62 THE PLIOCENE AND PLEISTOCENE DEPOSITS OF MARYLAND 



PLEISTOCENE. 



Above and unconformable on the Lafayette beds occurs a series of 

 gravels, sands, clays, and peats of Pleistocene age belonging to the 

 Columbia group. The Pleistocene deposits of this region form part of a 

 more extensive series which are developed over the entire Coastal Plain 

 from Earitan Bay southward to Florida and around the border of the 

 Gulf of Mexico. With the exception of Eecent sediments they are the 

 youngest of the Coastal Plain deposits and lie on the surface, constituting 

 the mantle which has just been referred to as concealing the Miocene and 

 older deposits from view. The Columbia deposits wherever found along 

 the Atlantic border are developed in more or less clearly defined terraces, 

 and consist of clay, sand, gravel, or ice-borne blocks, either separately 

 deposited or intermixed in indefinite proportions. The gravels and 

 coarse sands frequently are very much decomposed showing that they have 

 been resting for a considerable period in the position in which they are 

 now found. As a whole, these deposits have suffered but little from 

 erosion, although locally in the immediate proximity of streams, the older 

 members of the group have been eroded more extensively than the 

 younger. Up to the present time the Columbia has received more atten- 

 tion in Maryland, Delaware, New Jersey, and the District of Columbia 

 than elsewhere although it is now being carefully studied in New York 

 State. 



KECENT. 



The materials which constitute the Eecent deposits consist of mud, clay, 

 sand, and gravel. These are deposited in deltas, flood-plains, beaches, and 

 dunes, in the valleys of rivers and estuaries, and along the ocean front. 

 The deposition of deltas and flood-plains has been going rapidly forward, 

 at least since the settlement of the country by Europeans. Men are still 

 living who distinctly remember when vessels moored and discharged their 

 cargoes in places which are now occupied by extensive marshes or meadow 

 lands. Such rapid deposition would doubtless not have occurred if the 

 forests had been allowed to remain undisturbed, but the advent of the 

 white man and the consequent destruction of the forests exposed the loose 



