The Origin of the Mima Mounds of Western Washington

Abstract

The Mima mounds occur on certain prairies of glacial outwash in western Washington. The mounds are closely spaced, round or oval, from 10 to 40 feet in diameter, and from 1 to 7 feet in height. The typical mound is a double-convex lens of loose, unstratified, black silt-gravel set in a shallow pit in stratified yellow outwash gravel. Mounds are found only where a thin layer of soil overlies a compact bed of gravel, not on deep prairie soils. The Mima mounds are formed by pocket gophers (Thomomys talpoides) over long periods of time. Gopher activity in any particular place destined to become a mound site starts with intensive burrowing, such as that required in the construction of a nest, which loosens the soil and stimulates the growth of vegetation. The vegetation, in turn, furnishes food for the gophers and encourages them to concentrate their activities in the vicinity. A stage is reached where the gophers find sufficient food on the mounds to maintain them the year around, making it unnecessary for them to forage, except at rare intervals, into the intermound depressions. In deep burrowing to create living quarters gophers dig a shallow pit in the stratified gravel beneath each mound. The smaller elements in the stratified gravel removed by deep burrowing become mixed with silt to form the substance of the mound lens. Stones too large to be moved by the gophers are undermined and settle to the bottom of the mound. In shallow exploratory burrowing in the peripheral zone the gophers do not undermine large stones but remove soil from about them, eventually leaving them exposed on the surface of the ground.