32 THE PLIOCENE AND PLEISTOCENE DEPOSITS OF MARYLAND 



gravels, extended up from the south and occupied the divides and higher 

 portions of the Coastal Plain of Virginia as far north as Fredericksburg. 

 It was not recognized in Maryland. Distinct from these deposits both in 

 origin and age was another series of loams, sands, and gravels which 

 McGee designated as the Columbia formation. These filled the valleys of 

 the present streams and covered the divides between them. This forma- 

 tion was divided into two phases, fluvial and inter-fluvial. The fluvial 

 phase was composed of deltas which were deposited under water by those 

 streams in whose valleys they now lie, when the land stood lower than it 

 does at the present time. The inter-fluvial phase was developed on the 

 divides and was a littoral deposit made by the waves which beat against 

 the coast at the same time the rivers were building their deltas. The two 

 phases were, therefore, contemporaneous and graded over into one an- 

 other. The fluvial phase exhibited a distinct bi-partite division. The 

 upper member consisted of a brick-clay and loam, and the lower member 

 was composed of sand, gravel, and huge boulders. The material as a 

 whole was coarser near the mouths of the gorges where the rivers leave 

 the Piedmont Plateau to pass into the Coastal Plain than in the more 

 remote portions of the deltas. The inter-fluvial phase possessed no such 

 regularity of bedding, but was indiscriminately composed of clay, sand, 

 and gravel largely of local origin. These delta deposits were identified 

 in all the principal rivers of the Middle Atlantic slope and were found 

 particularly well developed in the valleys of the Potomac, Susquehanna, 

 and Delaware. Due to the presence of these huge boulders, which 

 were evidently ice-borne and indicated a climate much colder than exists 

 to-day in the same region, as well as to the fact that the Columbia, when 

 traced northward, was found to pass under the terminal moraine, McGee 

 concluded that it was Quaternary in age and belonged to the earlier 

 glacial advance. These beds, since their deposition, have been raised and 

 tilted so that they now lie higher in the regions to the north than they do 

 farther south. Their present elevation was found to be about 500 feet 

 on the upper Susquehanna and 245 feet at its mouth; 400 feet on the 

 upper Delaware; 145 feet on the Potomac; 125 feet on the Rappahanr 

 nock; 100 feet on the James, and 75 feet on the Eoanoke. McGee also 



