Washington’s Mysterious Mima Mounds

I want to share a grievance. I was researching ideas for fun stops along I-5 on the drive from Portland to Seattle and found pages of suggestions that were not even remotely on the way (for example, Seaside, Oregon). Meanwhile, Mima Mounds Natural Area Preserve, an extremely cool park just minutes from I-5 was not mentioned once. Hence, this article.

On your next Pacific Northwest road trip, consider checking out a geologic mystery while stretching your legs on trails through a beautiful prairie. Here is everything you need to know about visiting Mima Mounds Natural Area near Olympia, Washington.

Know before you go

Parking at the Mima Mounds Natural Area Preserve requires the Washington Discover Pass. It is $11.50 for a day pass or $35 for an annual pass. There is no pay station at the park, although you can buy a pass online ahead of time or when you’re there.
Mima Mounds Natural Area is open April - September 8:30 AM - 8:30 PM and October - March 8:30 AM - 5:30 PM.
The road to reach the Mima mounds is paved and accessible to low-clearance vehicles.
There are two small parking lots near the Mima mounds. The second has a restroom. I was one of two cars there when I visited and I wouldn’t expect any difficulty finding a parking spot.
There are a few picnic tables in the woods near the parking lot.
Dogs are not allowed in Mima Mounds Natural Area except for service dogs.
Mima Mounds Natural Area is a good destination year-round, although the best time to visit is in spring when the wildflowers are at their peak.

About Mima mounds

Mima mounds are small hills made of gravel, sand, and silt that form over areas of glacial outwash in the Mima Prairie near Puget Sound. Similar mounds occur elsewhere in the United States and around the world, where they are known by various names, such as Mima-like mounds, prairie mounds, pimple mounds, or hogwallows.

Mima mounds have puzzled geologists for generations. To this day, no one knows how they form. One thing that seems clear about them is that they are a natural phenomenon, not a manmade formation like burial mounds. They have been investigated both by questionable means (digging into them to look for human remains) and more culturally-sensitive techniques like ground-penetrating radar.

In terms of geologic time, the Mima mounds are young. There is debate about their exact age, but they formed after the last glacial retreat in Washington, which occurred around 16,000 years ago. There is evidence suggesting that at least some mounds may be much younger than that, perhaps only a few hundred years old.

You can explore the mounds of the Mima Prairie at Mima Mounds Natural Area Preserve, which contains thousands of mounds. They are about 3-7 feet tall and 8-40 feet wide. While no two mounds are the same, they look surprisingly uniform and tightly spaced in the 5 square miles of the park.

Mima Mounds Natural Area trail — *South Loop Trail through the mounds*

Theories of Mima mound formation

One of my favorite things to do while exploring Mima Mounds Natural Area is to consider how the mounds might have formed. Rather than a single explanation, geologists, biologists, and idle speculators like me have offered up many ideas. Here is a quick and dirty summary of the stronger suggestions over the years.

Runoff erosion-root anchor hypothesis was one of the first theories about Mima mound formation, proposed by George Gibbs in 1854. It suggests that plant roots anchored patches of soil and that the surrounding soil was worn away by erosion. The anchoring plants are now gone, but the soil pattern remains.
Sun cup hypothesis was suggested by prominent Pacific Northwest geologist J Harlan Bretz in 1913. According to this model, sunlight created small depressions in a melting ice sheet. These depressions got filled in with soil and when the ice melted, only these piles of soil remained.
Fluvial deposition hypothesis suggests that flood waters may have brought volcanic rocks and soil to the Mima prairie and deposited them in mound formations.
Earthquake shaking hypothesis is a newer theory of Mima mound formation from 1990 that suggests the seismic waves create a shaking pattern that sifts the soil into piles. When plants grow on the piles, they prevent erosion and preserve the mound-like shape.
Permafrost cracking hypothesis suggests that cracks formed in frozen soil and were filled with water. This water froze, widening the cracks. When the weather warmed, the water flowed away and carried eroded soil with it, leaving behind mounds.
Pocket gopher hypothesis is an idea suggested and championed by biologists. Pocket gophers dig holes on the prairie, moving the earth they dig into a pile and then packing it down to form a mound. The mound creates an elevated place where the gopher can stay dry when the soil gets wet from the rain. Pocket gophers do not live in the Mima Prairie today, although they may have at one point.

All of these theories have weaknesses. None of them is universally accepted or even a clear frontrunner. In classic human fashion, I want to know the answer but would be less interested in the mounds if I did.

To learn more about Mima mound formation, look for:

Dave Tucker’s book, Geology Underfoot in Western Washington, which gives a thorough yet approachable summary of the leading ideas.
The interpretive shelter at Mima Mounds Natural Area when you visit.
This online pamphlet from the Washington Geologic Survey.

Hiking at Mima Mounds Natural Area

Distance: 0.5 - 3 miles

Elevation: Minimal

Difficulty: Easy

Pass required: Washington Discover Pass

Dog-friendly: No, dogs are not allowed on the trails

ADA access: Yes, there is a 0.5-mile paved loop through the mounds

Trails through Mima Mounds Natural Area let you get up close and personal with the Mima mounds while enjoying some time alone in nature. I saw exactly zero other hikers during my visit. Starting from the parking area, follow the path that leads out of the forest to the Mima Prairie. You start seeing mounds before you leave the trees. Look for firs and cedars growing straight from the mounds around the edge of the prairie.

As soon as you leave the tree cover, you’ll see the small interpretive shelter, delightfully shaped like a Mima mound to blend in with its surroundings. It’s worth a stop here to read up on the theories of Mima mound formation as well as the park’s history and ecology. Look for the staircase to the top of the interpretive shelter. The roof of the shelter is your best chance for a bird’s eye view of the Mima Prairie. Note that you will probably hear gunfire here from a nearby shooting range. The noise improves as you head south from the interpretive shelter.

