628 A. W. GRABAU TYPES OF SEDIMENTARY OVERLAP 



the streams from the mountains and deposited on the plains adjoining. 

 Such a subaerial fan will, of course, grow year by year ; and in so growing 

 the latest deposits, whether derived from the mountains or whether 

 obtained through the reworking of the previously deposited portion, will, 

 as a rule, extend farther out on the plain than did the deposits of previous 

 periods. In other words, each later formation will overlap the previous 

 ones by a margin commensurate with the increase in the size of the fan, 

 and beyond the margin of the previously formed bed it will come to rest 

 directly on the floor of the plain. This overlapping of later formed over 

 earlier beds will, of course, be progressive, if the growth of the fan is 

 continuous. The essential point of difference between this type of overlap 

 and that formed in a transgressing sea is that in the subaerial fan the 

 formations will overlap one another in the direction away from the source 

 of supply of the material; while in marine progressive overlap (trans- 

 gressive) the overlap is toward the source of supply of the material. The 

 following diagrams will illustrate this difference, the source of supply in 

 each case being on the left. 



Source of Supf/;, Sou rce of Ju/>/>/</ 



Figure 10. — Non-marine progressive Overlap. Figure 11. — Marine progressive Overlap. 



The coarsest material of the subaerial fan will, of course, be deposited 

 near the head of the delta. Finer material may be carried out for 

 hundreds of miles across such a delta, as is plainly shown by the delta- 

 plains of the Indus, Ganges, and Yellow rivers. Occasionally pebbles 

 well rounded may be carried out to great distances, and this is especially 

 true of the well rounded pebbles derived from older conglomerates. When 

 the surface of the delta has become very flat, drainage obstructions may 

 take place, in which case swamps and deposits of carbonized plant remains 

 will form. Thus a fossil delta of this type may include coal seams, the 

 tops of which may again be eroded or covered with a moderately coarse 

 river deposit. 



Another type of non-marine overlap is that connected with a retreating 

 seashore, in which case the overlapping of the non-marine beds will be, 

 not on the old plain surface, but on previously deposited and all but con- 

 temporaneous marine beds. Along the border line the two, marine and 

 non-marine, will blend, and it will appear as if the non-marine overlies 



