The Mima Mounds formed sometime after Ice Age glaciers began receding 16,500 years ago, and their origin has puzzled scientists and visitors since the mid-1800s. Research has established the glacial history of the area and the structure and pattern of the mounds. About 2.2 million years ago, as earth’s climate cooled, massive ice sheets formed far to the north, advanced southward, and melted northward many times. Thousands of feet thick, they gouged the land beneath them and carried immense loads of gravel and rock. The last advance of ice into western Washington ended about 16,500 years ago, with Mima Prairie at the edge of its southernmost reach. As the glaciers retreated, torrents of meltwater spread across the landscape and deposited gravel and rock, creating gravelly outwash that became the well-drained, dry soils of today’s prairies. Mounded landscapes also occur in several parts of North America and in different habitats, where they are known by other names such as pimple mounds, prairie mounds, and hogwallow mounds, and mounds in each location may have a unique origin. In South Puget Sounds, Mima Mounds occur only on top of layered sediment left by glacial meltwater and seem to be limited to a single terrace level along the Deschutes River south of Olympia. The mounds are circular or elliptical, occur at a density of 8 to 10 mounds per acre, and are usually about the same size and shape within any one area, though some very low mounds are found among larger ones.