THE PLEISTOCENE OR GLACIAL PERIOD. 365 



But it is the association of the " knobs" or " short hills" with the 

 " kettles," and not either feature alone ; which is especially character- 

 istic of terminal moraine topography. 



The " knobs " vary in size, from low mounds but a few feet across, 

 to considerable hills half a mile or more in diameter, and a hundred 

 feet or more in height. If they attain such heights while their bases 

 are small, their slopes are steep. Not rarely they are about as steep 

 as the loose material of which they are composed will lie. 



The " kettles " are the counterparts of the elevations. They may 

 be a few feet, or many rods, or even furlongs in diameter. They may 

 be so shallow that the sagging at the center is scarcely observable, 

 or they may be scores of feet in depth. If steep-sided depressions 

 are closely associated with abrupt hillocks, the topography may be 

 notably rough, and the total relief within a few rods may be nearly 

 equal to the total height of the moraine above its surroundings. The 

 topography of the terminal moraine is often strongly developed, even 

 where the moraine as a whole does not appear as a distinct ridge. 1 



The surface of the terminal moraine, where well developed, is gen- 

 erally rougher than that of the ground moraine, but more because 

 the sags and swells are of smaller area and steeper slopes than because 

 the relief is notably more. It is not to be understood, however, that 

 this peculiar topography always affects terminal moraines, or that 

 it is strictly confined to them. The elevations and depressions of the 

 moraine may grade from strength to weakness, and locally may even 

 disappear, while features closely simulating those characteristic of ter- 

 minal moraines are sometimes found in other parts of the drift. 



Development of terminal moraine topography. — The first condi- 

 tion for the development of a terminal moraine is that the edge of 

 the ice remain approximately stationary in position for a time suffi- 

 ciently long for the submarginal accumulation to become sensibly 

 thicker than the drift within or without. If the margin of the ice 

 remained constant in position over a region of uniform topography 

 during the formation of a terminal moraine, and if the ice bore equal 

 amounts of material at all points along its margin, the terminal moraine 

 would be developed with some regularity. It would be about as high 



1 The terminal moraines of various regions are described in various state 

 reports and in various reports of the U. S. Geol. Surv., especially the 3d Ann. Rept., 

 and in Monographs XXXVIII and XLI. 



