386 W. J. McGee — Three Formations of 



v 



Lyell, Mather, the Rogers brothers, Tuomey, Desor, Conrad, 

 Booth, Tyson, Kerr, Fontaine, Cook, Lewis, Chester, Merrill, 

 Britton, and others, has recorded the conviction that such part 

 at least was waterlaid. The physiographic relations of the 

 phenomena and the area over which they prevail are such that 

 the evidence of sub-aqueous origin is cumulative, and now 

 that definite observations have been extended over the greater 

 part of the area it can only be regarded as decisive ; the coarse- 

 ness of the northern deposits as compared with the southern, 

 and the occasional presence of evidently ice-dropped bowlders 

 indicates that the period of submergence was one of refrigera- 

 tion ; and the limited volume of the formation indicates that 

 the period of deposition was short. 



So the interfluvial deposits corroborate and extend the testi- 

 mony of the deltas; and the phenomena conjointly record a 

 brief period of submergence of the entire Coastal plain in 

 the Middle Atlantic slope reaching 100 feet in the south and 

 over 400 feet in the north, with coeval cold, long anterior to 

 the terminal moraine period. 



The Terraces and Shore Lines — Every Middle Atlantic 

 slope river embouches through a narrow gorge in the Pied- 

 mont escarpment into a widely flaring shallow valley partly 

 occupied by the estuarine portion of the river ; and each of 

 these valleys is notably terraced. The city of Weldon is lo- 

 cated on a broad terrace of the Roanoke sixty feet above its 

 tidal waters ; Petersburg is built upon two or three distinct 

 terraces flanking the Appomattox, of which one is 110 feet in 

 altitude and many miles in extent ; the higher portions of Rich- 

 mond, including the public park, are located upon an extensive 

 terrace overlooking James river from a height of 180 feet above 

 tide on the north, while another terrace eighty feet in altitude 

 has an area of at least twenty-five square miles on the south 

 side of the river; the terraces on the Rappahannock at Freder- 

 icksburg are as distinctive and more extensive than those of the 

 lacustral basins of Bonneville and Lahontan, as are also those 

 about Washington ; the valleys of the Patuxent and Patapsco, 

 of the Susquehanna, and of the Brandywine and Schuylkill, 

 exhibit wide reaching series of terraces of progressively in- 

 creasing altitude ; at Trenton the Delaware has cut a narrow 

 gap in an otherwise continuous terrace of brick clay and gravel 

 of great extent and uniformity, and through that gap the 

 Trenton gravels have been swept ; and at Metuchen, N. J., 

 wonderfully broad terraces extend to the very base of the 

 hillocky terminal moraine, which has evidently been pushed 

 out upon their plains. 



While the best developed terraces about the mouths of the 

 rivers form independent systems, the higher members fre- 



