BOUNDARY OF THE GLACIATED AREA, 163 



buried valley as deep as that underlying the swamp and the 

 gravel at Stoneham. But while the Stoneham deposit con- 

 tains Canadian material, that from Tiona to Sheffield consists 

 wholly of local material which has been brought down from 

 the sides of the mountain to the east and south. 



Here again an interesting thing occurs. The gravel 

 deposit between Sheffield and Barnesville rises to the 'level 

 of a col which leads through a precipitous gorge into and down 

 the Tionesta River reaching the Allegheny at the town of the 

 same name. But the northern branch of the present Tionesta 

 rises a considerable distance west of Stoneham; the deposit 

 at Stoneham being the watershed between that place and 

 the Upper Allegheny only a few miles to the north. From 

 Clarendon to Barnesville this branch flows over a preglacial 

 trough nearly 200 feet in depth, then the present stream 

 occupies for the rest of its course a narrow rocky trough bor- 

 dered only in a slight degree with gravel deposits. 



The explanation of this, as already intimated, is that upon 

 the obstruction of the northerly flowing preglacial stream 

 through the Conewango Valley, deep slack water was produced 

 in the Upper Allegheny Valley, the depth being regulated by 

 the height of the highest col at the south. The depth of the 

 water at Clarendon and Warren was about 500 feet, as shown 

 by the depth of the buried rock bottom of the river at Warren 

 added to the height of the gravel terraces in the vicinity of 

 Clarendon. In such depth of water, held in check by an 

 obstructing col somewhere at the south the lower portions 

 were stagnant so that blue clay and fine quicksand would 

 settle and this is what is found to a thickness of about 200 

 feet. Above this approximate level coarse sand and gravel, 

 with occasional large pebbles, was spread in dumps and fans 

 wherever tributary streams brought in material, which was 

 pre-eminently the case where streams entered from the 

 glaciated region. Such are the terraces of glacial gravel 

 already described just east of Warren, and a still larger and 



