1884. 51 Tracts. N. V. Ac. Set. 



A paper was then read by Dr. John S. Newberry, illustrated by 

 lantern views and a map, on the subject of 



THE EROSIVE POWER OF GLACIER-ICE, AND ITS INFLUENCE ON 

 THE TOPOGRAPHY OF NORTH AMERICA. 



(Abstract.) 



Ice being a comparatively soft substance, seems at first sight inca- 

 pable of doing much toward the wearing away of solid rock or changing 

 topography. But the facts observed in connection with all glaciers 

 prove that ice is a powerful eroding agent. W^ere several hundred or 

 several thousand feet in thickness, as some glaciers have been, the ice 

 rests upon the underlying surface with a weight of hundreds and even 

 thousands of pounds to the square foot. Such a mass in motion, 

 impinging against obstacles, has often crushed and removed them, 

 gathering beneath it fragments of all sizes, from sand to boulders ; these 

 have been the instruments of powerful grinding action, which over 

 great surfaces has planed down the rocks, removed or rounded over 

 asperities, tilled valleys with debris, and thus produced a marked 

 effect upon the topography. Local glaciers broaden and deepen the 

 valleys in which they move, and having a positive excavating power, 

 have often increased the irregularities of topography ; while broad, con- 

 tinental glaciers have produced just the opposite effect, as we have 

 evidence on a stupendous scale. Over all that portion of North 

 America lying north and east of Bismarck, St. Louis, Cincinnati and 

 New York, the surface bears marks of extensive erosion by ice, con- 

 sisting of planed, scratched, and undulated rock surfaces, the inscrip- 

 tion made by glaciers and nothing else, and sheets of transported 

 material, which has been brought a greater or less distance from the 

 north southward. This sheet of drift material extends from the Banks 

 of Newfoundland, which are formed of glacial debris, over Nova Scotia, 

 Canada, New England, New York, Northern Pennsylvania, over most 

 of Ohio, Indiana, Illinois, Michigan, Wisconsin, and Minnesota. From 

 the British line it extends northward, parallel with the Canadian High- 

 lands, probably to the Arctic Sea. Over all this area, which embraces 

 not less than a million of square miles, the surface of the eroded rocks 

 is covered with a sheet from ten to three hundred feet in thickness, 

 averaging thirty or forty feet of glacial debris, boulder clay, boulders, 

 gravel, sand, etc., all of which have been moved a greater or less dis- 

 tance southward. North and east of the Canadian Highlands, which 

 extend from Labrador to the great Lakes, and then to the Arctic Sea, 

 the country has been but partially explored ; yet it apparently every- 

 where bears evidence of having been covered with glaciers, and it would 



