The old lake plain is part of the Tamarack Lowlands Subsection within the Northern Minnesota Drift Plains Section of the Laurentian Mixed Forest. Lacustrine sediments deposited by the glacial lake are parent to the present soils of peats, silt, and sand; the peats are in marshes which form large parts of the park. These wet lowlands adjoin rolling hills which are end moraines of the recent glaciation. The moraines are part of the Saint Louis Moraines Subdivision of the Northern Minnesota Drift Plain.