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.

Mima mounds in North America are primarily known from the western states of Washington, Oregon, and California; the Rocky Mountains; the mid-lower Mississippi Basin; and Louisiana-Texas Gulf Coast. By contrast, their former extent and abundance across the Upper Midwest prairie belt has never been systematically established due to their destruction by agriculture and historic confusion as to whether they were natural or anthropic mounds. Recent maps showing their distribution identify only two small moundfields, one centered on Waubun Prairie in western Minnesota, the other on Kalsow Prairie in north-central Iowa. But in fact, natural mounds were once a common feature of many Upper Midwest prairies, having extended from Kansas, Missouri, and Illinois north into Wisconsin, Iowa, Minnesota, the Dakotas, and across the prairies and parklands of Canada. Several remnant tracts, intact and preserved, bear witness to their former much greater extent. This chapter documents the original distribution across the prairie belt, which has implications for their origin insofar as it falls more or less entirely within the range of the Geomyidae (pocket gopher) family of fossorial rodents. Natural prairie mounds in the Upper Midwest invariably are found where limitations to vertical burrowing occur, or did occur, which leaves lateral burrowing as the only option to these and other soil animals. Owing to extensive overlaps between natural mounds and morphologically similar prehistoric “Moundbuilder” mounds, the idea is advanced that prairie mounds were opportunistically used for prehistoric interments, and later as ideation templates for prehistoric burial, effigy, and other mounds and utilitarian structures.

Abstract Since its discovery in 1993, the mid-Irvingtonian (0.78–.55 Ma) Fairmead Landfill locality has produced thousands of specimens representing 72 taxa (2 fish, 2 amphibians, 3 reptiles, 6 birds, 29 mammals, 1 bivalve, 1 gastropod, 12 plants/palynomorphs, and 16 diatoms). Fossils occur in sediments representing distal alluvial fan channel, distal fan overbank flood or sheetflood, and marsh/lacustrine deposits of the upper unit of the Turlock Lake Formation. A broad range of taphonomic conditions is represented. Overall, the biota suggests a predominantly grassland habitat. The field trip will make three stops, the Madera County Paleontology Collection repository in downtown Madera, California; the Madera County Fairmead Landfill site; and the Fossil Discovery Center of Madera County.