Mima mounds are not a phenomenon particular to the Northwest, although they occur here in their greatest numbers. Mima mounds remain a mystery, despite the best efforts of hundreds of years of scientists and other theorists.
One of the best examples of Mima Mounds can be found just south of Olympia, a few miles off I5 outside Littlerock at the Mima Mounds Natural Area Preserve. Imagine a broad prairie field. Now imagine that instead of being relatively flat, it is humped with a vast cluster of mounds, each about 30 feet around, and up to eight feet high. These mounds go on for hundreds of acres, nestled side by side with a narrow path between them.
We have ample scientific data on what the mounds are. But we know nothing about how they came to be.
Most of the mounds are basically giant piles of gravel and sandy soil. They sit atop a layer of flat, compacted clay soil. This is much the same kind of geology that many of us find in our back yards - but not as evenly dispersed.
The first theory I heard to explain the formation of the Mima Mounds was that the local native tribes had built them, as some kind of religious ritual, fertility rite, or burial rite. This theory was no doubt fueled by the thought of the giant burial mounds in England and Ireland which were built by the Picts as monuments.
This theory is quickly debunked by:
A) The widespread nature of the Mima Mounds, which can be found through Washington, Oregon, and Idaho, and even occur in other places throughout the country, including sites outside San Diego, in East Texas, and in central Wyoming.
B) The lack of any human remains (either skeletal remains or buried objects) inside the mounds.
B) Just asking the native tribes themselves, which have kept a historical record of their culture going back thousands of years, and have no such ritual involvement.
Take the pocket gopher theory. When the gophers hit the hardpan, so the theory goes, they give up and start building upwards. It would take millions of years - billions of generations of pocket gophers - to create these mounds. And although there are pocket gophers in the area, there aren't that many of them. Nor are the mounds notably hollow with pocket gopher tunnels, as one would expect from such an artifact of pocket gopher culture.
One thing is certain: the prairie landscape of the Mima Mounds is under threat. The native tribes used to keep the prairies clear by setting fires every season to burn the brush. Encroaching development has made this unwise, and the prairies are only barely being defended from Himalayan blackberry and Scotch broom by a tireless group of park rangers and volunteers.
Photo credit: WAStateDNR