46 THE PHYSICAL FEATURES OE CECIL COUNTY 



little later, the Appomattox formation of McGee was correlated with 

 Hilgard's Lafayette, and the extension of the latter across Maryland 

 into Cecil county was thus established. 



The Columbia Group. 



The presence of surficial deposits in Cecil county and neighboring 

 regions was noted by the early geologists and discussed in a desultory 

 manner by them, but the differentiation of these deposits and the 

 determination of their natural sequence has been the work of the 

 last twenty years. To Professor W J McGee is due the credit for 

 first grouping these surficial deposits by themselves under the name 

 of Columbia and of pointing out many of their leading characteristics. 



He divided the Columbia 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 to-day. The inter-fluvial phase was found 

 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 another. 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. 

 McGee found the material as a whole, 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 delta. 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 are 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, it was concluded that it 

 was Quaternary in age and belonged to the earlier glacial advance. 



