MARYLAND GEOLOGICAL SURVEY XXXI 



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 uncomformably 

 upon the underlying Chesapeake and are in turn unconformably over- 

 lain by the Pleistocene, they have been thought to represent the Plio- 

 cene. The apparent similarity of these deposits to those in Mississippi, 

 described by Hilgard under the name of the Lafayette formation, lias 

 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 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 exten- 

 sion 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 are the Pleis- 

 tocene formations, which lie at various elevations from near sea-level to 

 200 feet in the different portions of the region. From their typical 

 development in the District of Columbia all the Pleistocene deposits of 

 the Middle Atlantic Slope received the name of Columbia formation 

 by McGee, who described three distinct phases, viz., the fluvial, the 

 interfluvial, and the low-level. Later Darton recognized high-level and 

 low-level phases which he called earlier and later Columbia. More re- 

 cently Shattuck, of the State Geological Survey, has shown that greater 

 complexity exists in the Maryland Pleistocene deposits than had been 

 before recognized and that the later Columbia will have to be further 

 divided. 



The Pleistocene deposits consist of gravel, sand, clay, and loam, the 

 material in general becoming finer and more fully stratified with dis- 

 tance from the old shore-line and river-channels. In the latter instance 



