124 A MANUAL OF TOPOGRAPHIC METHODS. 



edly. In the western part of this region it has scoured the surface, cutting 

 away the soft rucks, and leaving the hard ones in projecting knobs, as in the 

 Marquette Iron range of Michigan. This, work was dune so recently that 

 the drainage systems have not yet been well developed. The streams are 

 tortuous and arc interrupted by lakes, swamps, and rapids. 



In northern New England and New York the glacier covered a region 

 of considerable relief, in which streams had established deep courses. Much 

 corrasion was done by it, but upon its retreat the streams reoccupied their 

 former beds. 



Most of the gorges of the Rocky mountains and Sierra Nevada, which 

 had previously been excavated by streams, have been occupied by glaciers, 

 and here and there small glaciers may still be found at their heads. These 

 glaciers, when they were in their prime, occupied the gorges from side to 

 side, and by their erosion broadened them from the sharp almost V shape 

 which water corrasion had given them to a ^_ shape, similar to that of the 

 bed of a stream, but many times larger. 



. At the heads of the main gorge and many of its branches, where the 

 neve fields formerly united and were crowded together into a glacier at the 

 heads of the gorges, there is commonly an amphitheater with steep, even 

 precipitous, walls, curving around in a semicircle. In the middle of this is 

 sometimes a lake or pond, with a rim of rock inclosing it on the lower side. 

 This lake basin was scooped out by the glacial ice, as it came together 

 down the steep slopes of the amphitheater. Here the ice has only modified 

 and shaped a gorge originally carved by water. Where the little streams, 

 flowing toward one another down the steep mountain side, had cut many 

 little gorges, with sharp spurs between them, the ice has pared away the 

 spurs, producing an amphitheater. PI. xvm illustrates the cirque in the 

 Rocky mountains of Colorado. 



DEPOSITION FROM THE ATMOSPHERE. 



The winds transport sand and deposit it in drifts, known as dunes, 

 They commonly appear as lines of hills, like hogbacks, with the gentle 

 slope toward the prevailing winds. Not having been shaped by erosion, 

 they present great inequalities of surface. 



