The wetland depressions in Glacial Park formed, about 14,000 years ago, around the tail end of the Pleistocene ice age. In time, without any stream recharge or channel outflow, these shallow depressions, partially dried up to form fens, bogs, and marshes. Plant communities such as skunk cabbage, for example, prefer soggy, alkaline conditions and thrive in fens. Alkalinity in fens can occur when groundwater percolates up through soil or bedrock and picks up salt ions that later precipitate near the surface. In contrast, bogs are acidic environments.
Bogs often contain sphagnum moss and leather-leaf shrubs.
Marshes often are inhabited by cattails and by bulrushes. Another important aspect of Glacial Park is the ongoing one-mile long dechannelization project in the Nippersink Creek. In 1951, before the park district bought the land, a 1.8-mile long section of the creek was channelized to reduce its floodplain. This allowed excess water to run off the fields so they could be used for agriculture and grazing. Figure 3-3 shows a reproduction of the original channel.