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In hopes of shedding light on their genesis, Mima-type soil mounds were investigated at two environmentally and geologically disparate gravelly prairies, Diamond Grove Prairie in southwestern Missouri, and Mima Prairie in the southern Puget Sound of Washington. Mound soils were described, with large volume samples collected at narrow depth increments and laboratory analyses conducted. Results reveal, as predicted, that the soils contain small gravels (≤6 cm) scattered throughout the mound above a basal stone layer of large clasts (>6 cm). The stone layer is exposed across the intermound areas as a pavement.

Biomantle principles predicted that mound soils would be texturally biostratified by small burrowing vertebrates into locally thickened, two-layered biomantles, and they are. What was not expected, but might have been predicted by the principles, was the revelation from laboratory data of a second, upper, weakly expressed stone layer of pebbles (≤6 cm). To explain the secondary stone layer we introduce the concept of dominant bioturbator. At both prairies the dominant bioturbator was probably, until geologically recently, a species of the Geomyidae family of rodents, pocket gophers. This animal does not presently inhabit either prairie, but is present nearby. We attribute the secondary, apparently incipient stone layer to new dominants, almost certainly invertebrates, whose textural effects were regularly erased by the earlier (gopher) dominants. These effects are evident and expressed as a new upper biomantle that is now being superimposed upon the old.

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