In the deposition of stratified drift beyond the edge of the ice, the latter was concerned only in so far as its activity helped to supply the water with the necessary materials.

C. Deposits at and beyond the edge of the ice in standing water.—The waters which issued from the edge of the ice sometimes met a different fate. The ice in its advance often moved up river valleys. When at the time of its maximum extension, it filled the lower part of a valley, leaving the upper part free, drainage through the valley stood good chance of being blocked. Where this happened a marginal valley lake was formed. Such a lake was formed in the valley of the Baraboo when the edge of the ice lay where the moraine now is (Plate [II]). The waters which were held back by the ice dam, reinforced by the drainage from the ice itself, soon developed a lake above the point of obstruction. This extinct lake may be named Baraboo lake. In this lake deposits of laminated clay were made. They are now exposed in the brick yards west of Baraboo, and in occasional gullies and road cuts in the flat bordering the river.

At the point marked s (Plate [XXXVII]) there was, in glacial times, a small lake having an origin somewhat different from that of Baraboo lake (see p. [133]). The former site of the lake is now marked by a notable flat. Excavations in the flat show that it is made up of stratified clay, silt, sand and gravel, to the depth of many feet,—locally more than sixty. These lacustrine deposits are well exposed in the road cuts near the northwest corner of the flat, and in washes at some other points. Plate [XXXVIII] shows some of the silt and clay, the laminæ of which are much distorted.

Deltas must have been formed where well-defined streams entered the lakes, and subaqueous overwash plains where deltas became continuous by lateral growth. The accumulation of stratified drift along the ice-ward shores of such lakes must have been rapid, because of the abundant supply of detritus. These materials were probably shifted about more or less by waves and shore currents, and some of them may have been widely distributed. Out from the borders of such lakes, fine silts and clays must have been in process of deposition, at the same time that the coarse materials were being laid down nearer shore.

WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XXXVIII.

Distorted laminae of silt and clay.
[See larger image]

Good examples of deltas and subaqueous overwash plains do not appear to exist in the region, although conditions for their development seem to have been present. Thus in the lake which occupied the valley of the Baraboo, conditions would seem to have been ideal for the development of such features; that is, the overwash plains previously described should, theoretically, have been subaqueous overwash plains; but if this be their character, their distinctive marks have been destroyed by subsequent erosion.

During the maximum extension of an ice sheet, therefore, there was chance for the development, at its edge or beyond it, of the following types of stratified drift: (1) kames and kame belts, at the edge of the ice; (2) fluvial plains or valley trains, in virtual contact with the ice at their heads; (3) border plains or overwash plains, in virtual contact with the ice at their upper edges; (4) ill-defined patches of stratified drift, coarse or fine near the ice; (5) subaqueous overwash plains and deltas, formed either in the sea or lakes at or near the edge of the ice; (6) lacustrine and marine deposits of other sorts, the materials for which were furnished by the waters arising from the ice. So far as this region is concerned, all the deposits made in standing water were made in lakes.

Deposits Made by Extraglacial Waters During the Retreat of the Ice.