MARYLAND GEOLOGICAL SURVEY 169 



mont Plateau, but as the base occupies a changing elevation in both 

 districts it is impossible to fix on any one locality as the typical posi- 

 tion. It would seem that 400 feet was a fair average for the eleva- 

 tion of the base on the Piedmont Plateau and 200 feet on -Elk Neck. 

 If these averages are approximately correct they indicate a dip of the 

 Lafayette formation to the southeast at a rate of 25 feet per mile. 

 It will be sufficient to say that the dip is probably not less than 20 

 feet, nor more than 30 feet per mile. 



The, Pleistocene. 



the columbia group. 



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

 clay, loam, sand and gravel, which are stratigraphically younger than, 

 and lie topographically below the Lafayette formation. They are 

 Pleistocene in age and are the last formations made in the region be- 

 fore the recent deposits. Several years ago Mr. "W* J McGee out- 

 lined the geology of the beds which now constitute the Columbia 

 group, and gave to them the name Columbia formation because of 

 their typical development within the District of Columbia. Subse- 

 quent study, however, has shown that the beds are divisible into three 

 well-defined formations, which have received separate names and the 

 term Columbia is now retained to designate the group. The forma- 

 tions which constitute the Columbia group are as follows, beginning 

 with the oldest: the Sunderland, the Wicomico and the Talbot. A 

 more definite correlation than this is not possible at the present time, 

 as the determination of this question depends on the relation of the 

 respective formations to the glacial deposits of neighboring regions. 

 When this relation has been more carefully worked out, no doubt the 

 correlation can be determined more accurately between the various 

 members of the Columbia group and the various epochs which have 

 been proposed in the Pleistocene period. It is possible that the Tal- 

 bot may be in part recent. The formations of the Columbia group 

 lie unconformable on whatever rocks are beneath them. They con- 

 sist of clays, loams, sands and gravels, which run in irregular beds, 



