The Erosional Origin of the Mima Mounds of Southwest Washington

Abstract

The Mima Mounds of southwestern Washington are from 10 to 70 feet in diameter and range from barely perceptible forms to 7 feet in height. The mounds are round or oval in plan and in cross-section commonly resemble a segment of a circle. They lie upon glacial outwash gravels and are composed of a structureless dark pebbly-sand and silt; many contain ice-rafted erratics. The mounds are restricted to the outwash valleys of the Vashon glacial stage, and in this region nearly all the outwash valleys bear mounds. The mounds probably were formed from a partially thawed pebbly-silt mantle, possessing a polygonal-fissure ice network, and by running water, which removed the thawed material from around the hemispheroidal frozen cores of each polygon. Afterward, the frozen cores thawed in place and formed the Mima Mounds. A mainly hexagonal pattern of fissure ice in a pebbly-silt mantle determined the mound spacing and is represented by most of the intermound spaces and by the remains of branching trenches in unmoun...