DRAINAGE CHANGES OF THE SHENAJSTDOAH 355 



flowing streams. The largest of these streams were able to lower their 

 channels across these barriers more rapidly than the smaller and less 

 able bodied ones. As a consequence, this gave the tributaries of the 

 larger streams flowing over soft limestone beds the advantage over the 

 tributaries of the smaller streams flowing at higher levels, and it was 

 therefore onty a matter of time till the tributaries of the large streams 

 had captured the small streams. 



CYCLES OF EROSION. 



Kittatinny cycle. The erosion cycle begun at the time of the Appa- 

 lachian revolution reduced the entire region to a peneplain, except some 

 minor areas in the Alleghany Ridges and the Blue Ridge regions, which 

 remained as low monadnocks above the general level of the plain. The 

 plain which resulted from this first cycle of erosion, completed during 

 Cretaceous times, has been called the Kittatinny after the mountain of 

 that name in New Jersey. Remnants of this plain in the Shenandoah 

 Valley region are preserved in the crests of the bordering mountains and 

 in ilassanutten ^Mountain (see pis. II, III, and V). The drainage con- 

 chtions of the region during this entire cycle remained very much as they 

 were at the beginning, as indicated by the large number of wind gaps in 

 the Blue Ridge, below the level of the Kittatinny plain. These were 

 eroded by streams that survived the first cycle of erosion. The abundance 

 of these gaps in the Blue Ridge shows with reasonable certainty that 

 whatever stream captures may have occurred during the Kittatinny cycle 

 of erosion were of minor importance only. The supposed general drain- 

 age plan during this cycle is shown in figure 3. 



The essential absence of piracy during the first cycle was doubtless 

 due to the more or less uniform hthologic character of the beds on which 

 the streams were working in the region now occupied by the Shenandoah 

 Valley. The overlying sandstone beds which covered the Shenandoah 

 limestones were removed only in part during this cycle of erosion. Instead 

 of a belt of soluble hmestone rocks in the Valley district, as at present, 

 there doubtless existed only sandstone beds outcropping during the greater 

 part of this cycle in the region west of the Blue Ridge. In the absence 

 of alternating beds of resistant and nonresistant rocks, structure alone 

 does not seem to greatly favor stream capture. 



It is quite probable also that during part of this cycle the belt now 

 occupied by the Blue Ridge was a limestone belt. Since the structure 

 of the Blue Ridge is anticlinal and Cambrian quartzites are known to 

 extend entirely across the mountain in several places it is reasonable to 



