The Biodynamic Significance of Double Stone-Layers at Diamond Grove Mima Moundfield, Southwest Missouri

Gravelly Mima-type mounds dot unplowed, gently rolling, poorly drained prairie uplands on the Springfield Plateau at and around Diamond Grove Prairie near Joplin, Missouri. The mounds here formed in loess-enhanced, chert-rich, strongly weathered residual gravelly soils derived from cherty Warsaw Limestone of Mississippian age. The mounds average 45 cm high and 14 m in diameter, with densities of 7.6 mounds per hectare, with intermound areas episodically wet and crayfish chimneys common.

To gain an understanding of mound-intermound soil stratigraphy, a trench 2 m deep by 21 m long was cut across a typical mound-intermound unit. One of the more remarkable discoveries was the presence of two stone-layers, one just below the mound surface comprised of small gravels (<6 cm dia.), and a much thicker stone-layer at the mound base comprised of large gravels (>6 cm dia.). These data, backed by supplementary geomorphic and biologic data, support a multi-stage polygenetic origin of the gravelly soil mounds of the region. We propose that initial mound formation occurred in the Quaternary, or possibly pre-Quaternary, owed to centripetal bioturbation by Plains Pocket Gophers (Geomys bursarius). As Geomys burrowed centripetally outward from their main activity centers (nest-and-food storage centers), back-transported soil was limited to particles < 6 cm. In the centripetal burrowing-biosorting process, larger clasts were both left behind in intermound areas, and down-settled under mounds to form the basal stone-layer. For reasons unknown, Geomys, though still present regionally, was extirpated locally during the late Holocene. Subsequent bioturbation by smaller animals -- mainly invertebrates (ants, worms), have produced a second, near-surface, post-Geomys incipient stone-layer in the process of actively forming. This biodynamic model is supported by recent work at Mima Prairie near Olympia, Washington, where near-identical double stone-layers in likewise recently gopher-extirpated gravelly soil mounds, with large-clast surface pavements in intermound areas, also occur.