The Origin of Mima Mound (Hogwallow) Microrelief in the Far Western States

The origin of the mound microrelief known by such names as “hogwallows,” “pimple mounds” or “mima mounds” has long been a mystery; although many solutions have been proposed, none has yet received widespread acceptance. During the course of the soil surveys of eastern Merced and Stanislaus Counties of California, where mound microrelief is widespread, the phenomenon was studied over a period of eight years. All known theories were evaluated and compared with the evidence which could be observed in the field. The evidence includes the nature of the mounds such as shape, size, internal structure, distribution of gravel, the burrows of ants and rodents; the distribution of the microrelief with respect to soil types and the pattern of occurrence of mounds under various conditions. The theories of origin which were tested included erosional theories based on wind action and water action, fluviatile, submarine and sub‐lacustrine currents; eruptional theories suggesting gas‐vents, sand boils and hydrostatic pressure; and biological theories based upon the activity of ants, ground squirrels and pocket gophers. It is concluded that the evidence points clearly to a biological explanation, and that the pocket gopher is responsible for Mima mound microrelief.