PLEISTOCENE RIVER-DEPOSITS. 



131 



while the similar deposits at lower levels were accumulated 

 after the valleys had been deepened to a greater extent. Pro- 

 fessor Prestwich illustrates his theory of the origin of the 

 gravels and their accompanying flood-loams by a diagram which 



Fig. 4. — Diagram representing one side of a valley, with a series 

 of gravel- and loss-beds. (Prestwich.) 



I borrow from his paper (Fig. 4). The diagram is meant to 

 represent one side of a valley with a series of gravel- and brick- 

 earth-beds. When the river flowed on the level d it formed the 

 gravel-bed indicated by that letter. During flood-seasons, how- 

 ever, when the water reached to the dotted line 0, its loamy mud 

 was deposited at h. Lower down the slope, that is to say between 

 the loamy mud I and the gravel-bed d, the scour of the river 

 would prevent any of the finer sediment accumulating. By 

 and by the river excavated its channel to a greater depth, and 

 flowed at the level c, where of course another bed of gravel was 

 formed. Being still subject to floods, it is evident that when 

 such was the case, and the surface of the water rose to the dotted 

 line n, a second deposit of loam (b f ) would be laid down upon 

 the old river-bed d, and would slope up against the side of the 

 valley to the level of the line n. The river still continuing to 

 deepen its channel, a time would come when it would be flow- 

 ing on the level d ', and when loam would be deposited at b" on 

 each occasion that the flood- waters rose to m, or to any point 

 between c and m. 



Thus we see how a deposit of loam would eventually come 

 to be spread over all the gravels from the highest to the lowest 

 levels. It may be as well, however, to warn the reader that he 



