782 THE POPULAR SCIENCE MONTHLY. 



concludes that several hundred feet of the ridge have been worn 

 away by the ice. 



The crowning example of bowlder transportation is, however, 

 afforded by the blocks of light gray gneiss discovered by Prof. 

 Hitchcock on the summit of Mount Washington, over six thou- 

 sand feet above sea-level, and identified with Bethlehem gneiss, 

 whose nearest outcrop is in Jefferson, several miles to the north- 

 west, and three or four thousand feet lower than Mount Wash- 

 ington. 



These varied phenomena of erratic blocks and rock striations, 

 together with the enormous quantity of bowlder clay and glacial 

 drift spread over the whole of the Eastern States, terminating 

 southward in a more or less abrupt line of mounds having all the 

 characteristics of an enormous moraine, have led American geolo- 

 gists to certain definite conclusions in which they all practically 

 agree. It may be well first to give a notion of the enormous 

 amount of the glacial debris under which a large part of the East- 

 ern States is buried. In New England these deposits are of less 

 thickness than farther south, averaging from ten to twenty feet 

 over the whole area. In Pennsylvania and New York east of the 

 Alleghanies the deposits are very irregular, often sixty or seventy 

 feet thick and sometimes more. West of the Alleghanies in New 

 York, Pennsylvania, and Ohio the thickness is much greater, be- 

 ing often one hundred and fifty or two hundred feet in the wide 

 valleys, and forty or fifty feet on many of the uplands. Prof. 

 Newberry calculates that in Ohio it averages sixty feet deep over 

 an area of twenty-five thousand square miles. 



The direction of the striae and of the traveled bowlders to- 

 gether with the form of the great terminal moraines show that 

 there must have been two main centers of outflow for the ice- 

 sheet, one over Labrador, the other over the Laurentian High- 

 lands north of Lake Superior. The southern margin of the drift 

 may be roughly represented by portions of circles drawn from 

 these two points as centers. The erratics on the summit of 

 Mount Washington show that the ice-sheet must have been a 

 mile thick in its neighborhood, and much thicker at the centers 

 of dispersion, while the masses of drift and erratics on plateaus two 

 thousand feet high near its southern boundary indicate a great 

 thickness at the termination. The Laurentian plateau is now about 

 two thousand feet above the sea-level, but there are numerous in- 

 dications from buried river channels, filled with drift and far 

 below the sea, which lead to the conclusion that during the Ice 

 age the land was much higher. That snow can accumulate to 

 an enormous extent over land of moderate height when the condi- 

 tions are favorable for such an accumulation is shown by the 

 case of Greenland, the greater part of whose surface is a vast 



