The Mysterious Mima Mounds

In the prairies near Olympia, in Washington, in northwestern United States, thousands upon thousands of grass-covered humps bulges out of the ground like an enormous bubble wrap. These humps are called mima mounds, named after the Mima Prairie, and they range in size from near imperceptible to more than two meters tall, and several meters across. Since their discovery by Charles Wilkes, a US naval officer and explorer, in 1841, these mysterious mounds have intrigued scientists and provoked curiosity, speculation, and debate.

Wilkes initially thought that the mounds were graves of ancient Indians but when he ordered his men to dig up, they found no bones. We now know that these mounds are thousands of years old but we still don’t know who or what created them. Over the years dozens of theories have been advanced implicating everything from earthquakes to glaciers to gophers to even aliens.

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Photo credit: Washington DNR/Flickr

Mima mounds are not unique to Washington. Similar formations have been found in other US states and all across the globe, except Antarctica. In California and Oregon they are known as hogwallow mounds, in New Mexico and Colorado as “prairie mounds”, and in the southeastern states as “pimple mounds”. The South Africans call them "heuweltjies", or little hills, and the Brazilians call theirs "campos de murundus", or mound fields.

The shape, size and composition of the mounds vary from place to place. They are typically circular to oval in shape, and composed of darker soil mixed with organic matter, including charcoal, on top of the gravel bed, with dark intrusions extending downward and usually mixed with small boulders.

One of the most popular theories that seems to have gained some traction among scientists credit pocket gophers, a small, burrowing rodent that build tunnels underground. According to this theory, each mima mound develops through the effort of many generation of gophers. However, pocket gophers are solitary and fiercely territorial and there is no evidence that the animals ever reinhabit old mound sites. Another issue with this theory lies in the big rocks commonly found in and on top of the mounds. These rocks are too large for gophers to move. Besides, some mima mounds are found in areas where gophers are not found and never lived in the past.

A competing theory suggest that the mounds resulted from the accumulation of wind-blown sediments around clumps of vegetation. They are known as nabkhas, and are commonly seen in arid regions such as the Arabian Desert, and New Mexico in the United States.

Another theory suggests that gravel and stones washed upon a melting glacier collected in depressions known as “sun cups”. When all the ice melted away, the sediment collected in the sun cups were left behind, forming the mounds that remain today.

The mima mounds in Washington originally covered about 30,000 acres and once had an estimated 900,000 mounds, but housing development, road building, farming and other development projects destroyed nearly all of them. The remaining mounds are now designated a National Natural Landmark and protected under the Mima Mounds Natural Area Preserve. Mima mounds are also present at the Scatter Creek Unit, located in southern Thurston County, Washington.

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