Mysterious Mima Mounds

In western Washington State, just south of Puget Sound, a series of dirt mounds are scattered along the Mima Prairie. These seemingly arbitrary extrusions of earth are the subject of great debate, as their origins are not quite clear. A number of hypotheses have been formed in the hundred years since the mounds were first studied, though none have been proven to date.

The Mima Prairie lies in the Puget Sound basin west of the Cascade Range. The area was covered by the Puget lobe of the Cordillera Ice Sheet during the Vashon Stade of the Fraser Glaciation, approximately 15,000 to 13,000 yr BP. The surficial terrain consists of sandy outwash sediments [Thorson 1980]. The mounds themselves are six to eight feet high and about thirty feet across [Nelson 1994], and though similar masses appear elsewhere—Wyoming, Arkansas, Mexico, Argentina, Kenya—the mounds on the Mima Prairie are the largest known to exist [Geiger 1998].

One of the first answers to the question of the mounds’ origins was glacial process. It was proposed that the mounds remained after softer sediments were wiped out by the retreating Cordillera Ice Sheet, which would make the mounds some 13,000 years in age [Geiger 1998].

Another theory proposes that the mounds are the result of soil-mounding activity by gophers. This would occur over a long period of time as gophers dig series of burrows, thus loosening soil and pushing it up to the surface. The theory was first presented by researchers Dahlquest and Scheffer of the University of Washington, who observed the burrowing habits of gophers. They pointed out that when gophers come upon an object too large to move, they burrow around it, and often the earth they displace when burrowing is ejected to the surface. Over time these small accumulations of soil would form larger piles and eventually create Mima mounds [Mielke 1977].

Various features of the mounds provide supporting evidence for this theory. To begin with, gophers either currently inhabit or at one inhabited all locations where Mima Mounds are found, and by the same token, mounds are missing from otherwise similar plains where there is no evidence of gopher habitation. In addition, the mounds are made up entirely of materials small enough to be moved by gophers, despite the existence of much larger materials underneath and between mounds; this correlates to the observations of gophers tunneling around large objects, as they eventually sink to the bottom of the mounds’ loosened soils [Dahlquest & Scheffer 1942]. Furthermore, evidence lies in the A-horizon soils within the mounds, which have been greatly redistributed since the parent soils were initially deposited, as indicated by the pollen record and radiocarbon dating of the sediments in the mounds. In the inter-mound areas the A-horizon is less than six inches thick, but is found to be as thick as seven feet within the mounds [Washburn 1988].

Moreover, remnants of certain types of beetles that reside exclusively within the burrows of small mammals—namely pocket gophers—have been found within mounds in the Mima Prairie [Nelson 1994]. By collecting ten kilograms of sediment and examining insect remains found therein, it was determined that three different species of beetles, two of which are known to be obligate burrow inhabitants, were present in the mounds. Nelson determined that all fauna found within the mound were “exclusively consistent with rodent burrows in a grassland environment.”

In another theory, it has been suggested that the combination of water runoff and the anchoring of vegetation in a once present forest could have caused the mounds to exist as they do today. The shape and distribution of the mounds supports the theory that tree roots held soils in place while fluvial processes eroded the surrounding sediments. Because the forest that once existed was burned by aboriginals, seedlings were destroyed and the forest did not grow back, leaving the prairie that exists today [Washburn 1988].

Another theory which has come up in recent years and is regarded by many as correct is not biological but seismic in nature. It has been suggested that in areas where loose soil thinly overlays either rock or harder soils, seismic activity (specifically earthquake waves) could cause uplift and create mima mounds. It is also proposed that once the mounds are formed they are stable and therefore would not be disturbed by future seismic activity. Geologist Andrew Berg performed an experiment using a plywood board covered with a thin layer of loess. He simulated seismic activity on the surface (which was similar in nature to the thin soils overlaying harder material on the Mima Prairie) and noted that the loess formed small mounds which strikingly mirrored those found on the prairie. As in the real mounds, it was noted that in the experimental mounds coarser material remained at the edges