496 H. E. Gregory — Formation and Distribution of 



strewn along the present stream channels would probably be 

 rapidly eroded or swept entirely away into the master stream. 

 The supply of waste, on the other hand, would probably 

 decrease in amount because of the newly developed cover of 

 vegetation and the decrease in concentration of runoff result- 

 ing from desert showers. Decrease in amount and increase 

 in fineness of waste furnished at the headwaters would result 

 also in an increasing ability of streams to erode their beds and 

 grind up bowlders. The area of deposition of coarsest mate- 

 rial would, therefore, probably not be nearest the source of 

 supply but would be followed, upstream, by deposits of waste 

 distinctly finer in texture. An up warp which steepens grade 

 might produce the same result. 



Gravel at River Mouths. 



The interstratifications of river gravels with marine sedi- 

 ments is not a normal relation. Gravels are prevailingly asso- 

 ciated with young or mature topography and the seaward 

 reaches of streams, especially those of large size, are character- 

 ized by fine waste. Deltas of rivers, therefore, may contain 

 no gravels resulting from erosion within a single cycle. The 

 inauguration of a new cycle as the result of climatic or tec- 

 tonic changes may produce, at river mouths, a stratigraphic 

 series, the origin of whose various members may not be defi- 

 nitely determined on the basis of published field study. 



In this connection the deltas of vanished lakes are worthy of 

 consideration. In cases which have come under my observa- 

 tion the deposition of gravels over finer lake muds appears to 

 be a normal feature of river work conditioned by crustal 

 upwarp or its equivalent in climatic change. By analogy it 

 follows that conglomerate overlying marine sandstones and 

 shales may be of fluviatile origin. 



In a section of an abandoned lake bed in Kayenta Valley, 

 iYrizona, a stratum of gravel is embedded in lacustrine deposits, 

 which suggests that fine-grained marine sediments may both 

 overlie and underlie conglomerate of continental origin. 

 Likewise if the finer-grained beds in a series of shale, sand- 

 stone and conglomerate show evidence of fluviatile origin, 

 there is little reason to assume that the conglomerate has a 

 different mode of origin. Fans of steep gradient immediately at 

 the lake shore, whose building is conditioned by fluctuating 

 stream discharge and changing lake level, would be expected 

 to show various combinations of coarse and fine strata, both of 

 lacustrine and fluviatile origin. The same conditions doubt- 

 less exist on certain marine deltas. 



