The Grays Lake quadrangle in northern Illinois lies within the Valparaiso (Wisconsin) moraine. The late glacial history of this region embraced (1) deposition of moraines of the Tazewell substage, (2) deposition in late Marseilles time of an outwash plain inside the Tazewell moraines, (3) a post-Tazewell erosion interval in which the Fox River valley and tributaries were cut to present depth, and (4) glacial advance in the Cary substage, with deposition of the Minooka, Valparaiso, Tinley, and Lake Border moraines.

The Grays Lake quadrangle contains the eastern edge of the late Marseilles gravel plain, deeply trenched and later mantled by Valparaiso drift. A lowland east of this plain, probably a fosse, is now partly filled with moraine and low outwash of Valparaiso age. The Valparaiso till sheet is thin and rests on older drift. Widely distributed throughout the Grays Lake area below the 830-foot level are deposits of thinly laminated lacustrine clay and silt. Some of the silt rests on Valparaiso moraine and originated in shallow postglacial lakes, but much of it is marked by morainal topography, and some is overlain by Valparaiso till. Thus is indicated a pre-Valparaiso lake formed during the deposition of the Minooka moraine, first of the Cary substage. The Minooka glacier crossed Fox River valley at Elgin, Illinois, forcing it into a new 820-foot channel farther west. From Elgin the Minooka moraine trends northeastward and, although buried by Valparaiso drift, lies near the east margin of the Grays Lake quadrangle all of which west of the Minooka moraine was flooded to the 820- to 830-foot level. Silt and clay deposits of the lake thus formed were later overridden by the glacier in Valparaiso time. To this lake the name “Glacial Lake Wauconda” is given.

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