the Middle Atlantic Slope. 465 



and while it is yet impossible for that reason to correlate the 

 records of land and sea, it will eventually be shown that the 

 broad base-level plain corresponds to an important marine for- 

 mation somewhere in the Coastal plain series. The uncon- 

 formity in deposits corresponding to the rise of land closing 

 the base-level period has not been certainly identified, but it 

 seems probable that the deep and broad estuaries of the 

 Coastal plain were then excavated, or at least deepened ; and 

 their depth suggests that for a time the land stood higher than 

 now.* 



The Columbia formation reposes upon the less elevated por- 

 tion of the Piedmont- Appalachian base-level plain and within 

 the newer gorges dissecting it, as well as upon the Coastal 

 plain and within its estuaries to considerable depths (generally 

 undetermined but known to exceed 140 feet in Chesapeake 

 Bay). It is evident from the relations of deposit to sub- 

 terrane in the Piedmont region that the deposition of the for- 

 mation occurred long posterior to the rise of the land by which 

 the old base-level was disturbed ; for despite the high declivity 

 of the stream post-Columbia erosion has not sufficed to lay 

 bare the bottom of the pre-Columbia gorge or to remove more 

 than half or two-thirds of the Columbia deposits in the Sus- 

 quehanna and Delaware Valleys ; and the post-Columbia ero- 

 sion of the Potomac is measured by a gorge but 15 miles long, 

 half a mile wide, and 75 feet in average depth, while the post- 

 base-level erosion is represented by an outer gorge more than 

 200 miles long, over a mile in width, and fully 200 feet in 

 average depth, and by corresponding gorges extending to the 

 very sources of all its tributaries. Indeed, when post-glacial 

 erosion is measured in yards and post-Columbia erosion in rods, 

 post-baselevel erosion must be measured in furlongs if not in 

 miles. 



So the direct record of the Columbia formation goes back to 

 an era 3, 5, or 10 times as remote as that to which the Quater- 

 nary has commonly been carried, while its indirect record ex- 

 tends far into the Tertiary and affords part of the data required 

 for equilibrating Tertiary and Quaternary time — the data from 

 the deposits being yet lacking. 



* It should be noted that, as indicated by rapid corrasion on the one hand and 

 the failure of equally rapid deposition to fill the estuaries on the other hand, the 

 Piedmont region is now rising, while the Coastal plain is sinking — the displace- 

 ment coinciding with the fall line ; that this movement has been in progress since 

 the Columbia period at least; and that in consequence the records of continental 

 oscillation found on opposite sides of the fall line are inconsistent. There is evi- 

 dence, too. that the hydrography of the Coastal Plain, and especially the deflection of 

 the rivers at the fall line, was determined during the Columbia period ; and hence 

 that the estuaries were cut not by the rivers which occupy them but by those 

 which more nearly coincide with their courses. 



