ABSTRACT

Quantitative data on the sediments are presented, along with a discussion of their environment, sources, and agents of transportation. Maps show bottom topography and areal distribution of sediments, and diagrams illustrate relations between mechanical composition and bottom topography.

Bottom sands of both the eastern and western shore terraces of the lake show a coarsening in size as the depth of water and distance from shore increase. A gravel-veneered till bottom is common in the western and southern parts of the lake; the gravel is interpreted as a lag concentrate of the coarser constituents of the till, produced by wave and current action. Sand derived from the till is apparently carried toward shore in most localities, while the finer fraction of till is deposited as a soft gray clay in the deeper parts of the lake. In the Chicago area, sand occurs in broad low ridges which are nearly parallel with shore, while gravel veneered till is found in the hollows between the ridges. This topography may be largely inherited from a time of low water, but the characters f the sediments indicate important present-day action on the bottom.