Mima mounds

At the Washington type locality, Mima mounds are domelike or elliptical hills composed chiefly of sandy loam. Geological Society of America reporting describes them as locally up to about 2 metres high and 12 metres across, clustered on terraces associated with the region's glacial history. The Washington Department of Natural Resources describes the preserve's mounds as generally 3–8 feet high and 8–60 feet wide. The figures differ in scope and should be treated as descriptive ranges rather than universal dimensions.

The term “Mima mounds” specifically refers to the landforms of the southern Puget Lowland. Similar-looking mounds elsewhere are usually called Mima-like mounds or by other regional names, and their resemblance does not establish that they formed by the same process.

The mounds occur in a landscape shaped by past glaciers and meltwater. Washington's Department of Natural Resources reports that the mounds rest on glacial outwash, sediment deposited by meltwater, and that their structure differs from the underlying outwash. This indicates that the mound forms developed after that material was laid down, although it does not by itself identify the process that built them.

The mounds' placement on terraces and near former glacier margins is part of the evidence considered by geologists. The exact sequence of sediment movement and mound growth, however, remains open to interpretation.

More than one mechanism has been proposed. Bioturbation models argue that repeated activity by pocket gophers and other burrowing animals moved soil toward favored activity areas, gradually creating or enlarging mounds. A 2014 numerical modelling study argued that a biotic mechanism could produce mound patterns under suitable conditions. A USGS publication reviewing the gopher idea suggested that gophers may rework mound material without necessarily having built the mounds in the first place.

Other proposals invoke glacial processes, including sediment collecting in depressions on or around ice, or cold-climate soil processes. Aeolian models emphasize wind-blown sediment stabilized around vegetation; a seismic hypothesis attributes mound formation to earthquake-related sediment movement. These explanations draw on different observations and may not apply equally to every landscape with mound-like hills. Researchers have also considered whether multiple processes acted together.

The debate persists because the landforms combine biological and geological clues, and because similar surface patterns can arise through different processes. The Geological Society of America review describes a long-running, interdisciplinary controversy and notes that the origin of one mound field need not explain all Mima-like mounds elsewhere. The Washington DNR likewise presents the origin as unresolved, while noting that much evidence points toward a glacial setting.

The Mima Mounds Natural Area Preserve protects examples of the landforms along with Puget prairie habitat. Its value is therefore both scientific and ecological: it offers a place to investigate how soils and landforms develop while conserving a distinctive prairie landscape.