184 GEOLOGY. 



coarser. These materials are largely deposited at the bases of the 

 slopes, forming basal accumulations of greater or less extent, com- 

 parable in origin to alluvial fans. A relatively small amount of the 

 slope wash is carried far out from the base of the declivities. It is 

 not easy to realize the extent to which this process is taking place. 

 There is hardly a slope without loose material, and there is hardly an 

 acre of low land below a slope on which running water has not 

 deposited sediment washed down from above. When it is remembered 

 that this is as true of gentle slopes and their surroundings as of steep 

 slopes, though perhaps not to the same extent, and that a very large 

 part of the earth's surface is made up of sensible slopes, or of flats at 

 their bases, some idea of the aggregate effect may be gained. 



There is another way of looking at the same question. Earthy 

 matter is being continually transferred from land to sea, and chiefly 

 from high land. Rarely does it start from any point distant from 

 the shore and move uninterruptedly to it. It is transported a short 

 distance and lodged, to be again picked up, carried forward another 

 step in its journey, and lodged again. For a very large part of the earth's 

 surface it would be true to say that its mantle rock is material in transit 

 from higher land to the sea. 



Alluvial plains. — Most streams, whether heading in mountains 

 or not, have gentler gradients in their lower courses than in their upper, 

 and in spite of increasing volume are usually unable to carry to their 

 debouchures all the material gathered above. The excess of load is 

 dropped chiefly on the flood-plains of the streams and constitutes 

 them alluvial plains. 



The making of an alluvial plain usually involves both erosion and 

 deposition. When a stream has cut its channel to grade, downward 

 erosion ceases, or more exactly, downward cutting is, on the average, 

 counterbalanced by deposition. So long as a stream is cutting down- 

 ward rapidly, it carries away whatever debris descends the side slopes. 

 When it approaches grade, the debris which descends the side slopes 

 tends to accumulate at their bases, and the V-shaped cross-section 

 of the valley becomes U-shaped (see Fig. 172). At about the same time 

 the stream begins to meander, for, having lost something of its former 

 velocity, it is more easily turned from side to side. As it begins to 

 meander, it widens the bottom of its valley. This is the initial stage 

 in the development of the valley flat (2 and 3, Fig. 172). In its mean- 



