RECENT FINDINGS OF STABILITY OF THE ARCHETYPAL MIMA MOUNDS IN WASHINGTON: A LONG-TERM ASSESSMENT OF SURFACE DIFFUSION

We acknowledge Don Johnson for this tireless and stimulating research topic that included the Mima Mound in Washington State. The nature and formative mechanism of the archetypal Mima Mounds in Washington State remain elusive. To better understand the timing and formative mechanisms of the mounds, we measured surface diffusion for 19 years. There is negligible motion suggesting that the current mounds are stable features. In light of these findings, various proposed mechanisms including, gopher burrowing, tree root anchoring, seismically induced, aeolian, and periglacial processes are reviewed. While a conclusion is not apparent, we can suggest whether certain processes are possible or unlikely. Our findings suggest that these features may have persisted for long-periods of time. The age of these features is constrained by the deglaciation timing, and the time needed to develop the current organic carbon content. While our diffusion studies don’t rule out root anchoring soon after deglaciation, the organic carbon content would require at least several thousand years to form. The published age of the carbon within the mounds suggest that these features are at least several thousand years old. The current stability of the mounds also does not preclude a gopher-burrow model. We suggest that Mimi type mounds found at various locations are likely to involve several formative mechanisms.