THE WORK OF RUNNING WATER. 



185 



dering the stream encroaches on the talus accumulations at the bases 

 of its valley's slopes. The side-cutting may remove all the loose debris 

 and even undercut the bluff as at a, Fig. 173. The stream's meanders 

 shift their positions from time to time so that the valley flat is succes- 

 sively widened at different points. By lateral planation, therefore, a 

 stream tends to develop a flat as soon as it reaches grade. This is the 

 initial part of erosion in the making of a river flat, but a flat developed 

 by erosion alone is not an alluvial plain. 



So soon as the flat developed by a stream exceeds the width of 

 its channel, the water (except in times of flood) does not cover it all 

 at the same time. On any part which it temporarily abandons, some 

 debris (alluvium) is Hkely to be left. This deposit of alluvium con- 



FiG. 172. — Diagram illustrating the transformation of a V-shaped valley into a U- 



shaped valley. 



stitutes the valley flat an alluvial plain (Fig. 174). It will be seen 

 that the valley flat is commonly an alluvial plain from the beginning. 



Fig. 173. — Diagram to illustrate the widening of a valley fiat by erosion. Compare 3, 



Fig. 172. 



Once the valley flat and alluvial plain are begun, their further 

 development is easily followed. The stream in flood overflows the 

 banks of its channel. The velocity of the overflowing water is reduced, 



Fig. 174. — An alluvial plain. The diagram suggests the relative importance of lat- 

 eral planation and alluviation in the development of the flat. 



and if it has much load a part of it will be dropped and the plain ag- 

 graded. Meantime meandering and lateral planation continue. Thus 

 the flood-plain is widened by erosion, and aggraded by alluviation, 

 the two processes going on simultaneously. 



