406 A. W. GRABAU PALEOZOIC DELTA DEPOSITS OF NORTH AMERICA 



the growth of the delta, and the consequent pushing upstream of the zone 

 of first deposition. Such an overlap is, however, likely to be far less ex- 

 tensive and much more rapid than the overlap in the other direction, and 

 is not likely to be confused with the marine type of overlap which is also 

 toward the source of supply, but characterizes an extended sea -border 

 belt rather than the contracted strip of a river floodplain. 



When a delta largely of the subaerial type is building into the sea 

 which it gradually replaces, the non-marine layers will regularly merge 

 into a marine extension and a replacing overlap will result. As the con- 

 tinental layers extend outward, the marine layers will be shortened, and 

 as a result the two series will be separated by an upward-slanting divid- 

 ing line. Such a replacement may also occur where a submarine deposit 

 of terrigenous material, the seaward extension of a delta, gradually 

 spreads over one of thalassigenous or ocean-derived material, as in the 

 case of the Hudson beds of Normanskill, Schenectady, and Utica age, 

 which gradually replace the successive members of the Black Eiver- 

 Trenton limestone series from the east, the source of supply of the terrig- 

 enous material, westward. 



COARSENESS OF DELTA DEPOSITS 



This may vary from conglomerates and breccias, or even accumulations 

 of large boulders, to the finest lutytes or mud deposits. The coarse con- 

 glomerate is more characteristic of the subaerial part of the delta and the 

 fine mud of the subaqueous, but there is no especial restriction in the dis- 

 tribution of these deposits. Thus the alluvial fans of the Huang Ho and of 

 the Ganges and Indus are characterized by material nearly or quite as 

 fine as that generally accumulating in standing water, while on the other 

 hand coarse marine deposits are not uncommon. On the whole, the text- 

 ure of the material increases in coarseness toward the head of the delta 

 or the fan, this being more or less coincident with the increase in the 

 thickness of the deposit. 



Examples of ancient Delta Deposits from the Ordovictc and 



Silueic Conglomerates and Sandstones of the 



Appalachian Mountain Region 



in general 



Among the many deposits of America and Europe which by their 

 character and distribution suggest that they represent delta deposits of 

 one or the other type above noted, only a few American examples can be 

 selected and described in detail. Those will be chosen which, by their 



