DISSECTION OF THE COASTAL PLAIN 833 



bays which dissect the Atlantic coastal plain of North America. A 

 drowned subsequent valley or inner lowland is seen in Long Island 

 Sound, the northern edge of Long Island forming the more or less 

 ice-disturbed and moraine-covered escarpment, the inface of a 

 normal cuesta, now largely submerged. Deposits of the present 

 geologic epoch are being formed within these valleys, which were 

 cut in the partly consolidated Tertiary and Cretacic clays and sands, 

 the stratification of which is almost horizontal. The result of such 

 deposition will be that horizontally stratified modern deposits come 

 to rest upon the Cretacic or later strata of similar position which 

 form the bottoms of these eroded valleys, and that laterally they 

 will become continuous with or merge into the horizontal beds of 

 Cretacic or Tertiary age, of the valley sides and of the rewashed 

 material of which these modern strata will in part at least be com- 

 posed. Since these old drowned valleys have in places a width of 

 a score of miles or more, and since it is not at all unlikely that 

 conditions like the present one may have existed at diiferent stages 

 in the formation of the Atlantic coastal plain, of North America — 

 not to mention earlier coastal plains of this and other countries — 

 the significance of these facts becomes apparent. The commingling 

 of the older and newer organic remains is another feature charac- 

 terizing such deposits. Thus, in Chesapeake Bay, modern oyster 

 shells are found attached to oyster shells of Miocenic age. 



Effect of dissection and pcncplanation of coastal plain strata on 

 outcrop. Where normal deposition with continuous subsidence and 

 progressive overlapping of strata occurs the undissected coastal plain 

 will show on elevation the highest stratum only, which then rests 

 directly by overlap against the old land. The formation of' the 

 inner lowland results in the exposure of a belt of lower strata next 

 to the old land, while the edge of the higher stratum is farther and 

 farther removed from the old land. As the inner lowland is wid- 

 ened and deepened, lower strata appear by erosion of the over- 

 lapping ones, and the map of a strongly dissected coastal plain 

 region will show several belts of strata next to the old land, the 

 lowest exposed one being nearest it. These belts of strata will also 

 appear on the banks of the consequent streams, but will progres- 

 sively disappear below the valley bottoms in a seaward direction 

 and from the lowest to the highest. When the coastal plain has 

 been reduced to a peneplain, the various strata composing it will 

 outcrop in a series of more or less parallel bands from the lowest 

 next to the old land to the highest of the series. This last will ap- 

 pear as a belt near the ])oint where the coastal ])lain passed beneath 

 the sea-level at the time the peneplanation was completed. Thus a 



