MARYLAND GEOLOGICAL SURVEY 47 



McGee concluded that these beds, since their deposition, had been 

 raised and tilted so that they now lie higher in the regions to the 

 north than they do further south. Their present elevation was found 

 to be about 500 feet along the gorge of the Susquehanna and 245 feet 

 at its mouth; 400 feet on the upper Delaware; 145 feet on the Poto- 

 mac; 125 feet on the Rappahannock; 100 feet on the James and 75 

 feet on the Roanoke. A series of well-defined terraces distributed 

 over the entire region was also noted. Professor McGee also pub- 

 lished an interesting paper on " The Geology of the Head of Chesa- 

 peake Bay/' in which he called attention to some of the more striking 

 features of the Columbia formation within Cecil county. 



Mr. 1ST. H. Darton took up the work where McGee left it. The 

 Columbia was found to be divisible into an earlier and a later mem- 

 ber, which were developed in well-defined terraces, the former lying 

 normally above the latter. The land surface upon which the Colum- 

 bia was deposited had been raised and tilted at various times in such 

 a manner that only in that part of the Coastal Plain which lies near 

 the Piedmont was the normal sequence present, while in that portion 

 bordering on Chesapeake Bay the normal sequence was reversed. 

 This state of things was brought about in the following way: At the 

 close of the Lafayette deposition, the surface on which that formation 

 rested was raised and tilted so as to slope eastward toward the sea. 

 Later, after suffering considerable erosion, it was depressed in such a 

 manner that its eastern portion was submerged while its western 

 margin bordering the Piedmont Plateau remained above water. In 

 the estuaries thus formed and along the coast, the earlier Columbia 

 formation was then deposited. This formation, therefore, built up 

 a terrace below that of the Lafayette in the heads of the estuaries 

 near the Piedmont, but covered up the Lafayette surface where it was 

 submerged to the east. "While the deposition of the earlier Columbia 

 was still in progress, the Coastal Plain again tilted so as to bring that 

 portion of it lying to the northeast and against the Piedmont above 

 water, while the southeastern portion was still further depressed. 

 The later Columbia was in its turn deposited in the estuaries beneath 

 the earlier Columbia where the latter had been elevated, and above 



