MARYLAND GEOLOGICAL SURVEY 35 



terminated by an abrupt descent to the low Pleistocene terrace bordering 

 the bay to a width of several miles." 8 



The Columbia of McGee was found by Darton to be divisible into an 

 earlier and a later member, which were developed in well-defined terraces, 

 the former lying normally above the latter. The land surfaces upon which 

 the Lafayette and Columbia terraces were deposited were 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, and 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 estu- 

 aries 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 south- 

 eastern 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 it where it had been depressed. 

 Consequently the three formations near the Piedmont were developed in 

 separate terraces lying one above the other, the Lafayette at the top, with 

 the earlier Columbia in the middle and the later Columbia at the bottom, 

 while in the eastern submerged portion the formations were not devel- 

 oped in terraces, but in a continuous series, with an erosive break between 

 the Lafayette and the earlier Columbia. In this region the sequence 



3 Amer. Geol., vol. ix, 1891, p. 181. 



