At the north end of the lake, and again in the capacious valley leading east from its south end, there are massive terminal moraines. Followed southward, this valley though blocked by the moraine a half mile below the lake, leads off towards the Wisconsin river, and is probably the course of a large preglacial stream. Beyond the moraine, this valley is occupied by a small tributary to the Wisconsin which heads at the moraine. To the north of the lake, the head of a tributary of the Baraboo comes within eighty rods of the lake, but again the terminal moraine intervenes. From data derived from wells it is known that the drift both at the north and south ends of the lake extends many feet below the level of its water, and at the north end, the base of the drift is known to be at least fifty feet below the level of the bottom of the lake. The draining of Devil's lake to the Baraboo river is therefore prevented only by the drift dam at its northern end. It is nearly certain also, that, were the moraine dam at the south end of the lake removed, all the water would flow out to the Wisconsin, though the data for the demonstration of this conclusion are not to be had, as already stated (p. [132]).

There can be no doubt that the gorge between the East and West bluffs was originally the work of a pre-Cambrian stream, though the depth of the pre-Cambrian valley may not have been so great as that of the present. Later, the valley, so far as then excavated, was filled with the Cambrian (Potsdam) sandstone, and re-excavated in post-Cambrian and preglacial time. Devil's lake then occupies an unfilled portion of an old river valley, isolated by great morainic dams from its surface continuations on either hand. Between the dams, water has accumulated and formed the lake.

Changes in Streams.

In almost every region covered by the ice, the streams which established themselves after its departure follow more or less anomalous courses. This region is no exception. Illustrations of changes which the deposition of the drift effected have already been given in one connection or another in this report.

Skillett creek.—An illustration of the sort of change which drift effects is furnished by Skillett creek, a small stream tributary to the Baraboo, southwest of the city of that name. For some distance from its head (a to b, Fig. [43]) its course is through a capacious preglacial valley. The lower part of this valley was filled with the water-laid drift of the overwash plain. On reaching the overwash plain the creek therefore shifted its course so as to follow the border of that plain, and along this route, irrespective of material, it has cut a new channel to the Baraboo. The postglacial portion of the valley (b to c) is everywhere narrow, and especially so where cut in sandstone.

The course and relations of this stream suggest the following explanation: Before the ice came into the region, Skillett creek probably flowed in a general northeasterly direction to the Baraboo, through a valley comparable in size to the preglacial part of the present valley. As the ice advanced, the lower part of this valley was occupied by it, and the creek was compelled to seek a new course. The only course open to it was to the north, just west of the advancing ice, and, shifting westward as fast as the ice advanced, it abandoned altogether its former lower course. Drainage from the ice then carried out and deposited beyond the same, great quantities of gravel and sand, making the overwash plain. This forced the stream still farther west, until it finally reached its present position across a sandstone ridge or plain, much higher than its former course. Into this sandstone it has since cut a notable gorge, a good illustration of a postglacial valley. The series of changes shown by this creek is illustrative of the changes undergone by streams in similar situations and relations all along the margin of the ice.

Fig. 43. -- Skillett Creek, illustrating the points mentioned in the text.
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The picturesque glens (Parfrey's and Dorward's) on the south face of the East bluff are the work of post-glacial streams. The preglacial valleys of this slope were obliterated by being filled during the glacial epoch.

The Wisconsin.—The preglacial course of the Wisconsin river is not known in detail, but it was certainly different from the course which the stream now follows. On Plate [I] the relations of the present stream to the moraine (and former ice-front) may be seen. [10] As the ice approached it from the east, the preglacial valley within the area here under consideration was affected first by the overwash from the moraine, and later by the ice itself, from the latitude of Kilbourn City to Prairie du Sac.