Geography of Ohio 193 



bottoms of valleys in some localities. Sometimes, along the sides 

 of a valley, a block will be plucked bodily; but the slow scouring 

 and rasping process, such work as the glacier did last on a striated 

 surface, is the more usual method of wearing away rock. 



The fiords of Alaska and Norway, the U-shaped valleys of 

 Switzerland, the rock basins now holding lakes in England and 

 Scotland, the over-deepened valleys in the Finger Lake region oi 

 New York, the cirques and amphitheaters of all glaciated moun- 

 tainous areas, show the competency of ice to erode rock. There 

 may be a lack of agreement among students as to just how fiords 

 and rock basins were produced. In the studies of Gilbert in Alaska, 

 of Penck, Bruckner, and Davis in the Swiss region, of Reusch in 

 Scandinavia, of Marr in England and Scotland, of King and At- 

 wood in the Rockies, of Chamberlin and Salisbury in Greenland, 

 and of Tarr and others in central New York, we have an array of 

 evidence that takes the c^uestion of glacial erosion out of court. 

 It is no longer a discussed point. 



The conditions that obtained in the regions where active ice 

 can now be studied are not necessarily identical with the erosion 

 processes of the ice sheet in North America. The closest example, 

 perhaps, is found in Greenland, where a valley leading to tide 

 level now bears a tongue of ice, behind which is a large area of ice 

 whose forward movement is concentrated on this one valley as an 

 outlet ; erosion consequently is vigorous. In all of our over- 

 deepened valleys, fiords and basins, it is probable that ice action 

 was similarly concentrated. Such a tongue of ice, shod with tools, 

 slowly rasping the bottom and the lower side walls of the valley, 

 and continuing in action through man}^ centuries or perhaps 

 thousands of years, did not accomplish anything astounding, in 

 wearing away the rock many hundred feet. In addition to lower- 

 ing the bottom of the valley, its walls were cut back, thus chang- 

 ing its cross-section to a broad U. Tributary streams that former- 

 ly met the major stream of the valley at grade were left hanging 

 several hundred feet. ''The hanging valley is especially signifi- 

 cant in two lines of physiographic interpretation. It is a con- 

 spicuous earmark of the former presence of glaciers; and it helps to 

 a conception of the magnitude of the Pleistocene glacial erosion. "- 



2 Harriman Alaskan Expedition, vol. iii, (1904), Glaciers and Glaciatiori, page 

 115. 



