PENNSYLVANIA AND DEPOSITS THEREIN. 83 



of bergs into the torrent passing through. These grounded and 

 packed wherever a chance offered, and there were many chances in 

 so tortuous a valley. Pondings occurred after cols were degraded, 

 and as a finishing clearance from the wasting glacier, the final ice- 

 dams were comparatively feeble and against weak currents which 

 brought the last of the washings of the thin drift sheet, and floated 

 the remnants of the ice-cakes to form the clayey-sandy-silty cap- 

 ping which covers everything below the ponding level. 



Because the ponding from the untrenched cols extends north- 

 ward at high levels, and because we find this universal capping, it 

 does not follow that the boulder clay was dropped everywhere at 

 the same time. Its wide variations between clay, silt, and sand, as 

 well as the great difference in size of the cobbles and boulders in- 

 cluded, prove that in each portion of the Allegheny Valley it was 

 merely the final episode of the clearance of that portion, and as the 

 ice-dam at a given point was finally carried away, whatever pond- 

 ing extended over that point was from a lower dam to the south. 



This is proved to have been the case in the Allegheny Valley. 

 The ice-dam just below Emlenton was not the sole one in that 

 valley. Those to the north would be formed between walls reach- 

 ing to a higher elevation: those to the south to a lower one. The 

 sporadic deposits would be carried to elevations averaging above or 

 below a theoretical gradation plain ; but with wide variations there- 

 from on opposite sides of the valley at a given point that would not 

 obtain in a complete valley filling. There is more adherence to such 

 a gradation plain in the glacial outwash in the Juniata, as shown by 

 the extension of the river terrace up the valleys of the affluents, and 

 the strictness of the average elevation of 80 feet above present 

 stream level. It is safe to conclude that the Kansan gravels in the 

 valleys of the Lehigh, the Susquehanna, the Juniata, and the Alle- 

 gheny were dropped and sealed in their present shapes during the 

 final clearance from the Kansan glacier. There was no complete 

 valley filling. 



It seems also that this conclusion can be extended to gravels of 

 uncertain or disputed origin in this and other countries. We have 

 seen valleys never touched by the glacier, but adjacent thereto, and 

 separated by a high watershed therefrom, invaded by torrential 



