Uncovering the Mysterious Origins of the Mima Mounds

Lush greenery of the Mima Mounds
(Photo: Zack Frank/Shutterstock)

From the otherworldly rainforests of the Olympic Peninsula to the towering volcanic peak of Mount Rainier, the state of Washington is land of many grand natural wonders. While these larger-than-life destinations are bound to dominate a Pacific Northwest travel itinerary, one other fascinating yet little-known locale is Mima Mounds Natural Area Preserve.

Aerial view of Mima Mounds
(Photo: Dan Schreiber/Shutterstock)

This state-protected land, located just 20 minutes south of Olympia, is distinguished by its large concentration of grassy domes known as mima mounds. Composed of loose, gravel-like sediment and averaging about 6 feet in height, the mounds are a surreal sight, whether you're observing them from ground level or from a bird's-eye view.

Of course, the only thing more interesting than their funky, pimple-like appearance is the fact that scientists still aren't exactly sure how they were created.

When Western settlers arrived in the mid-1800s, they speculated that the odd grassy domes were burial mounds built by local Native American tribes, but subsequent digs revealed no human remains or artifacts. Several other theories have been floated over the years — seismic activity, the swelling and shrinking of soils and even extraterrestrials.

One of the prevailing theories is that pocket gophers built up the mounds over many generations. After one team of researchers built a computer model to test this theory a few years ago, it seemed like they had finally solved the mystery.

Camas on Mima Mounds
(Photo: Washington DNR/flickr)

That is, until a new study came out in 2014 that asserts the mounds are not the handiwork of gophers, but rather the result of natural, non-faunal processes involving the longtime "spatial patterning" of vegetation.

As LiveScience explains in its report on the study, this spatial patterning occurs when "individual or groups of plants spread their roots and drain surrounding areas of water and nutrients, while the soil in which they grow remains fertile. Resources become depleted between the vegetation patches and accumulate on the patches, essentially setting up islands of fertile areas that are regularly spaced out across a large region. The plants don't directly form the mounds, but they affect waterborne and windborne soil deposition and erosion, which can lead to mound formation."

Different mounds, different theories

Australia also has its own variation on the mounds, though the ones in New South Wales are made of small pebbles, but the underlying bedrock is not made of the same material. Because of this, Geologist Leigh Schmidt suggests that this is the handiwork not of geological forces but a bird, specifically the Australian malleefowl (Leipoa ocellata), which builds mounds instead of nests. However, the size of the mounds doesn't match the size of the modern bird. Schmidt has a theory for that too, suggesting the bird's ancestors — which were much bigger — displayed the same behavior with a bigger result. Schmidt goes into more detail in a May 2018 study for the Australian Journal of Earth Sciences.

Regardless of how they came to be, there's no denying that this pimply stretch of land is breathtaking.

Yellowed grasses on the Mima Mounds
(Photo: Catie Leary)
Green Mima Mounds
(Photo: Jeffrey M Frank/Shutterstock)