48 THE EOCENE DEPOSITS OF MARYLAND 



The greater portion of the Chesapeake^, however^ is composed of variously 

 colored sands and clays, with which are frequently mingled vast numbers 

 of molluscan shells. Sometimes the shelly materials form so large a 

 proportion of the deposits as to produce nearly pure calcareous strata, 

 which in a partially comminuted state may become cemented into hard 

 limestone ledges. The organic remains are very numerous and show 

 the Neocene age of the deposits. Their great number early attracted 

 the attention of geologists, in whose Avritings descriptions of them are 

 frequently found. Several faunas have been distinguished on the basis 

 of which, as well as on stratigraphic and structural grounds, a number of 

 well-defined formations have been recognized by the State Geological 

 Survey. 



Covering the Chesapeake deposits in places, is a formation composed 

 of gravel, sand, and clay, which thus far has afforded no distinctive 

 fossils upon which to base a determination of its geologic age. From 

 the fact that the deposits rest unconformably upon the underlying 

 Chesapeake, and are in turn imconformably overlain by the Pleistocene, 

 they have been thought to represent the late Neocene or Pliocene. 

 The apparent similarity of these deposits to those in Mississippi, des- 

 cribed by Hilgard under the name of the Lafayette formation, has led 

 to the adoption of the same name for the strata of the Atlantic Coast. 

 The beds of the Lafayette are very irregularly stratified, and often 

 change rapidly within narrow limits. Toward the ancient shore-line the 

 deposits are of coarse gravel, through which is scattered a light-colored 

 sandy loam, the whole cemented at times by hydrous iron oxide into a 

 more or less compact conglomerate. The eastward extension of the 

 formation shows a gradual lessening of the coarser elements and a larger 

 admixture of loam. Arkosic materials are also present throughout the 

 formation, while the coloring and manner of weathering are highly 

 characteristic, the exposed surfaces presenting what is known as case- 

 hardening. 



PLEISTOCENE. 



Superficially overlying the deposits hitherto described, and with 

 marked variations in thickness, composition, and structure, is the Pleis- 

 tocene, which lies at various elevations from near sea-level to 200 feet 



