40 SOILS: FROFMITIBS AND MANAGEMENT 



soil is a direct result of both thebe activities. The carry- 

 ing power of water varies directly as the sixth power of 

 its velocity ; so that a doubling of the velocity increases 

 the transportive ability sixty-four times. It is estimated ^ 

 that water flowing at the rate of three inches a second 

 will carry only fine clay, but if this rate is increased to 

 twenty-four inches a second, pebbles the size of an egg 

 wall be moved along the stream bed. Any checking of 

 the velocity of a stream will cause it to deposit the ma- 

 terial carried in suspension, the larger particles first and 

 the finest when the current becomes very sluggish. This 

 brings about one of the important characteristics of an 

 alluvial soil, its stratification. Wherever material is 

 being laid dow^n by w^ater this phenomenon is exhibited, 

 due to the rapid changes in velocity. As a stream ap- 

 proaches nearer and nearer to its outlet, its bed becomes 

 less and less inclined and the current more and more 

 sluggish. This tends toward an aggrading of the stream 

 bottom from the deposited material. Such a condition 

 naturally increases the probability of overflow in high 

 water. Overflow at a time when the stream is carrying 

 its maximum of sediment causes the deposition of a thin 

 layer of soil over the areas covered by the water. This 

 soil is stratified according to the conditions under which 

 it was laid dowTi, the finer particles usually being carried 

 farther and often deposited in slack water or lagoons. 

 Also, a stream on a gently inclined bed may begin to 

 swing from side to side in long, gentle curves, due to the 

 deposition of alluvial material on the inside of the curve 

 and the cutting by the current on the opposite bank. 

 This results in oxbows, lagoons, and similar inclosures, 



iGeikie,A. Text Book of Geology, p. 380. New York. 1893. 



