84 REOONNAIBSANOE IX NOBTHERSJ U.Aska IN L901. 



PLEISTOCENE DEPOSITS. 

 GLACIAL DEPOSITS IM' GLACIATION. 



Glaeiation in geologically recent time in aorthern Alaska bus not been of a con- 

 tinental character comparable to that of the northern United States and of the Cor- 

 diiieran glaciei", which overspread the vast field of the Pacific coast mountains in 



southern Alaska between the Yukon and the coast. It lias been far inoi - e extensive, 

 however, than has generally been supposed. In the valleys nearly everywhere 

 within the Endicott Range evidence of ice action is shown by bed-rock scorings, 

 local roches moutonuees, terminal moraines, and deposits of till on the lower valley 

 slopes and benches. It has modified preexisting topographic forms, both by erosion 

 of the rocks and by deposition of drift. In some cases, as revealed in places by 

 erosion of the glacial deposits, steep-faced bed-rock benches in the sides of the val- 

 leys were covered and concealed by the drift, whose surface slopes gently down from 

 the steep mountain side a distance of one-half mile or more, nearly to the present 

 stream (PI. XIV, B). 



In the lower part of the intramontane section of John River, where the valley 

 occasionally widens, it is sometimes floored by a till sheet, from 50 to 100 feet 

 thick, containing ponds and lakelets, some of which seem distinctly to be of glacial 

 or-igin. Till is prominent also in the mouths of some of the side valleys, as Till Creek 

 Valley, which opens into that of John River on the east at about the sixty-eighth 

 parallel. Here good exposures show the deposit to be typical till. It has a thick- 

 ness of at least 100 feet, and forms a broad terrace or sort of small, triangular 

 plateau a mile or more in extent, occup} r ing the mouth of the side valley. It slopes 

 westward, to where John River has been crowded to the bluffs on that side of the 

 valley. On the east it overspreads and conceals the lower benching and bed-rock 

 topography. Till Creek, a torrential stream, flows in a small ca^^on whose steep 

 banks are formed of the bowlder clay or ground moraine. 



In the region of Fork Peak, near the head of John River, at the confluence of 

 several of its tributaries, the valley is wide and open and is floored by a till sheet at 

 least 150 feet in thickness. The surface declines to the southwest, denoting that the 

 deposit has largely come from the northeast, a view which is supported by bed-rock 

 striae farther down the valley. The till ascends the northwest slope of Fork Peak to 

 an elevation of 2,500 feet, and may continue much higher, but becomes obscured by 

 younger talus and coarse debris of doubtful character, which, however, also exhibits 

 an ill-defined terminal moraine topography. At the elevation of about 3,500 feet is 

 a bench overlain by talus and drift, which, judging from topography, was probably 

 largely deposited by a local hanging glacier. 



