What We Know for Sure

What We Know for Sure. . The Mima Mounds were formed some time after ice age glaciers began receding 16,500 years ago. Their origin has puzzled scientists and curious visitors since the mid-1800s. Research has revealed what we know so far about the glacial history of this area and the structure and pattern of the mounds. , , Evidence of Ice Ages Beneath the Mounds , , About 2.2 million years ago as earth’s climate cooled, massive ice sheets formed far to the north. These glaciers advanced southward and melted northward many times. Thousands of feet thick, they gouged underlying land and picked up immense loads of gravel and rock. , , The last advance of ice into western Washington ended about 16,500 years ago. Mima Prairie is at the edge of its southernmost reach. , , As the glaciers retreated, torrents of meltwater fanned across the landscape and deposited a load of gravel and rock. This gravelly outwash became the well-drained, dry soils of today’s prairies. , , What is a Mima Mound? , , Mounded landscapes are found in several areas of North America and in a variety of habitats. They are called pimple mounds near the Gulf of Mexico, prairie mounds in the northern Great Plains, and hogwallow mounds in California. Mounds in each location may have a unique origin. , , In South Puget Sounds, Mima Mounds are located only on top of layered sediment deposited by glacial meltwater. , , They appear to be restricted to a single terrace level along the Deschutes River south of Olympia. Mounds are circular or elliptical 8 to 10 mounds per acre. Mounds are usually about the same size and shape in any one area, but some very lot mounds are found among larger mounds.