G. F. Wright — Conitnuity of the Glacial Period. 173 



Opposite Parker City there occurs the deserted river bed so 

 fully described by Mr. Chance,* and whose descriptions are 

 reproduced by Professor Chamberlinf to controvert my views 

 concerning the preglacial origin of the rocky gorge of the 

 river at this point. But Mr. Baldwin and I discovered some 

 things not heretofore noticed which in themselves strongly con- 

 firm my previous views concerning the preglacial date of the 

 rocky erosion. The details can be clearly and briefly stated in 

 connection with the accompanying map (slightly modified 

 from that of Mr. Chance4 The elevation of the Allegheny 

 River at low water is here 840 feet, and the width of the rocky 

 gorge is 1500 feet. The rock bottom is fifty feet lower. The 

 summit of the gravel upon the rock shelf which we have been 

 considering is 250 feet above low water, or 1090 feet above 

 tide. Upon the east side of the river there is a deserted rock 

 channel in the shape of an oxbow extending out about a mile 

 and a half from the main stream, but at an elevation of about 

 200 feet above it. This is partially filled, to a depth of thirty 

 or forty feet, with river gravel of Glacial origin. Mr. Chance 

 and Prof. Chamberlin supposed that this deposition took place 

 previous to the lowering of the rock gorge from the level of the 

 ox-bow to the present rock bottom of the river, a perpendicu- 

 lar distance of 250 feet. But the additional facts which I now 

 have to present will, even in themselves I think, disprove this 

 hypothesis, and show that the existing rock erosion of the main 

 channel had taken place, to a large extent at least, before the 

 deposition of Glacial gravel in the ox-bow. 



That the ox-bow was at an early period eroded by the river 

 appoximately to its level at the point farthest away from the 

 present stream cannot well be doubted ; but there is evidence 

 also that after its desertion by the main stream the small 

 streams occupying the two prongs of the bow had accomplished 

 a good deal of erosion before the deposition of the gravel ; for 

 there is a considerable slope of the rock bottoms of these small 

 tributaries in their progress towards the river, and it is evident 

 that their beds had finally assumed the shape which results 

 from the erosion of such small streams rather than from the 

 passage of a great river. This would go to prove that the 

 undercutting of the neck had been fully accomplished long 

 before the partial filling of the ox-bow with gravel. Further- 

 more, the upper prong of the bow is filled in with gravel 

 around to its farthest extremity at A ; while the lower prong 

 of the bow is filled in with gravel only a short distance, namely, 



* Second Geol. Surv. Pa., TV, 1880, pp. 17-20. 

 •j- Introd. Bull. U. S. Geol. Surv. No. 58, p. 30. 



\ Map of old channel at Parker partially filled with high-level Glacial gravel. 

 See section on lower right hand corner of Fig. 1 . 



