HISTORIC FORMATION AND RE-FORMATION OF MIMA MOUNDS

Few topics in Earth sciences are as interesting, or provocative, as the origin of mima mounds. While many theories have been advanced, all can be reduced to either biological or physical models, or some combination. Nevertheless, evidence is slowly and inexorably accumulating that some combination fits the evidence, and that a polygenetic explanation is indicated. Evidence further suggests that one process plays a more ‘common denominator’ role than any other -- animal bioturbation. In fact, evidence is overwhelming that mima mounds are, in the first instance, simply point centered, locally thickened biomantles produced at animal ‘activity centers’ -- living-reproduction-food storage and over wintering sites. Many studies inclined towards this conclusion implicate rodents as the primary process vectors, in North America commonly one or more members of the 40+ species of Geomyidae – pocket gophers. Adherents to this view, including us, generally explain mound formation under the Dalquest-Scheffer-Cox ‘centripetal-soil-movement-to-activity-centers’ model. One problem with this approach has been an inability to demonstrate historic mound formation beyond near-anecdotal instances.

We have discovered many cases where mima mounds are actually in the process of being formed by pocket gophers along highway right-of-ways following either road grading and or highway construction. Examples are new low mounds along U.S. Highway 287 on granite southeast of Laramie, Wyoming, and along State Highway 139 on basalt near Canby on Modoc Plateau, California. We have also documented instances where pocket gophers have actually re-formed mima mounds on glacial outwash above long-abandoned ranch and historic roads. Examples are in Shasta Valley northwest of Weed bordering Interstate 5, and where Butte Creek crosses Tennant Road, both in Siskiyou County, California.

Marine geologists have long recognized the key and important role of animal bioturbations in subaqueous sediments, and are successfully quantifying their effects in spite of difficulties and efforts in doing so. We strongly encourage students of subaerial soils and sediments -- future and in-training Quaternary geologists, geomorphologists, and pedologists -- to do likewise. New research pathways and discoveries are certain to accrue.