156 MOUNTAIN AND MOORLAND 



by the air, the rain, the frost, and other carving tools 

 is unequal. Some parts stand out more than others, 

 and the rain begins to follow particular lines which 

 twist about seeking the paths of least resistance. 

 These lines may, indeed, be determined in some 

 measure by original ups and downs on the crust, or 

 by a line of weakness such as a fault; but it seems 

 that most valleys began in a commonplace way, such 

 as we may often see mimicked when the water of a 

 pool on a gently sloping sandy shore flows out after 

 the receding tide, or when a heavy fall of rain makes 

 a flood on soft soil. It seems, then, that the beginning 

 of the carving out of valleys is due to the atmospheric 

 weathering agencies operating on a crust which is 

 more resistant in some parts than in others. 



When a valley has got started it is deepened and 

 widened in various ways. The water of the stream 

 may dissolve the rocks over which it flows, or it may 

 carve at the bed and the sides by means of the pebbles 

 and sand it rolls along. It may undercut the banks so 

 that they fall in, or it may undercut below a waterfall 

 so that the harder rock above comes tumbling down. 

 " The water wears the stones " according to the adage, 

 but it is more correct to say that the water's tools are 

 the stones, large and small, which it is able to move. 



At its head a stream usually tends to cut farther and 

 farther back into the hill. The stream's "curve of 

 erosion," as it is called, is steepest there, and the 

 rivulets made by the heavy rain hurry downwards, 

 carrying the weathered-out pebbles with them and 

 carving as they go. The rivulets cut backwards into 

 the hill and they enable the stream to tap fresh 

 springs, beyond those with which it started. Springs 



