572 



SCIENCE 



[N. S. Vol. XLII. No. 1086 



streams which have happened to head opposite 

 each other. 



It is one of the commonest phenomena in a 

 maturely dissected region, whether this be 

 mountainous or simply a plateau. 



In the Blue Eidge Eegion covered by Har- 

 pers Ferry sheet, there is nothing in the ar- 

 rangement of the drainage or in the disposi- 

 tion of the contours which would suggest that 

 all of those wind gaps — Snickers, Ashby, 

 Crampton, Turners and a number unnamed — 

 might not be as satisfactorily explained in 

 this way as by a "diversion" — reversion — 

 and " beheading " process. To my mind, the 

 simple notching process affords by far the 

 better explanation, since it fits in with the 

 general and notable characteristic of the topog- 

 raphy which militates against the Willis 

 theory. This is the total lack of that "barb- 

 ing " arrangement in the tributaries of the 

 streams alleged to have been reversed which 

 would seem to be necessary as conclusive evi- 

 dence that these wind gaps are corollaries re- 

 sulting from cases of " river piracy." 



There is one line of evidence, namely, a 

 progressive deepening of the wind gap notches 

 in the Blue Eidge from the Water Gap of the 

 Potomac at Harpers Ferry southward, which, 

 if this were pronounced enough, might be 

 alleged in support of the " Eiver Capture 

 Theory." However, Professor Willis barely 

 hints at this evidence in calling attention to 

 Manassas Gap — the most remote from the 

 Potomac water gap of the wind gaps in the 

 Blue Eidge south of the Potomac as well as 

 the deepest. 



The paragraphs from the Willis monograph 

 which have become the sources of " the accepted 

 test-book theory of wind gap formation " are 

 as follows : 



On the Kittatinny Plain many smaller streams 

 flowed across the ranges; and they also, persisting 

 in their courses during the upheaval, cut water 

 gaps in the hard beds. But they could not deepen 

 the gaps as rapidly as did the great rivers, and 

 the work of the smaller streams is now repre- 

 sented by the notches in the ridges high above the 

 Shenandoah Plain. No streams now flow through 

 these little Vs: they are wind gaps from which a 



rivulet descends on each side of the ridge. . . . 

 The Potomac traverses the Blue Eidge at Harpers 

 Ferry. South of this water gap are several wind 

 gaps, such as Snickers Gap, which mark the chan- 

 nels of ancient streams, now diverted. The 

 Shenandoah River enters the Potomac above the 

 water gap at Harpers Ferry, flowing northward 

 along the western base of the Blue Eidge. The 

 streams which passed through Snickers Gap and 

 the other wind gaps ran above the present course 

 of the Shenandoah, crossing it about at right 

 angles. The two drainage systems could not exist 

 at one time; therefore it is evident that the older 

 one has been replaced by the younger river, the 

 Shenandoah. This diversion took place by the 

 gradual growth of the Shenandoah from its mouth 

 southward. The Potomac, the large stream, cut its 

 water gap faster than Snickers Gap was cut. The 

 Young 'Shenandoah of the Kittatinny Plain, a 

 small tributary of the Potomac where the mouth 

 of the present Shenandoah is, acquired consider- 

 able fall as the Potomac deepened its gorge and 

 sawed its channel down rapidly in the limestone, 

 which offered no great resistance. But the stream 

 in Snickers Gap, with perhaps less fall and not 

 much greater volume than the Shenandoah, had to 

 saw much harder rock in crossing the Blue Eidge. 

 Its channel remained high, therefore, as compared 

 with that of the Shenandoah. The latter, extending 

 its headwaters backward as a tree puts out new 

 twigs, eventually tapped the channel of the other 

 stream above Snickers Gap. The waters above the 

 point of attack joined the Shenandoah; the section 

 between the point of attack and Snickers Gap was 

 reversed as the Shenandoah rapidly deepened the 

 channel of its new conquest; and the lower portion 

 of the stream, now called Beaverdam Creek, hav- 

 ing lost its original head waters, took its rise at 

 Snickers Gap. Thus the ancient stream which once 

 flowed through the gap was divided into three sec- 

 tions, the diverted, the inverted and the beheaded, 

 while the Shenandoah, the diverter, was strength- 

 ened. 



Thomas Jefferson, the only one of our presi- 

 dents, except Eoosevelt, who ever showed 

 marked interest in science, also tried his hand 

 at explaining topographic features of the Blue 

 Eidge. It occurs in his notes on Virginia 

 written in 1781, and the passage is as follows : 



The passage of the Potomac through the Blue 

 Eidge is perhaps one of the most stupendous 

 scenes in nature. You stand on a very high point 

 of land. On your right comes up the Shenandoah, 



