Where a stream flowed parallel or approximately parallel to the edge of the advancing ice it was sometimes shifted in the direction in which the ice was moving, keeping parallel to the front of the ice. All of these classes of changes took place in this region.

Wisconsin lake.—Reference has already been made to certain lakes which existed in the region when the ice was there. The largest of these lakes was that which resulted from the blocking of the Wisconsin river. The ice crossed its present course at Kilbourn City, and its edge lay to the west of the river from that point to Prairie du Sac (see Plate [I]). The waters from the area now draining into the Wisconsin must either have found an avenue of escape beneath the ice, or have accumulated in a lake west of the edge of the ice. There is reason to believe that the latter was what happened, and that a great lake covered much of the low land west of the Wisconsin river above and below Kilbourn City. The extensive gravel beds on the north flank of the quartzite bluff at Necedah, and the water-worn pebbles of local origin on the slope of Petenwell peak (Plate [XXXII]), as well as the gravels at other points, are presumably the work of that lake. The waters in this lake, as in that in the Baraboo valley, probably rose until the lowest point in the rim of the basin was reached, and there they had their outlet. The position of this outlet has not been definitely determined, but it has been thought to be over the divide of the Black river. [8] It is possible, so far as now known, that this lake was connected with that of the Baraboo valley. Until topographic maps of this region are made, the connections will not be easily determined.

Even after the ice had retreated past the Wisconsin, opening up the present line of drainage, the lakes did not disappear at once, for the ice had left considerable deposits of drift in the Wisconsin valley. Thus at f, Plates [II] and [XXXVII], and perhaps at other points, the Wisconsin has made cuts of considerable depth in the drift. Were these cuts filled, as they must have been when the ice melted, the drainage would be ponded, the waters standing at the level of the dam. This drift obstruction at f would therefore have prolonged the history of the lake which had come into existence when the ice blocked the drainage of the Wisconsin. As the drift of the valley was removed the level of the lake sank and finally disappeared.

Baraboo lake.—Another lake which existed in this region when the ice was here, occupied the valley of the Baraboo and its tributaries when the ice blocked the valley at Baraboo. This lake occupied not only the valley of the Baraboo, but extended up the lower course of every tributary, presumably rising until it found the lowest point in the rim of the drainage basin. The location of this point, and therefore the height of the lake when at its maximum, are not certainly known, though meager data on this point have been collected. At a point three miles southeast of Ablemans on the surface of a sandstone slope, water-worn gravel occurs, the pebbles of which were derived from the local rock. On the slope below the gravel, the surface is covered with loam which has a suggestion of stratification, while above it, the soil and subsoil appear to be the product of local rock decomposition. This water-worn gravel of local origin on a steep slope facing the valley, probably represents the work of the waves of this lake, perhaps when it stood at its maximum height. This gravel is about 125 feet (aneroid measurement) above the Baraboo river to the north.

Further evidence of a shore line has been found at the point marked t, Plate [II]. At this place water-worn gravel of the local rock occurs in much the same relationship as that already mentioned, and at the same elevation above the Baraboo river. At a point two and one-half miles southwest of Ablemans there is local water-worn gravel, with which is mingled glacial material (pieces of porphyry and diabase) which could have reached this point only by being carried thither by floating ice from the glacier. The level of this mixed local and glacial material is (according to aneroid measurement) approximately the same as that of the other localities.

When the ice melted, an outlet was opened via the Lower narrows, and the water of the lake drained off to the Wisconsin by this route. Had the ice left no drift, the lake would have been promptly drained when the ice melted; but the lake did not entirely disappear immediately after the ice retreated, for the drift which the ice left obstructed drainage to the east. The moraine, however, was not so high as the outlet of the lake while the ice was on, so that, as the ice retreated, the water flowed over the moraine to the east, and drew down the level of the lake to the level of the lowest point in the moraine. The postglacial cut through the moraine is about ninety feet deep.

Besides being obstructed where crossed by the terminal moraine, the valley of the Baraboo was clogged to a less extent by drift deposits between the moraine and the Lower narrows. At one or two places near the City of Baraboo, such obstructions, now removed, appear to have existed. Just above the Lower narrows (c, Plate [XXXVII]) there is positive evidence that the valley was choked with drift. Here in subsequent time, the river has cut through the drift-filling of the preglacial valley, developing a passage about twenty rods wide and thirty-five feet deep. If this passage were filled with drift, reproducing the surface left by the ice, the broad valley above it would be flooded, producing a shallow lake.

The retreat of the ice therefore left two well defined drift dams in the valley, one low one just above the Lower narrows, and a higher one, the moraine dam, just west of Baraboo. Disregarding the influence of the ice, and considering the Baraboo valley only, these two dams would have given rise to two lakes, the upper one behind the higher dam being deeper and broader, and covering a much larger area; the lower one behind the lower dam, being both small and shallow.

Up to the time that the ice retreated past the Lower narrows, the waters of the upper and lower lakes were united, held up to a common level by the ice which blocked this pass. After the ice retreated past the Lower narrows, the level of the Baraboo lake did not sink promptly, for not until the ice had retreated past the site of the Wisconsin was the present drainage established. Meantime the waters of the Baraboo lake joined those of Wisconsin lake (p. [129]) through the Lower narrows. If the lakes had been before connected at some point farther west, this connection through the narrows would not have changed the level of either. If they were not before connected, and if the Wisconsin lake was lower than the Baraboo, this connection would have drawn down the level of the latter.

Since the drainage from the Baraboo went to the Wisconsin, the Baraboo lake was not at first lowered below the level of the highest obstruction in the valley of the Wisconsin even after the ice had retreated beyond that stream. As the drift obstructions of the Wisconsin valley were lowered, the levels of all the lakes above were correspondingly brought down. When the level of the waters in these lakes was brought down to the level of the moraine dam above Baraboo, the one Baraboo lake of earlier times became two. The level of the upper of these two lakes was determined by the moraine above Baraboo, that of the lower by the highest obstruction below the moraine in either the Baraboo or Wisconsin valley. The drift obstructions in the Baraboo valley were probably removed about as fast as those in the Wisconsin, and since the obstructions were of drift, and the streams strong, the removal of the dams was probably rapid. Both the upper and lower Baraboo lakes, as well as the Wisconsin, had probably been reduced to small proportions, if not been completely drained, before the glacial period was at an end.