THE MIMA MOUNDS OF SOUTHWEST WASHINGTON AND OTHER MOUNDS LOCATED AROUND THE STATE WERE THE RESULT OF FREQUENT MELT WATER INUNDATIONS OF FORESTS ALONG THE MAJOR GLACIAL DRAINAGE CHANNELS DURING THE LAST ICE AGE

The Mima Mounds along the Black River near Olympia, Washington appear to have been created by surging glacial melt water flooding the drainage channel between Glacial Lake Russell and the Chehallis River. Lake Russell was created by the blockage of the Strait of Juan de Fuca by the continental glacier and the impoundment of melt water in present-day Puget Sound. When the glacier that occupied the north end of Puget Sound advanced even a few feet, displacement of water from Lake Russell that was 600 feet deep and more than 10 miles wide would have caused a massive surge of melt water through the Black Lake channel, inundating the forests that grew on the plateau along the channel.

Perturbation of the root system of trees, especially Douglass Fir, Western Red Cedar and Scrub Oak, causes the trees to create mounds around their bases. Flooding, unidirectional air circulation (constant wind) and animal migration all can initiate mounding.

Repeated flooding along the Black Lake Channel by glacial melt water surges would have cause the extensive mounding found on Mima Prairie. Absent modern-day logging, trees in the Puget Sound Basin will grow to have 6-foot diameters which would fit the unusually large mounds found on Mima Prairie. Numerous examples of groups of mounds with trees can be found today along creek channels in the Puget Sound Basin. The size and spacing of the mounds resemble the Mima mounds although on a smaller scale.

Glacial age mounds are found at other locations and elevations in south Puget Sound that correspond to various known glacial lake levels. LIDAR mapping of the Puget Sound Basin indicates that there were at least three separate advances and retreats of the Vashon Glacier and that the lakes were controlled by the location and direction of each advance.