RIVER-LAID CLASTICS 583 



a. Alluvial fans. 



b. Flood plains. 



c. The playa — a temporary expanse of certain rivers. 



Considering" the classes of streams mentioned in Chapter III 

 (p. 129),. it may be noted that the deposits of consequents and inse- 

 quents are essentially alike, those of overflow streams may be 

 neglected as insignificant, but deposits of glacial streams, from their 

 abundant supply, are of the greatest importance. Only the residual 

 clays of subterranean streams need to be considered. 



a. Alluvial Fans. Wherever the debris-laden stream leaves 

 its steep mountain bed and debouches upon the piedmont belt, or 

 the floor of a large valley, it changes from a degrading stream, or 

 one just at grade, to an aggrading stream, since the change in the 

 angle of the river bed brings with it decreasing velocity of the 

 current. In its steeper development the alluvial fan grades directly 

 into the talus and other atmoclastic deposits with which, indeed, it 

 forms a continuing sedimentary series, the river portion often being 

 difficult to separate from the purely atmoclastic type. 



Form and Extent of Modern Alluvial Fans. In extent alluvial 

 fans vary from an area of only a few square feet to one covering 

 thousands of square miles. Small alluvial fans are best described 

 as semi-cones with a surface slope which may be as high as 20° 

 or even 30°. As the fan increases in size its surface angle is 

 lowered, until, in the very large deposits of this type, the surface 

 seems almost to be horizontal. Small cones are seen to rise regu- 

 larly toward the notch in the hills through which the river de- 

 bouches, but in the very large alluvial fans there may result a con- 

 fluence of many adjoining deposits, which will obliterate the effects 

 of regularity. On all large fans, whether simple or confluent, ero- 

 sion channels abound, for the streams building the deposits divide 

 near the head of each fan into numerous distributaries, each of 

 which, when not depositing, will be eroding. Thus a succession of 

 contemporaneous erosion surfaces will result, and later beds will 

 be deposited on the eroded surfaces of the older. In this manner 

 the effect of a disconformity may be repeatedly produced within 

 the depositional unit, and such apparent disconformities might lead 

 to grave misinterpretations of the age and relationships of the ad- 

 joining beds. Should, by subsidence, the sea cover such an alluvial 

 fan of great extent, a decided break would appear between the non- 

 marine and marine strata, the latter gradually encroaching by over- 

 lap upon the eroded surface of the old subaerial fan. 



Among the large alluvial fans or dry deltas of modern times 



