24 THE PLIOCENE AND PLEISTOCENE DEPOSITS OP MAEYLAND 



The deposits of the Appalachian Mountain and Piedmont Plateau 

 regions are very ancient, extending from Archaean down through 

 Paleozoic to early Mesozoic and, as stated above, are all consolidated. 

 The deposits of the Coastal Plain, on the contrary, are much younger. 

 Their oldest formations do not date back further than the Jurassic period 

 and their youngest beds are now in the process of deposition. They are 

 all unconsolidated with the exception of a few ledges which are of local 

 and subordinate importance. The Pliocene and Pleistocene beds which 

 are among the latest of the Coastal Plain deposits are developed as a 

 series of terraces lying one above the other and covering the entire 

 Coastal Plain as a mantle, except along certain of the drainage lines 

 where the rivers have succeeded in partially stripping them away and 

 revealing the older beds beneath. It is to the study of this surficial mantle 

 cf clays, loams, sands, and gravels that this monograph is especially 

 devoted. 



Although fossils are not wanting in the Pleistocene deposits of Mary- 

 land, yet they are not uniformly distributed, but are confined to a few 

 localities which are grouped for the most part near the margin of 

 Chesapeake Bay and its estuaries. It is, therefore, impossible to rely on 

 fossil evidence alone for the correlation of the various formations, and 

 although this line of evidence has been used whenever available, it has 

 been found more helpful to employ the criteria of topography than that 

 of paleontology. This is correlation by what McGee has called the 

 method of " homogeny." 



Prom the bottom to the top of the Coastal Plain deposits of Maryland, 

 the formations are separated by a large number of unconformities. 

 Studies which have been prosecuted by those who have worked in this 

 region have shown that the eastern border of North America has been 

 alternately below water receiving deposits and above sea level undergoing 

 erosion. This uneasiness of the sea margin exhibited throughout Meso- 

 zoic and early Cenozoic time is still more manifest and striking through 

 later Cenozoic and Kecent time. Among the most interesting discoveries 

 which have come to light in prosecuting the work on the surficial deposits 

 is the undoubted oscillations of the land border. These results have an 

 important bearing on the theory of isostasy. 



