Larger relations of deltas 893 



Where rivers flowing from high or broad lands are more than able to 

 keep pace with the rising sea and build ii}) their deltas, there will result 

 the construction of continental deposits on the margins of the lands, 

 illustrated by the transgressive overlapping of the fresh-water Potomac 

 formations onto the crystalline floor of the Piedmont plain. Even local 

 and profound subsidence of a geosyncline may not be able to admit the 

 sea, provided the sediment is sufficiently abundant, as seen during the 

 Tertiary history of the Indo-Gangetic plain. Here more than 15,000 

 feet of fresh-water sediments accumulated and are now exposed through 

 becoming involved in the Himalayan mountain movements. 



In the great majority of cases, however, subsidence of the laud has 

 resulted in marine inundations and marine transgressive overlap, the 

 waves working directly against the land. This was favored when the 

 lands were low or small in area and the rivers consequently were unable 

 to dam back the rising sea. Such sediment as they have supplied, sup- 

 plemented by that from marine denudation, lias been diwstributed by 

 waves and currents in the forjn of marine deposits. 



The results of local changes in baselevel are seen in interior basins, 

 where the upper beds extend farther toward the mountains. Among 

 older deposits which have become generally accepted as continental in 

 origin may be cited the Newark formations of New Jersey. Kiimmel 

 has shown that the basal formation, the Stockton, does not reach north 

 to the limits of the basin, and he regards the coarse northern beds as 

 belonging probably entirely to the uppermost or Brunswick formation, 

 wliich in the more southern parts is normally a red shale. ^ 



Eiver grades are sensitive indicators of crustal or climatic movements. 

 Any change which causes that part of the river bed between the head- 

 waters and the mouth to fall below grade will cause the building of 

 river deposits which will give the appearance of transgressive overlap. 

 Any change which causes that part to be above grade will result in a 

 shifting downstream of the deposits and give rise to regressive overlap. 

 Illustrations are seen in the alternate filling and cutting during the 

 Pleistocene of rivers flowing across Piedmont slopes. Transgressive or 

 regressive overlap toward the source of supply consequently can not be 

 used in itself as a criterion of either the marine or continental origin of 

 deposits. The nature of these must be determined by other criteria, and 

 then the nature of the overlap takes on great significance in regard to 

 the conditions which supplied the waste. 



Overlay atcai/ from the source of supply. — Overlap away from the 

 source of supply, which Grabau regards as establishing the fluviatilc ori- 



8 Annual Report of the State Geologist of New Jersey, 1898, p. 48. 

 XXIX— Bull. Geol. Soc. Am., Vol, 23, 1911 



