84 the geology of st. ilaey s county 



The , Pleistocene. 

 The Columbia Group. 

 The Columbia Group is the name applied to a series of beds of clay, 

 loam, sand, gravel, and ice-borne boulders, which are stratigraphically 

 younger and lie topographically below the Lafayette formation. They are 

 widely distributed over the surface of the Coastal Plain from Atlantic 

 Highlands southward to Mexico and are Pleistocene in age, being the last 

 formations which have been laid down in the region before the Recent 

 deposits were formed. The formations which constitute the Columbia 

 Group are, beginning with the oldest, the Sunderland, Wicomico, and 

 Talbot. These deposits were laid down for the most part during the 



Fig. 4. — Diagram showing ideal arrangement of ttie various terrace forma- 

 tions in tlie Maryland Coastal Plain. 



glacial period, but a definite correlation of them with the glacial deposits 

 of New Jersey and other regions is not practicable in the present state 

 of knowledge. When the field relations w-hich exist between these two 

 great classes of deposits are more accurately known, a correlation will, 

 no doubt, be possible. The various formations of the Columbia Group 

 lie unconformably on whatever rocks are beneath them. The clay, peat, 

 loam, sand, gravel, and ice-borne boulders, out of whicli they are com- 

 posed, occur in irregular beds or are developed in lenses. They are 

 mixed together in varying amounts and grade over into each other both 

 horizontally and vertically. Two of the formations, the Sunderland and 

 Talbot, carry determinable vegetable remains, and the latter has yielded 

 in addition fragments of fossil insects. The various members of the 

 Columbia are developed in terraces lying one above the other in order 

 of their age, the oldest occupying topographically the highest position 



