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UNIVERSITY OF VIRGINIA PUBLICATIONS 



The region has been exposed continuouslj' to erosion since the close 

 of the Paleozoic era. It shows evidence of at least three distinct cj'cles 

 of erosion with a fourth well inaugurated. 



It is the purpose of this paper to trace the major drainage changes 

 which have occurred since the beginning of the first erosion cj^cle in late 

 Paleozoic or earlj^ Mesozoic times. 



THE IMPORTANT PHYSIOGRAPHIC FEATURES. 



The main ph3rsiographic features of the region trend in a northeast- 

 southwest direction in conformity with the strike of the folds of the rocks 

 out of which they have been developed. These are the Shenandoah 

 A''alley, the Blue Ridge, and the Alleghany Ridges, which form the main 

 subdivisions of the Appalachian Mountain province. 



Fig. 1. Index Map showing Position of the Shenandoah Valley Region. 



The principal features of the Shenandoah Valley are: (1) Massanutten 

 Mountain (pis. II and III) which divides the vallej^ into two parts 

 between the latitudes of Strasburg and Harrisonburg, (2) the rounded 

 monadnocks and low linear ridges (pi. IV, fig. 1) which mark the cherty 

 beds of the Shenandoah group of limestones, (3) the Shenandoah pene- 

 plain (pi. VII), and (4) the valleys of the present cycle of erosion sunk 

 100 to 200 feet below the Shenandoah surface. 



The Blue Ridge consists of a main ridge composed largelj^ of igneous 

 rocks, with an uneven and knobby crest cut at close intervals by num- 

 erous -mnd gaps, and flanked on the northwestern side by ridges of sili- 

 cious Cambrian sediments (pi. V). The Alleghanj' Ridges subpro\dnce is 



