NONMARINE TRANSGRESSIVE OVERLAP 741 



out for hundreds of miles across such a delta, as is abundantly 

 shown by the delta plains of the Indus and Ganges and of the 

 Huang-ho River. Occasionally pebbles well rounded may be car- 

 ried out to great distances, and this is especially true of the well- 

 rounded pebbles derived from older conglomerates. When the sur- 

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

 take place, in which case swamps and the deposit of carbonized 

 plant remains will form. Thus a fossil delta of this type may in- 

 clude coal seams, the tops of which may again be eroded, or covered 

 with a moderately coarse river deposit. 



An essential feature of this type of overlap in purely nonmarine 

 sediments is the relation which the beds have to the surface on 

 which the overlapping edges rest. If this is an old land surface, 



Img. 154. Diagram showing the westward overlap of the various members of 

 the Pottsville series of eastern North America, upon the old 

 erosion surface of pre-Pottsville age; the surface of supply is in 

 the southeast. 



i. e., without contemporaneous sediments of a marine origin, the 

 evidence of the nonmarine origin of this overlapping series is com- 

 plete. For it is manifestly inconceivable that clastic sediment will 

 be carried across a body of water and be deposited upon the oppo- 

 site shore in such a relationship as to suggest marine transgressive 

 overlap. 



The Pottsville Series, a Tyi-ical Example of Nonmarine 

 Progressive Overlap. (Fig. 154.) 



This is a deposit of quartzose elastics, consisting for the most 

 part of rounded quartz pebbles embedded in a matrix of sand. In 

 the eastern area the conglomerate is coarsest, the pebbles often 

 reaching a diameter of several inches. They consist also of a 

 greater variety of material, but westward, where the pebbles are 

 smaller, scarcely any but those of quartz remain. Several distinct 

 coal beds with associated shales occur, making identification of 



