maryland geological survey 57 



General Stratigrapiiic Kelations. 



Throughout the Coastal Plain of the Middle Atlantic slope, the Lafay- 

 ette and Columbia deposits rest on the eroded edges of the older forma- 

 tions which are buried beneath them. The materials of which this 

 younger mantle is composed have been supplied in great measure by the 

 destruction of the older beds on which they rest. The appearance of 

 these older deposits is frequently so characteristic and striking that it is 

 not a difficult matter to recognize materials which have been derived from 

 them and are now re-worked in a younger formation. As so much de- 

 pends on a clear understanding of the older deposits of this region, they 

 will be briefly described before proceeding farther. 



pre-cambrian and paleozoic. 

 The older rocks on which the surficial deposits rest are found along 

 the eastern border of the Piedmont Plateau and range in age from pre- 

 Cambrian into Paleozoic. They consist of igneous rocks of various kinds, 

 such as granite, gabbro, diabase, etc., and highly crystalline metamor- 

 phosed sedimentaries, among which schists and marble are important. 

 When these rocks are fresh, there is no difficulty in distinguishing them 

 from the overlying sands and gravel, and even when they are decayed, a 

 little practice suffices to differentiate the two. If we add to the mantle 

 which covers the Coastal Plain the river terraces of western Maryland 

 which are believed to be, in part at least, of Pleistocene age, it is an easy 

 matter to distinguish between the shales, sandstones, and limestones of 

 the mountains and the unconsolidated deposits which cover them along 

 the river channels. 



JURASSIC ( ?) AND CRETACEOUS. 



Above the pre-Cambrian and Paleozoic rocks lie the unconsolidated 

 deposits of Jurassic ( ?) and Lower Cretaceous age. These deposits, 

 which consist of arkose, clay, sand, and gravel, are divisible into four 

 formations which have been named, beginning with the oldest, Patuxent, 

 Arundel, Patapsco, and Earitan. Of these the two former are believed 



