DRAINAGE CHANGES OF THE SHENANDOAH 351 



characterized by a series of parallel linear ridges separated from each other 

 by narrow vallej's which var}^ in width depending on lithologic and struc- 

 tural conditions. Massanutten Mountain, geographically belonging to 

 the "\'alle3' province, is, in respect to lithology and structure as well as 

 topograph}', an outlier of the Alleghany Ridges (see pis. V and VI). 



THE PRESENT DRAINAGE PLAN. 



The main stem of the Shenandoah River does not follow the direction 

 of greatest continental slope, but has developed a course parallel with the 

 chief physiographic features of the region. It occupies a position abnor- 

 mal to the Valley and region which it drains, since it does not follow 

 a central position in the Vallej^, but maintains a course along its extreme 

 southeastern margin (see map, pi. I.). 



The Shenandoah River illustrates an extreme case of an asymmetrical 

 river system. All important tributaries enter from the northwest side; 

 only small streams from the northwest slope of the Blue Ridge enter from 

 the southeast. The sj^stem is made up of two distinct stream types, 

 antecedent and adjusted. The main stem is an ideal representative of 

 the latter; while the larger tributaries. North Fork, North River, and 

 Middle River, belong to the former type, except that part of North Fork 

 between New Market and Strasburg, which is of the adjusted type. 



PHYSIOGRAPHIC RELATIONS OF THE REGION TO DRAINAGE. 



While the main stem of the Shenandoah ijarallels the chief physio- 

 graphic features and the strike of the rocks, its tributaries are entirely 

 independent of these features so far as their direction of flow is concerned. 

 The west side tributaries head well back within the Alleghany Ridges in 

 the same manner as the James and the Potomac and other important 

 streams of the Northern Appalachians, except New River which presum- 

 ably follows a course antecedent to the Appalachian revolution. West 

 of their junction with the Shenandoah, these tributaries are identical in 

 type with those parts of the James, Potomac, and Susquehanna in the 

 Appalachian Mountains i^rovince. They traverse the Alleghany Ridges 

 through numerous water gaps in courses transverse to the axes of the 

 ridges and cross the Shenandoah Valley in the same direction regardless 

 of resistant rock barriers, reaching the foot of the Blue Ridge where they 

 join the Shenandoah and turn abruptlj^ in a northeasterly direction (map, 

 pi. I). The larger streams of the Northern Appalachians do not change 

 their course on reaching the Blue Ridge but continue in the same direc- 

 tion through water gaps in the mountain. 



