The Pacific Northwest has its enigmas: Bigfoot, D.B. Cooper, a long history of UFO sightings. But sometimes mysteries are literally underfoot. Just south of Puget Sound is a vast pimpled prairie featuring over 600 acres of earth-sculpted goosebumps. They have been the subject of study, speculation and scientific theorizing for nearly 200 years. Yet we still don’t know what — or who — made them. How hard can it be to figure out a lump of dirt?
We’re here to ponder what we call the Mima mounds. Explorers, traders and settlers moving into the area noted these odd hillocks covering hundreds and hundreds of acres of prairie land in southwest Washington.
In 1841, American Naval officer Charles Wilkes, who led an exploring expedition around the globe, became fascinated by the mounds. While trekking overland from Fort Nisqually on Puget Sound to Fort Vancouver on the Columbia River, he saw the curious humps. He thought they might be Native burial mounds. So he brought shovels and dug into a few hoping to prove it, but he and his men found no relics. Still, he thought Indigenous mound builders were the most likely source. His probing is considered the first scientific attempt to figure out their origin.
In 1847, Canadian artist Paul Kane came through and painted and a picture of what the prairie looked like at the time — or at least a rather beautiful and Romantic image of the phenomenon.
The Chehalis people of the area said they were created suddenly by water, perhaps a flood. The name Mima is said to mean “newness” in their language. Water and ice have been major factors in dozens of theories about where they came from. The southern extent of the Puget Lobe ice sheet stopped just a bit north of where the mounds started. Were they the result of pockmarks — called “sun cups” — in the ice that filled with soil, and when the ice melted, lumps were left behind? Were they formed by the alternate freezing and thawing of permafrost? Did a flood of melted ice shape them? Did the mounds mark where floodwaters flowed around thick vegetation which then attracted piles of silt?
In 1862, an unnamed Oregon reporter traveling by stagecoach surmised that the prairie marked the remainder of a vast forest ravaged by tornado, then wildfire. Each mound, he speculated, marked where an old tree had stood. “I have the vanity to think this theory of their formation more plausible than any other,” he wrote. Ah, vanity, vanity, all is vanity. Everyone has a theory.
Flood, fire, wind, ice, volcanic eruption and human engineering have been put forward. But the more we seem to know, well, certainty seems as elusive as ever. The hillocks themselves are fairly simple: They lie on a flat underlying surface of cobbles that probably formed at the bottom of a post-glacial lake. As the massive Puget Lobe ice sheet started melting about 16,000 years ago, the meltwater was epic. And the Lobe’s ice blocked the melting ice from Mount Rainier’s Carbon glacier, creating what geologists call Glacial Lake Carbon. When that ice dam failed, a flood and a debris flow swept into the Puget Sound lowlands near the mounds prairie. A substantial percentage of the rock in the mounds is andesite — volcanic rock washed down from Mount Rainier.
One version of the Chehalis story says the bumps are large sea mammals that the flood left behind, like whales. Oddly, whale bones have been found on the banks of the nearby Chehalis River, but those fossils were deposited millions of years earlier. Still, that’s how the Carbon flood waters drained out to the Pacific.
The mounds are relatively evenly sized and spaced, about seven to 40 feet in diameter and four to seven feet high. They consist of soil mixed with silt, sand and gravel and are covered by prairie soil and vegetation — grasses, mosses, wildflowers, even trees. Interestingly, the mounds themselves are thought to be relatively young — formed after the Puget Lobe retreated.
In the 1940s, a new theory emerged suggesting they were made by burrowing animals. The idea is that gophers could have built up mound communities. People scoffed at the idea. Were these some super-sized prehistoric gophers? Research biologists more recently have suggested that small common pocket gophers, which still reside in the area, are capable of creating expansive Mima-like mound communities. Computer models show it could take a couple of hundred generations of gophers over 500 years to build them. That would make the mounds something like the gopher equivalent of medieval cathedrals!
In the 1990s, a new theory was forwarded: an earthquake! Under the right conditions, vibrations from seismic waves could redistribute loose soil into evenly spaced mounds. A geologist in Spokane came up with that notion when he was hammering plywood with volcanic ash on it after the eruption of Mount St. Helens. The vibrating ash turned into mini-mounds.
There are Mima-like mounds elsewhere in the country, even where glaciers have never been. Most are in areas where there’s earthquake activity and where pocket gophers can live. So there is no scientific mounds consensus. Fortunately, many of the Mima mounds here are protected within a special prairie preserve. You can wander among them. Perhaps a new generation of STEM-educated students will visit and finally crack the Mima code. Doing so might not cure cancer or get us to Mars, but solving an enigma that’s in plain sight and underfoot? That would be very satisfying.
