LAND WATERS STREAMS 



161 



and softness of their beds. It is primarily the slope about the head 

 of a gully which determines its line of growth, and, once established, 

 streams tend to hold their courses. The streams on the weaker 

 rock will deepen their valleys more rapidly than others, and those 

 which flow across stronger and weaker rocks alternately, will 



Fig. 121. A monadnock; a mass of igneous rock isolated by erosion, and 

 remaining high because of its superior hardness. Matteo Tepee, Wyo. 

 (Detroit Photo. Co.) 



deepen their valleys more rapidly than those which run on hard 

 rock all the time. 



The streams on the weaker rocks, therefore, have an advantage 

 over the others. Being deeper, their tributaries may be lengthened 

 until their heads reach the other valleys, with the results shown in 

 Figs. 122-124. 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. 125-127. An 

 actual case is shown in Figs. 87 and 88. Though piracy may take 

 place where streams do not flow over rock of unequal resistance, 



