Thus the drainage changes effected by the drift after the ice was gone, concerned both lakes and rivers. In this region there are several illustrations of these changes.

Lakes.—The lake basins of drift-covered regions are of various types. Some of them are altogether in drift, some partly in drift and partly in rock, and some wholly in rock. Basins in the drift were likely to be developed whenever heavy deposits surrounded thin ones. They are especially common in the depressions of terminal moraines.

Another class of lake basins occurs in valleys, the basins being partly rock and partly drift. If a thick deposit of drift be made at one point in a valley, while above there is little or none, the thick deposit will form a dam, above which waters may accumulate, forming a pond or lake. Again, a ridge of drift may be deposited in the form of a curve with its ends against a rock-ridge, thus giving rise to a basin.

In the course of time, the lakes and ponds in the depressions made or occasioned by the drift will be destroyed by drainage. Remembering how valleys develop (p. [46]) it is readily understood that the heads of the valleys will sooner or later find the lakes, and drain them if their bottoms be not too low.

Drainage is hostile to lakes in another way. Every stream which flows into a lake brings in more or less sediment. In the standing water this sediment is deposited, thus tending to fill the lake basin. Both by filling their basins and by lowering their outlets, rivers tend to the destruction of lakes, and given time enough, they will accomplish this result. In view of this double hostility of streams, it is not too much to say that "rivers are the mortal enemies of lakes."

The destruction of lakes by streams is commonly a gradual process, and so it comes about that the abundance and the condition of the undrained areas in a drift-covered region is in some sense an index of the length of time, reckoned in terms of erosion, which has elapsed since the drift was deposited.

In this region there were few lakes which lasted long after the ice disappeared. The basins of the Baraboo and Wisconsin lakes (p. [129]) were partly of ice, and so soon as the ice disappeared, the basins were so nearly destroyed, and the drift dams that remained so easily eroded, that the lakes had but a brief history,—a history that was glacial, rather than postglacial.

The history of the little lake on the East quartzite bluff (p. [133]) as already pointed out, came to an end while the ice was still present.

The beds of at least two other extinct ponds or small lakes above the level of the Baraboo are known. These are at v and w, Plate [XXXVII]. They owed their origin to depressions in the drift, but the outflowing waters have lowered their outlets sufficiently to bring them to the condition of marshes. Both were small in area and neither was deep.

Existing lakes.—Relatively few lakes now remain in this immediate region, though they are common in most of the country covered by the ice sheet which overspread this region. Devil's lake only is well known. The lake which stood in this position while the ice was on, has already been referred to (p. [132]). After the ice had melted away, the drift which it had deposited still left an enclosure suitable for holding water. The history of this basin calls for special mention.