The current landscape of the northeast corner of Illinois was shaped principally by glacial activity, particularly when the Wisconsin glacier began its final stages of melting thousands of years ago. As it receded, it deposited a blanket of unsorted debris, including clay, sand, gravel and boulders, collectively called glacial till. Embedded in the till were large chunks of ice that broke off the melting glacier. As the climate continued to warm, the ice blocks melted, forming depressions which developed into lakes, bogs and marshes. Volo Bog was originally a deep 50-acre lake, with steep banks and poor drainage. Research on pollen grains preserved in the bog indicates that the lake began filling with vegetation approximately 6,000 years ago. A floating mat, consisting primarily of sphagnum moss formed around the outside edges among the cattails and sedges. As these plants died and decomposed, the peat mat thickened, forming a support material for rooted plants. Because of the lack of drainage and the presence of sphagnum moss, the water in the bog became acidic. This limited the types of plants that could survive and thus created the unique plant communities found in the bog. Volo Bog is significant in that it exhibits all stages of bog succession. A floating mat of sphagnum moss, cattails and sedges surrounds an open pool of water in the center of the bog. As substrate material thickens, a shrub community dominated by poison sumac and leatherleaf invades the mat. This is eventually replaced by tamarack forest. Surrounding this forest is a second, more extensive shrub zone which abruptly ends and becomes a marsh/sedge meadow community.