OVERLAP OF RIVER DEPOSITS 



597 



acter. or one composed of a heterogeneous aggregate. On the 

 whole, destruction of all but the resistant quartz seems more as- 

 sured on the flood plain of large rivers than on the seashore; and 

 certainly the wide transport of such material is more readily ef- 

 fected by streams. 



The sorting action of streams on sands has already been dis- 

 cussed. (Chapter V, p. 252.) 



Overlap Relations of River Deposits. In a growing subaerial 

 river delta, the later formed portions will progressively overlap 

 the earlier formed parts, coming to rest upon the basement beds, 

 beyond the margin of the older beds of the delta. Since, however, 

 this can be recognized only as an overlap of formations of definite 

 chronologic value, and not an overlap of continuous beds, this sub- 

 ject is better discussed after the simpler type of marine overlap 

 has been described. The general theoretical relationship is ex- 

 pressed by the following diagram (Fig. 124), from which it will 

 be seen that the principal overlap is away from the source of supply. 

 There may, of course, be a slight headward overlap due to aggrada- 



FiG. 124. Diagram showing normal non-marine progressive overlap, each 

 later stratum resting upon the old land surface beyond the edge 

 of the preceding one. A slight headward overlap is also indi- 

 cated. 



tion of the upper part of the river channels as the grade is lowered. 

 This, as a rule, will be slight and local as compared with the over- 

 lap at the other end, though it is not to be denied that at times 

 such a headward overlap may be considerable. 



Flood Plains of Glacial Streams. These are of exceptional 

 character owing to the abundant supply of detritus as well as water 

 from the melting ice. As already pointed out in Chapter III, p. 136, 

 such streams when overloaded will form extensive deposits in their 

 valleys, aggrading these to a considerable depth. Subsequent change 

 in the character of the stream, either from diminution of supply, 



