THE CONTINENTAL GLACIERS OF THE "ICE AGE" 303 



point it was assumed that the ice traveled southward up the 

 northern slope of the Laurentian divide in Canada, and thence 

 to the Ohio river, a distance of over 1300 miles. If such a mantle 

 of ice be represented in its natural proportions in vertical section, 

 to cover the distance from center to margin we may use a line 

 six inches in length, and only y^g- of an inch thick. Upon a reduced 

 scale these proportions are given in Fig. 329. Obviously the 

 force of gravity acting within a viscous mass of such proportions 



FIG. 329. Cross section in approximate natural proportions of the latest North 

 American continental glacier of Pleistocene age from its center to its margin. 



would be incompetent to effect a transfer of material from the 

 center to the periphery, even though the thickness should be 

 doubled or trebled. Yet until the fixed glacial anticyclone above 

 the glacier had been proven and its efficiency as a broom recog- 

 nized, no other hypothesis than that of viscous flow had been 

 offered in explanation. The inherited conception of a universal 

 plucking and abrasion on the bed of the glacier is thus made un- 

 tenable and can be accepted for the marginal portion only. 



Not only do the rock scorings show the lines of ice movement, 

 but the directions as well may often be read upon the rock. Wher- 

 ever there are pronounced irregularities of surface still existing on 

 the pavement, these are generally found to have gradual slopes 

 upon the side from which the ice came, and relatively steep falls 

 upon the lee or " pluck " side. If, however, we consider the irregu- 

 larities of smaller size, the unsymmetrical slopes of these protruding 

 portions of the floor are found to be reversed it is the steep slope 

 which faces the oncoming ice and the flatter slope which is upon the 

 lee side. Such minor projections upon the floor usually have their 

 origin in some harder nodule which deflects the abrading tools and 

 causes them to pass, some on the one side and some upon the other. 

 By this process a staple-shaped groove comes to surround the 

 nodule, leaving an unsymmetrical elevated ridge within, which is 

 steep upon the stoss side and slopes gently away to leeward. 



Younger records over older the glacier palimpsest. Many 

 important historical facts have been recovered from the largely 

 effaced writing upon ancient palimpsests, or parchments upon 

 which an earlier record has been intentionally erased to make room 



