RIVER DEPOSITS 401 



of deposits which owe their origin and characteristics not so much to the 

 ordinary forces active in marine sedimentation, but rather to the domi- 

 nance of the influence of water on land. Most noteworthy among these 

 are the deposits formed by rivers, those formed by lakes or other standing 

 water bodies being of secondary and limited significance. 



River deposits may be divided into (1) interstream deposits, or those 

 formed along the course of the stream — that is, on its flood plain — and 

 (2) terminal deposits, or those formed at its debouchure either at the 

 foot of the mountains or at its junction with the sea or a lake. Consider- 

 ing only the last type, we find that in the first case a dry delta or alluvial 

 fan or cone results, while in the second a delta of the ordinary type, 

 partly subaerial and partly subaqueous, is formed. The general charac- 

 teristics of these two types of deposits when formed by modern streams 

 are well known and have been fully described. 2 They may therefore be 

 passed over briefly : 



CHARACTERS OF THE ALLUVIAL FAN 



The essential difference between the alluvial fan and the delta lies in 

 the simple character of the first as compared with the complex one of the 

 second. The beds of the alluvial fan will be essentially of one kind — 

 that is, topset beds — though they may vary in texture, thickness, con- 

 tinuity, and other characters. These beds are chiefly of subaerial origin, 

 though the fan's foot may dip into the sea or a body of standing water. 

 The increase in size will be a semi-radial one, though showing more or 

 less irregularity according as one or the other of the distributaries will 

 build faster than its neighbors. In grain the individual beds will vary 

 from coarsest along the distributaries, especially near the head of the fan, 

 to finer in the spaces between the distributaries and finest as a whole near 

 the periphery of the fan. Owing to the frequent shifting of the dis- 

 tributaries, these features will vary in harmony with such shifting and 

 the variation will increase in magnitude as the alluvial Tan grows. 



CURRENT BEDDING OF THE ALLUVIAL /■' A \ DEPOSITS 



Two kinds of current bedding may be observed in alluvial fan deposits: 

 (a) compound oblique bedding and (b) complex cross-bedding. The 

 former is characterized by a succession of oblique layers, all of them in- 

 clining in the same direction, at approximately uniform angles, and sepa- 

 rated by horizontal beds of less thickness (figure 1). This type of cross- 



- Herman Credner : Die Delten. Petermano Geographlsche Mltthellungen. ElrgHo m 

 ii.'ti No. 56, pp. 1-74, :s tafeln. 1878. 



Joseph Barrell: Bull. Geol. Boc. Am., vol. in:, pp. -tt 146. 1912, 



XXVIII— Bull. Qrol. soc. am., vol. U4, 1912 



