Washington’s Ancient Prairie Domes Have Stumped Geologists Since the 1900s

Mima Mounds, Washington

Scattered across Washington’s prairies sit thousands of grass-covered bumps that have puzzled scientists for decades.

These ancient domes, some as old as 4,600 years, refuse to give up their secrets despite countless studies and theories.

Here’s what we do know about these stubborn little mysteries.

The Mysterious Mounds

The Mima Mounds pop up from southwest Washington soil like giant anthills. These domes stand 3-7 feet high and stretch 8-40 feet wide, dotting 637 acres in Thurston County.

“Mima” comes from Chinook language meaning “a little further along.”

Scientists dated them to 4,600 years ago, well after the glaciers disappeared.

For 200 years, scientists have cooked up over 30 theories but still can’t agree on what made them.

Gophers As Builders

In 1942, scientists Dalquest and Scheffer first pointed fingers at pocket gophers. They figured these little diggers built mounds to stay dry when the ground got soggy.

Since wet soil kills gophers, they pushed dirt uphill over generations. In 2013, Manny Gabet created a computer model showing how gophers could make these patterns over centuries.

The mounds grow about an inch yearly as dirt piles up, exactly what you’d expect from generations of busy gophers.

Earthquake Theory Shakes Things Up

Researcher Andrew Berg had a different idea in 1990: earthquakes made the mounds. He hammered a plywood board covered with dirt and created similar patterns.

During quakes, soil shifts into piles that plants later anchor with roots. Washington had a major earthquake 1,000 years ago that might have done the job.

Recent earthquakes elsewhere haven’t made new mounds, but geologist Larry Goldstein still thinks powerful quakes could form these bumps.

Other Guesses

Scientists haven’t stopped there. One theory suggests ice age freeze-thaw cycles cracked the soil, creating patterns that stuck around after thawing.

Another blames plant roots for moving soil into mounds over time. Some think wind piled dirt around prairie plants like sand dunes.

The Chehalis Tribe tells a better story: sea creatures got stranded after a great flood, their bodies becoming the humps we see today.

Inside The Mysterious Mounds

Slice open a mound and you’ll find loose, gravelly soil with thick topsoil. Under the grass lies sand, gravel, and rotted plants.

Some mounds show clear layers: black sandy topsoil darkened by ancient fires, gravelly middle, and extremely gravelly bottom. This stuff came from the melting Vashon glacier.

A Vanishing Prairie

The Mima Mounds support a rare prairie dominated by Roemer’s fescue grass. These grasslands once covered 160,000 acres in the region. Today, only 3,000 acres remain.

Native Americans maintained these prairies through controlled burning. This prevented forests from taking over and helped important food plants like camas thrive.

The site includes both dry prairie on well-drained soils and wet prairie in low spots where seasonal ponds form.

Spring Brings Color Explosion

Mid-April through May turns the Mima Mounds into a flower festival. First come Henderson’s shooting stars with their swept-back magenta petals.

Bright yellow buttercups, spring-gold, western saxifrage, and wild strawberries soon follow. By late April, camas lilies paint the prairie blue-purple.

Chocolate lilies add their red-brown petals freckled with yellow. Native peoples harvested camas bulbs for food, a prairie staple for thousands of years.

Butterfly Haven

Several endangered butterflies call this prairie home. The Hoary Elfin spends its whole life on kinnikinnick plants, and the adults sip flower nectar while caterpillars munch the leaves.

Summer brings Great Spangled and Zerene Fritillaries with wings as big as your palm. Four butterfly species face extinction here: mardon skipper, Puget blue, Wulge checkerspot, and valley silverspot.

Conservation efforts focus on growing more nectar plants while fighting invasive species.

Visiting Mima Mounds

Located near Littlerock, Washington (I-5 exit 95, then Maytown Road west to Waddell Creek Road), this Natural Area Preserve requires a Washington Discover Pass ($11.50 daily, $35 annual).

You’ll find ADA-accessible paved trails, two longer loop paths, interpretive center, and vault toilets. Dogs are not allowed. No special tours offered, but group tours available by request through site stewards.

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