rapidly than the others, and those which flow across stronger and weaker rocks alternately will deepen their valleys more rapidly than those which run on hard rock all the time. The former conclusion is self-evident. The latter appears from the fact that rapids will be likely to develop at the crossing of each hard layer, thus accelerating erosion at those points. Such a stream therefore not only has less hard rock to erode than one which flows on resistant rock all the time, but it erodes that which it does cross much faster. [Illustration: +Figs.+ 131, 132.—The capture of the head of Beaverdam Creek by the Shenandoah Va.-W. Va. (After Willis.)] Streams which do not cross hard layers therefore have an advantage over those which do, and the tributaries to such streams, since they join deeper mains, have an advantage over the tributaries to the others. The valleys of the former may lengthen until their heads reach the latter, and capture their streams. This sequence of events is illustrated in the accompanying diagrams (Figs. 125–27). Even where several streams cross the same resistant bed, piracy is likely to take place among them, for some are sure to deepen their valleys faster than others, because of inequalities of volume, load, or hardness. This is illustrated by Figs. 128–30. An actual case is shown in Figs. 131, 132. Though piracy may take place when streams do not flow over rock of unequal hardness (p. 103), it is much more common where unequal resistance of the rock puts one stream at a disadvantage as compared with another.