LAND WATERS STREAMS 143 



The above are among the commoner processes of weathering, 

 although they do not exhaust the list. The more active and 

 itangible processes by which surface rocks are broken up, such as 

 wave wear, river wear, and glacier wear, are processes of corrasion, 

 land are not usually regarded as processes of weathering. 



The importance of weathering in the general processes of 



'erosion is shown in many ways. In regions where the mantle rock 



!is the product of the decay of the solid rock beneath, and such 



regions constitute a large portion of the earth's surface, the soil and 



ubsoil represent the excess of weathering over transportation. 



Since most of the earth's surface is covered with soil and subsoil, 



t is clear that, on the whole, weathering keeps ahead of transpor- 



ation. Again, it is clear that the loosening of rock by weathering 



greatly increases the erosion which a given amount of moving 



7 ig. 108. Diagram of a valley, the top of which is ten times the width of 



the stream. 



vater can accomplish. Not only this, but weathering plays a 

 nuch more important role in the development of valleys than is 

 ommonly realized. This is illustrated by the valleys of young 

 wilt streams. The valley whose top is not ten times as wide as 

 ts stream, is rare. The stream which has such a canyon has been 

 cutting chiefly at its bottom. A swift stream would make a valley 

 is wide as itself. This is illustrated by Fig. 108. Weathering in 

 ts broadest sense is largely responsible for the width of such a 

 r alley, in so far as it exceeds the width of the stream. Weathering, 

 lope wash, etc., have widened the valley, especially in its upper 

 )art, and the stream has carried away the material which slope 

 vash, gravity, etc., brought down to it. The above illustration 

 vould not apply to old and sluggish streams, for they widen their 

 ralleys by meandering, independently of weathering. 



The central idea of the processes embraced under the term 

 ^Gathering is the loosening and the disrupting of rock, by which 



