An important rôle is played by the thaw water which emerges at the ice margin, especially within the reëntrants 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 Michigan 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 localities 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 hummocks or knobs, so that the surface has often been referred to as “knob and basin” topography (plate 17 C). [Illustration: FIG. 337.—Map of the vicinity of Devils Lake, Wisconsin, located within a reëntrant of the “kettle” moraine upon the margin of the Driftless Area. The lake lies within an earlier channel of the Wisconsin River which has been blocked at both ends, first by the glacier and later by its moraine. The stippled area upon the heights and next the moraine represents the clay deposits of a former lake (based on map by Salisbury and Atwood).] [Illustration: FIG. 338.—Moraine with outwash apron in front, the latter in part eroded by a river. Westergötland, Sweden (after H. Munthe).] [Illustration: FIG. 339.—Fosse between an outwash plain (in the foreground) and the moraine, which rises to the left in the middle distance. Ann Arbor, Michigan.]