COBBLES, BOULDERS, AND GRAVEL PEDESTALS AMONG MIMA MOUNDS AT MIMA AND ROCKY PRAIRIES, PUGET LOWLAND: IMPLICATIONS FOR MOUND FORMATION

Continuing to stimulate controversy after a century of debate, the Mima Mounds of the Puget Lowland are domelike ellipsoids of locally thickened diamicton composed of a sandy loam with sparse pebbles. Up to two meters high and elongated parallel to the downslope gradient of proglacial terraces, the Mima Mounds are separated by topographical lows (intermounds) of thinned diamicton nearly the same size and shape of the mounds themselves (Washburn, 1987). Though the origin of the mounds remain controversial, recent studies have largely ignored felsic plutonic cobbles and boulders found in intermounds at Mima and Rocky Prairies. Located in at least one third of intermounds, these clasts are found primarily in shallow intermounds or along the rim of mounds but are rarely positioned at the lowest point of an intermound, although some have been found on the upper half of mounds. The clasts are at least 5 cm on the shortest axis and up to 23 cm on the longest axis and commonly range between 2.5 to 10 kilograms. Some boulders have been interpreted as a “Intermound cobble-boulder pavement...that continues as a stone layer under the mounds” (Howarth Burnham et al., 2012), yet analysis of the clasts in the diamicton and underlying coarse-bedded gravels indicates that these boulders are not associated with the underlying gravels. Reaching a maximum diameter of 13 cm, most clasts in the underlying gravels are smaller than even the least of the intermound cobbles, and those that are of comparable size are generally local basalt rather than plutonic. Furthermore, the lithologies of the boulders and cobbles resemble those of smaller clasts within the diamicton, cumulatively indicating that these clasts were deposited along with the diamicton. At Mima Prairie, these cobbles sometimes comprise gravel pedestals at the base of Mima Mounds that are elongated parallel to the mounds. Though refuting an aeolian explanation of the mounds and challenges the fossorial rodent model, the presence of these gravels is explainable by the debris flow phase of the Tanwax flood.