Mima Mounds

What is it and why visit?

Purple camas blossoming at Mima Prairie

Photo by Dan Coe, WGS/DNR

In Thurston County, Washington, mysterious mounded landforms create a wavy surface at the 637-acre Mima Mounds Natural Area Preserve (NAP). This fascinating geological phenomenon will likely leave you wondering, “How did these mounds form?” Don’t worry, you’re not the only person asking this question. Researchers are also puzzled. For many, the appeal is more than the mystery—it’s the views and picturesque hikes. As you wander the trails of this secluded natural area, the raw beauty of the landscape will consume your senses. Enjoy views of Mount Rainier and Mount St. Helens to the east, and the green hills of Capitol State Forest to the west. In the spring, purple camas flowers speckle the lush green grass that blankets these puzzling landforms. Come visit the Mima mounds and see what theory you come up with while enjoying this beautiful Washington natural area.

Geologic story

More than 30 theories have been published about the origins of the mysterious dome-shaped mounds that speckle Mima Prairie. Some scientists think the mounds were built by pocket gophers. These animals dig tunnels, piling the scooped out earth into mounds. Others think geology played a role. According to the sun cup theory, streams dropped sediment into evaporation pits found on thin layers of ice. When the ice melted, it left behind mounds of sediment. The frost-polygon theory is based on the idea that water-soaked soil expands as it freezes, creating mounds. Perhaps earthquake shaking created these perplexing piles. Despite an abundance of popular theories, nobody knows for sure how the mounds formed.

Trail through the Mima mounds

Photo by Dan Coe, WGS/DNR

So if scientists can’t agree on one of these theories, what do we know? The Mima mounds are 3 to 8 feet tall and 8 to 60 feet wide. They typically sit close together, with as many as 10 mounds per acre. Cobbles of multiple rock types sit in the low spots between the mounds. Another piece of the puzzle is the fact that we seem to find Mima mounds where the edges of the great ice age glaciers used to be.

These are all things that can be observed above ground, but what secrets lie beneath the surface? Scientists have discovered that the mounds rest on 15,000-year-old glacial outwash. Outwash is a collection of sediments dropped at the edges of glaciers by meltwater streams. However, the mounds themselves do not have the layers characteristic of outwash—they are made of dark sand mixed with gravel. This tells scientists that the mounds began forming sometime after the outwash was deposited.

Geologist Tim Walsh examines a cross section of a Mima mound

Photo by Dan Coe, WGS/DNR

Much of the evidence seems to suggest a glacial origin for the Mima mounds, but does that mean pocket gophers or earthquakes didn’t play a role? We may never know for sure, but in the meantime scientists can still enjoy the debate!

We recently published an educational comic booklet about the Mima mounds. Check it out by clicking here or on the link below.

Mima mounds booklet link