CENOZOIC HISTORY. 581 



edge of the Chesapeake formation and many outliers of 

 Potomac formation. It was from the lower and marginal 

 beds of the Potomac formation that much of its material was 

 derived. 



Lafayette deposition was followed by relatively rapid uplift,, 

 during which there were carved the wide steep-sided troughs of 

 the river valleys and a wide shelf along the coast to the east and 

 north. It was at this time that the present topography of the 

 Coastal Plain region was outlined for, in previous emergences, 

 there had been only a general planing. It is thought that the 

 irregular course of the rivers across the Coastal Plain, notably 

 the southerly deflections of portions of the Delaware and Poto- 

 mac rivers and the head of Chesapeake Bay and the local drain- 

 age relations, were due to the original contour of the surface of 

 the Lafayette deposits. This surface had a peninsular configura- 

 tion similar to the submarine sand bars now existent along the 

 ocean coast, and, with uplift, the "sloughs" determined the loca- 

 tion of the water ways. The deflected courses of the rivers are 

 not related to any orogenic influence so far discovered, as sug- 

 gested by McGee, nor to the texture of the deposits they 

 traverse. Post-Lafayette emergence was greatest in amount 

 to the northward, and in northern Maryland, Delaware, and 

 New Jersey, the formation was widely eroded, together with 

 the underlying formations. All of this region was planed 

 to a terrace level, and a wide depression was excavated along 

 the western margin of the Coastal Plain region from Baltimore 

 to Washington. Wide valleys and series of terrace plains were 

 cut in the Piedmont region, especially in the areas of softer rocks 

 in the Jura-Trias and limestone valleys adjoining the Susquehanna 

 and Potomac depressions. 



In figure 3 there are given a series of sections which illus- 

 trate the conditions at a number of periods following the 

 Lafayette uplift. 



In the second section in this figure is shown the relative 

 amount of the erosion during the uplift following Lafayette 

 deposition, along a line passing near the latitude of Washington. 



