You Mean That Aliens Didn't Do It? New Research on Mima Mounds in the American West

Mima mounds south of Seattle, Washington

What's life without a little mystery? For years, geologists and geomorphologists have been curious about a landscape that shows up on prairie lands around the American West. It's a regular pattern of low hillocks called mima mounds. The hills are a few feet high and a few dozen yards across. They generally occur where there is a clay substrate that sometimes causes pools to collect between the mounds. These are called vernal pools, and for the few months that they hold water, they support a large variety of unique plants and animal species.

Mounds in the Yokohl Valley region in the Sierra foothills near Sequoia National Park.

The mima mounds are a mystery because their origin is not exactly clear. Early settlers thought they were burial mounds for Native Americans, but that is not supported at all by archaeological research. Some have suggested that they result from disruption of the soil by earthquake waves. The mima mounds I photographed from above (first picture) were in Washington near the margins of the vast continental glaciers, and some have speculated that they formed as a result of meltwater flooding in some fashion. Other less scientifically oriented observers suggest that aliens from space have something to do with their formations (landing pads, maybe?).

Mima mounds near Willms Road in the Sierra Nevada foothills near Knights Ferry on the Stanislaus River.

Of course, some people suggested that the origin might be a bit simpler (remembering Occam's Razor: among competing hypotheses, the hypothesis with the fewest assumptions should be selected). They suggested generations and centuries of activity by ground squirrels or gophers resulted in the formation of the mounds. In the prairie environment, the mounds would give the rodents a better view of approaching predators, and their burrows would stay drier, being above the impermeable clays.

I was pleased to run across some current research on the origin of these enigmatic mounds by Manny Gabet at San Jose State University, and the verdict is...gophers. Generation after generation of gophers, building their mounds in the same place over time. Gabet doesn't insist that this is the absolute explanation for all mounds, but the computer modeling provides strong support for the idea. The mounds would develop after around 500-700 years.

Snow patches outline mima mounds at Tehachapi Pass in southern California (photo by Mrs. Geotripper).

Sorry, alien lovers...maybe Roswell has some new evidence.