GLACIAL EROSION. 127 



In Buckskin Cunon this bench, which has been cut across by several 

 minor ravines, is not sufficiently regular to be defined by the contours of 

 the map, althouji^h it is readily apparent to the eye. That in Mosquito 

 gulch, however, which forms a practically continuous terrace nearly a mile 

 and a half in length on the north face of Pennsylvania Hill, is shown by the 

 topography of the map, and is about seven hundred feet above the bed of 

 the present stream. It has about the same slope in an easterly direction 

 as the present valley, and this slope carried upward corresponds with the 

 present bottom of the South Mosquito amphitheater above the London fault, 

 which is formed by gently-dipping quartzites and schists of the Weber 

 Grits formation whose angle over a considerable area is about the same as 

 that of the bottom of the basin. On the rock surfaces of the flat portion of 

 this basin glacial grooves and striag are still distinctly to be seen, showing 

 that but little erosion has taken place since the Glacial epoch. On the other 

 hand, in the neighborhood of the fault and in the Archean rocks below it, 

 the present stream-bed deepens very rapidly and the valley becomes a nar- 

 row, winding, V-shaped gorge. In the north Mosquito amphitheater, which 

 is entirely in Archean rocks, the upper part of the basin (which, owing to 

 its great elevation and the consequent low temperature that prevails in it, 

 suffers but little abrasion by running water) remains at essentially the same 

 level as tlie South Mosquito amphitheater, but the V-shaped cutting by 

 present streams extends back much farther than in the latter. The conclu- 

 sion to be drawn from these facts is that the eroding force of glacier ice is 

 a power .so great as to be comparatively independent of the materials on 

 which it acts, while that of running water varies very gi-eatly with the dif- 

 ferent forms and characters of these materials. Thus the original glacial 

 cutting of lower Mosquito gulch formed a comparatively straight and regular 

 valley, but the present stream-bed near the mouth of the canon makes a 

 bend to the soutli, around a boss of more resisting granite on the north side 

 of the valley, and then is deflected to the north by the upturned edges of 

 the Paleozoic strata which cross its course diagonally. 



The Mosquito glacier, as might be expected from its course, left its 

 moraine material mainly on the south side of the valley, where it forms 

 several wooded ridges opposite Park City. It was of greater extent than 



