[Illustration: Fig. 34.--Diagrammatic representation of the effect of a hill on the edge of the ice.]
After the ice has disappeared, the influence of the obstruction might be found in the disposition of the drift. If recession began during the first stage, that is, when the ice edge was separated into lobes, the margin of the drift should be lobate, and would loop back around the ridge from its advanced position on either side. If recession began during the second stage, that is, when the lobes had become confluent and completely surrounded the hill, a _driftless area_ would appear in the midst of drift. If recession began during the third stage, that is, after the ice had moved on over the obstruction, the evidence of the sequence might be obliterated; but if the ice moved but a short distance beyond the hill, the thinner ice over the hill would have advanced less far than the thicker ice on either side (Fig. 35), and the margin of the drift would show a re-entrant pointing back toward the hill, though not reaching it. All these conditions are illustrated in the Devil's lake region.
[Illustration: Fig. 35.--Same as Fig. 34, when the ice has advanced farther.]
The region under description is partly covered with drift, and partly free from it. The limit of the ice, at the time of its maximum expansion is well defined at many points, and the nature and position of the drift limit are so unique as to merit attention (see Plates II and XXXVII). They illustrate many of the principles already discussed.
The ice which covered the region was the western margin of the Green Bay lobe (Fig. 36) of the last continental ice sheet. Its limit in this region is marked by a ridge-like accumulation of drift, the _terminal moraine_, which here has a general north-south direction. The region may have been affected by the ice of more than one epoch, but since the ice of the last epoch advanced as far to the west in this region as that of any earlier epoch, the moraine is on the border between the glaciated country to the east, and the driftless area to the west (Plates I and II). That part of the moraine which lies west of the Wisconsin river follows a somewhat sinuous course from Kilbourn City to a point a short distance north of Prairie du Sac. The departures from this general course are especially significant of the behavior of glacier ice.