PHYSIOGRAPHY OF THE PLATEAU. 303 



boundary of the Piedmont region should be drawn considerably more toward 

 the east. At the base of Catoctin stretches a broad transgression of Newark 

 red sandstone, from beneath whose eastern border emerge the upturned edges 

 of the Frederick valley limestone, which we now know to be of the same 

 age (Trenton-Chazy) as the valley limestone of Virginia.* These two for- 

 mations produce a broad fertile valley, east of which rises the more broken 

 and rolling country composed of the crystalline and semi-crystalline rocks 

 of undetermined age, which geologically characterize the Piedmont region 

 proper. Through this valley meanders southward the Monocacy ("lazy- 

 river " of the Indians) on its way to the Potomac. Near the southern end 

 of its course there rises on its left bank the sandstone mass pf Sugarloaf 

 mountain. This is a thick monocline of easterly dipping beds, 1,360 feet in 

 height, which in appearance, composition, and structure resembles a part of 

 Catoctin. It would bound the Piedmont area of Maryland on the west 

 topographically as it does geologically, were it not for the fact that it is 

 abruptly cut off on the north and south, its isolated mass being represented 

 farther toward the north by only a few insignificant patches of sandstone 

 intercalated in the slates. 



Drainage. — We may roughly outline the Piedmont reigon proper within 

 the limits of Maryland as a trapezium, bounded on the north by the state 

 line, on the east by the Baltimore and Ohio railroad from Wilmington to 

 Washington, on the south by the Potomac, and on the west by the Monocacy 

 river. The surface of this area is nearly level, or slopes very gently east- 

 ward and westward from its median divide, known as Parr's ridge. This 

 well defined water-shed follows a course somewhat west of southward from 

 the Pennsylvania line through the towns of Manchester, Westminister, and 

 Mt. Airy, until, before reaching the Potomac, it becomes merged into the 

 steady eastern slope from Sugarloaf to Washington. The average height of 

 this ridge is about 850 feet. From it the streams flow westward into the 

 Monocacy, at a height of 250 feet, and eastward into the Chesapeake, at tide 

 level. It does not continue to be a prominent topographical feature far south 

 of the Patuxent river, for in Montgomery county the drainage is west-by- 

 southward, toward the Potomac. 



The even surface of the Piedmont region has been so recently elevated 

 that its streams are still excavating narrow precipitous channels. This im- 

 parts to the country a mild canon type — the rock gorges suddenly becoming 

 broad estuaries where they enter the coastal plain. There is, furthermore, 

 evidence of a superimposed character in the drainage of this area to be found 

 in the frequency with which even the smaller streams abruptly leave broad 



* A year ago the age of the Frederick limestone could only be surmised from its lithological 

 resemblance to the valley limestone of Virginia, but last June (1890) the welcome extension ot the 

 determined horizon east of the Trias, was accomplished by IMr. Charles R. Keyes, who obtained 

 Chazy-Trenton fossils some three miles east of Fredericlt. See Johns Hopkins University Circulars, 

 no. 84, vol. X, December, 1890, p. 32. 



