EVOLUTION OF A GRAVELLY MIMA-TYPE MOUNDFIELD IN SOUTHWESTERN MISSOURI

Unplowed silt- and gravel-rich Mima-type mounds dot remnant prairie uplands of the cherty limestone Springfield Plateau of southwestern Missouri. Geomorphic, soil, animal, vegetation, and other research was conducted on mounds and intermound areas of one of these prairie uplands, namely Diamond Grove Prairie and adjacent land, southeast of Joplin. Fieldwork included mound measurements (heights, diameters, densities/ha), mound spatial distributions and other measures on 6 one-hectare plots, and detailed internal soil assessments of one backhoe-dissected mound. The results justify, we believe, a biological and pedological evolution of this Mima-mounded landscape.

A biological hypothesis is proposed as the dominant process of mound origin, by which the foraging-nesting habits of the Plains Pocket Gopher (Geomys bursarius) create mounds as a consequence of episodic wetness due to water-impermeable substrates that perch water. Although pocket gophers do not currently inhabit the study site, evidence at Diamond Grove to support the gopher theory is: 1) presence of a basal mound stonelayer of clast sizes that Geomys cannot move; 2) small stones scattered throughout mounds of a size that Geomys can move (through their burrows); 3) Geomys-sized krotovina in the lower profile of mounds; and 4) vertical (homogenous) depth functions of fine fractions mound soils. Other processes such as small inputs of loess, water erosion, and physical and chemical weathering have also operated to produce the current mound morphology.