312 



EARTH FEATURES AND THEIR MEANING 



of the surface, and in part by the upturning of ice layers near the 

 margin (see ante, p. 277). 



An important role is played by the thaw water which emerges 

 at the ice margin, especially within the reentrants or recesses of 

 the outline. The materials of moraines are, therefore, till with 

 large local deposits of kame gravel, and these form in a series of 

 ridges corresponding to the temporary positions of the ice front. 

 Their width may range from a few rods to a few miles, their height 



may reach a hundred feet or more, 

 and they stretch across the country 

 for distances of hundreds or even 

 thousands of miles, looped in arcs 

 or scallops which are always convex 

 outward and which meet in sharp 

 cusps that in a general way point 

 toward the embossment of the 

 former glacier (Fig. 334, p. 308, and 

 Fig. 336). These festoons of the 

 moraines outline the ice lobes of 

 the latest ice invasion, which in 

 North America were centered over 

 the depressions now occupied by the 

 Laurentian lakes. There was, thus, 

 a Lake Superior lobe, a Lake Mich- 

 igan lobe, etc. With the aid of 

 these moraine maps we may thus 

 in imagination picture in broad lines 

 the frontal contours of the earlier 

 glaciers. At specially favorable lo- 

 calities where the ice front has 

 crossed a deep valley at the edge of 



the Driftless Area, we may, even in a rough way, measure the slope 

 of the ice face. Thus near Devils Lake in southern Wisconsin the 

 terminal moraine crosses the former valley of the Wisconsin River, 

 and in so doing has dropped a distance of about four hundred feet 

 within the distance of a half mile or thereabouts (Fig. 337). 



The characteristic surface of the marginal moraine is responsible 

 for the name " kettle " moraine so generally applied to it. '* The 

 " kettles " are roughly circular, undrained basins which lie among 



FIG. 336. Sketch map of portions 

 of Michigan, Ohio, and Indiana, 

 showing the festooned outlines of 

 the moraines about the former ice 

 lobes, and the directions of ice 

 movement as determined by the 

 striae upon the rock pavement 

 (after Leverett). 



