XXXll INTRODUCTIOX 



they at times contain large numbers of marine molluscan shells, form- 

 ing a characteristic calcareous marl. In general, however, the organic 

 remains consist largely of the branches and leaves of terrestrial plants, 

 many of which are exquisitely preserved. 



The Pleistocene deposits of the Middle Atlantic Slope, widely 

 known hitherto under the name of the Columbia group, have been 

 divided by the Maryland Geological Survey into the Sunderland, 

 Wicomico, and Talbot formations. The Sunderland formation, the 

 oldest member of the Colmnbia group, constitutes the highest of the 

 Pleistocene terraces and often covers the highest levels along the west- 

 ern shore of the Chesapeake Bay, and consists of gravel, sand and clay 

 overlain by loam often carrying ice-borne boulders. The Wicomico 

 formation occupies lower levels skirting the high lands capped with 

 Sunderland deposits. The Wicomico formation like the Sunderland, 

 is composed of gravel, sand and clay which is often capped with loam 

 and marly clay bearing here and there ice-borne boulders. The Talbot 

 formation occupies the lower levels of the Coastal Plain, seldom reach- 

 ing above 40 feet in altitude. It is composed of gravel, sand and clay, 

 with here and there large deposits of peat, and is generally overlain with 

 loam frequently carrying ice-borne boulders. The Talbot formation con- 

 stitutes the lowest of the series of terraces previously described and has a 

 very much larger areal extent than any of the other members of the 

 Pleistocene. 



