270 GEOLOGICAL EXCURSION TO I'm: ROCKY MOUNTAINS. 



Point of Rocks, a short distance beyond Washington Junction, marks 

 the entrance of the Appalachian Mountain bell proper. The river 

 herecnts throughahigh ridgecapped by a monocline of Cambrian sand- 

 stone,known as Oatoctin Mountain. This runs nearly due north and 

 forms the western boundary of the Frederick Valley. The road passes 

 for S miles across a valley of slate, drained byCatoctin Creek, and 

 called, from its principal town on the .Maryland side of the river, 

 the Middletown Valley. It then intersects another abrupt ridge nearly 

 parallel to Oatoctin Mountain at 



Weverton. Tins is the junction for a branch road northward to 

 Bagerstown, one of the largest cities of Maryland. The road now 

 crosses a slight depression occupied by crystalline rocks (granite-gneiss 



with some basic dikes) and soon reaches 



Harpers Ferry, a place which, both geologically and historically, is 

 of more than usual interest, From the railway bridge over the Poto- 

 mac may be seen on the right (north) a lofty ridge of contorted sand- 

 stone with underlying shales and slates, known as Maryland Heights. 

 I'pon the opposite side of the river rises the continuation of this ridge 



in Virginia, known as Londonn Heights. The course of the Potomac is 



for some distance above this point nearly south, and here it LS joined by 

 one of its most important tributaries, the Shenandoah River. In the 

 triangular space between these two streams lies the town of Harpers 

 Ferry, which, from its surroundings, possesses great strategic impor- 

 tance. 



The two ridges cut through at Weverton and Harpers Ferry well 

 illustrate a characteristic feature in Appalachian topography. The 

 former originates a short distance south of the river and continues its 

 course across Maryland as the "Blue Ridge," and. after its junction 

 with the Oatoctin Mountain, as "South Mountain," in Pennsylvania. 

 The Harpers Ferry elevation, on the other hand, soon dies out toward 

 the north, but continues its course hundreds of miles southward, across 

 Virginia and North Carolina, as the "BlueEidge." Thus, near the 

 Potomac, one important fold dies out and is continued by another, en 

 echelon, or offset somewhat to one side, as is a frequent occurrence 

 among- the long parallel ridges of the Appalachian system. 



The geology at Harpers Ferry is complex, and has given rise in for- 

 mer times to different interpretations by different investigators. To 

 the west are the contorted layers of the blue Valley limestone, known 

 from its fossils to be of Trenton-Cha/.y age (II). On the east of 

 these, and apparently overlying them, succeeds a thick mass of shales, 

 slates, and the contorted sandstones seen in the front face of Maryland 

 Heights which Walcott has recently shown to be Lower Cambrian. 

 Then follows toward the east, occupying the space between here and 



