SIGXS OF FORMER GLAGIATION. 133 



boundary-line rises from the level of the sea at New York 

 to the height of the Blue Mountains in New Jersey, and 

 descends into the valley of the Delaware, and rises again 

 over the Blue Ridge in Pennsylvania, crossing the valley 

 between it and Pocono Mountain, where it runs for many 

 miles at an elevation of 2.000 feet above the sea, and descends 

 in a nearly straight line to the East Branch of the Susque- 

 hanna at Beach Haven, where it is not more than 500 feet 

 above the sea. Thence it continues onward in a diagonal 

 course across the Alleghanies, with their various subsidiary 

 valleys, to its great turning-point in southwestern New York. 

 Thence to the trough of the Mississippi, for a distance of 

 700 or 800 miles, the line winds gradually down the west- 

 ern "flanks of the Alleghanies, paying little attention in its 

 deflections to the minor inequalities of the country. West 

 of the Mississippi the rise is equally gradual to northern 

 Dakota and Montana, where the glacial border is 2,000 or 

 3,000, and in British America 4,000, feet above the sea. 



Thus it is not possible, by the supposition of any conceiv- 

 able submergence, to account for this line of demarkation. 

 Water which would have floated ice from the north to almost 

 any point along that line would have floated it farther south. 

 Consequently, the line must have been determined not by a 

 barrier which restrained a fluid, but by the irregular losses 

 of momentum such as would take place in a semi-fluid mov- 

 ing in the line of least resistance from various central points 

 of accumulation. 



The reader will find the subject of this chapter treated 

 with great fullness by President Chamberlin in the " Third 

 Annual Report of the United States Geological Survey," pp. 

 291-402, and in the " Seventh Report," pp. 149-248, where 

 the subject of rock-scoring is most fully illustrated. Numer- 

 ous illustrations in the present volume have been reserved for 

 the latter part of Chapter X, on " Glacial Erosion and Trans- 

 portation," which the reader will do well to consult in this 

 connection as well as in the order in which it occurs. 



