A GEOLOGICAL SECTION ACROSS THE PIEDMONT 

 PLATEAU IN MARYLAND. 



BY CHARLES R. KEYES. 



{Read before the Society December 30, 1890, as a Supplement to the Memoir by Dr. 

 Williams on the Pet7'ography and Structure of the Piedmont Plateau in Maryland.) 



The broad elevation lying between the coastal plain and the first range 

 of the Appalachians is known as the Piedmont plateau. In Maryland its 

 median water-shed is called Parr's ridge, and rises from 600 to 900 feet 

 above the sea-level. To the northward, Parr's ridge forms a prominent 

 topographical feature ; but along the line of the section here considered, the 

 central water-shed has merged into the general level of the plateau. The 

 eastern flank of the Piedmont plain is made up chiefly of gneisses, with 

 numerous intrusive rocks. The western half consists principally of sericitic 

 schists, with some sandstone. On the extreme western border occurs a broad 

 limestone area. 



In connection 'with a recent attempt to determine, from paleontological 

 evidence, the age of this (the Frederick) limestone, which is the most easterly 

 of the unaltered calcareous formations within the limits of the state, a sec- 

 tion was made across the valley of Monocacy river from the Catoctin to 

 Sugarloaf mountain, a distance of ten miles. The results then obtained 

 suggested the continuation of the section eastward. This has lately been 

 done, with considerable detail, as far as Washington, where the more ancient 

 rocks are hidden from view by the coastal plain deposits. The course of the 

 section (illustrated in figure 3) is therefore approximately along the line of 

 the Metropolitan branch of the Baltimore and Ohio railway. The general 

 course is indicated on plate 12 (facing page 301). 



The finding of fossils in the Frederick limestone* has already been an- 

 nounced. The forms include several important types, of which the crinoids 

 and brachiopods are the leading representatives. Of the first group, there 

 are, however, only a few plates, perhaps belonging to Glyptocrinus. Of the 

 second, a good series of several species was obtained. They represent chiefly 

 the Leptcena group of Strophomena. The form is the type of a large assem- 

 blage of described species, having a wide geographic distribution and a very 

 considerable range in time. One of the varieties is regarded as representing 



* Johns- Hopkins University Circulars. 1890, vol. X, p. 32. (Separates distributed in advance, July, 



1890.) 



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