There are several choices for hiking from the interpretive shelter.

The North Loop is a 0.5-mile gravel loop trail that heads through the north section of the Mima Prairie before returning to the interpretive shelter.
There is a 0.5-mile paved universal access trail that makes a loop to the south and includes another viewing platform overlooking the Mima mounds, this time accessible by a ramp.
The paved trail will bring you to a connecting gravel trail that heads to the South Loop Trails. The full loop is a 2-mile trail with an option for a shortcut that makes it a 1-mile loop.

If you do all the trails, the total distance will be a little over 3 miles.

*Observation tower (private land, not accessible)*

The park is easy to navigate with maps at all the trail junctions, so no need to worry about studying the directions too much ahead of time. The only confusing bit for me was the large observation tower you can see from the trail. It’s tempting to investigate the tower, but it is on private land and not accessible from the park.

Garter snake in Mima Mounds Natural Area — *Northwestern garter snake in Mima Mounds Natural Area*

Plants and animals of the Mima Prairie

The Mima Prairie is a grassland that becomes carpeted with delicate wildflowers in spring. Flowers will vary throughout the season, beginning with early bloomers like shooting stars and desert parsley. Expect flowers into summer, with many varieties emerging like death camas, cinquefoil, and goldenrod. Spring until early fall is also the best time to look for butterflies. Around a couple dozen species visit the prairie, including monarchs, painted ladies, and swallowtails.

My pictures are from winter, the least attractive season to visit, although look for the pale green of reindeer lichen and the red berries of kinnikinnick bushes amongst the dormant grasses and ferns covering the mounds. Around the edges of the prairie, there is a forest that stays green thanks to pine trees like Doug firs and cedars. Look for them beginning to encroach on the mounds at the forest’s edge.

The Mima Prairie is a good place to watch for both songbirds and predatory birds. I saw a Northern Harrier eating some unfortunate creature on the viewing bench on the South Loop Trail. It flew away slowly when I approached, as though unsure whether I was worth the trouble of avoiding. You may also see red-tailed hawks, kestrels, turkey vultures, and songbirds like sparrows and bluebirds. Besides birds, a variety of other animals like deer, coyotes, meadow voles, garter snakes, and northern alligator lizards live in or visit the Mima Prairie.

Other places to see Mima-like mounds

One of the puzzling things about Mima mounds is that they appear in areas far apart without an obvious pattern. In the Pacific Northwest, there are mounds in the Channeled Scabland of eastern Washington as well as eastern Oregon and Idaho. One of the best places to see Mima-like mounds in the Pacific Northwest is outside of the intriguing ghost town of Shaniko, Oregon.

Mima-like mounds are not confined to the Pacific Northwest. In the United States, you can find them in California, the Great Plains, and near the Gulf of Mexico. Similar mounds also occur in other places around the world from Canada to Argentina, covering a diversity of climates and geologic features. In places outside the Puget Lowland, mounds form on other surfaces besides glacial outwash.

Things to do near Mima Mounds Natural Area

There are plenty of things to do near Mima Mounds Natural Area that can turn a short visit into a day trip. These stops are also about halfway between Portland and Seattle if you are planning a road trip and don’t want to make a detour to, say, Tillamook, Oregon (which was honestly on a list of suggestions I found for the Portland to Seattle drive).

Just down the road from Mima Mounds Natural Area, you’ll find the trailhead for a hike to Mima Falls, a 25-foot waterfall in a forest of pine trees, ferns, and salal. The hike can be done as a 6.5-mile loop or a 4-mile out-and-back hike to the falls and back.
Mima Mounds Natural Area is only 20 minutes from Olympia, Washington, which is a good place to stop for lunch, see the state capitol building, walk around Heritage Park, and do a little sightseeing.
One of my favorite places in Olympia is Brewery Park at Tumwater Falls. It is free to visit and has a playground and views of Tumwater Falls just steps away from the parking lot. You can continue down an easy gravel trail along the Deschutes River for a 0.5-mile out-and-back hike (or return along the other side of the river for a loop hike when conditions permit). Look for salmon in the fish ladder if you visit during September or October.
Head north on I-5 and look for birds from the boardwalks at Billy Frank Jr Nisqually National Wildlife Refuge.

Getting to Mima Mounds Natural Area

To reach Mima Mounds Natural Area from Seattle, take I-5 South until Exit 95 for Maytown Road. Continue on Maytown Road for about 10 minutes until you arrive at the park, which will be on your left. The drive will take about 70 minutes.

From Portland, stay on I-5 North for about 1.5 hours, then take Exit 95 toward Littlerock/Maytown. Continue on Maytown Road for about 10 minutes until you arrive at the park, which will be on your left. The drive will take about 1 hour and 45 minutes.

Enjoy your trip to the baffling Mima mounds!

With love,

Emma

Looking for more to do in western Washington? Check out the best things to do on the Long Beach Peninsula.

In winter, try out this easy snowshoeing adventure near Snoqualmie Pass.

References:

Guide to the Mima Mounds. Washington Geological Survey, https://www.dnr.wa.gov/publications/ger_mima_mounds_booklet.pdf.

Miller, Marli B., and Darrel S. Cowan. Roadside geology of Washington. Mountain Press Publishing Company, 2017.

Tucker, Dave. Geology Underfoot in Western Washington. Mountain Press Publishing Company, 2015.

Zetner, Nick, host. “Mima Mounds.” The Nick Zetner Geology Podcast, episode 45, Spotify, 30 July 2020.