Gravelly soil mounds less than 1 m high and up to 20 m in diameter, generally referred to as “pimple mounds,” are found in the prairies and aspen parklands of southern Saskatchewan, Canada. These low-relief mounds are the northernmost occurrence in North America of this geomorphic feature documented to date. When truncated by cultivation, former mounds are evident on air photos as a pattern of small, light-toned patches called “mound scars” that contrast with the surrounding darker soil. In many cases, these photographs are the only existing evidence of former mound topography. Using air photos and direct field observation for identifying both intact and truncated mounds, the spatial distribution and characteristics of 10 pimple mound sites were examined, with mounds at the Little Manitou site studied in detail. Examination of the morphology and stratigraphy of multiple mounds indicates that bioturbation by burrowing animals has had, and continues to have, a major impact on the size, shape, nature, and origin of present-day mounds. Statistical analysis of 124 intact and 190 truncated mounds at the Little Manitou Lake site indicates that mounds form a more regular than random pattern, with strong biological implications. Saskatchewan mounds were compared to 30 other documented mound sites in North America. While most Saskatchewan mounds have greater relief and denser spacing, mounds from Texas, Wyoming, and Colorado are spatially most similar, also occur in predominantly prairie landscapes in areas with shallow soils, and are heavily bioturbated by burrowing animals.

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.

We studied low prairie (Mima) mounds and ridges with sorted stone borders separated by broad rubbly soil intermounds in areas near Mount Shasta, northern California. An earlier study ascribed a purely physical origin for these soil features based on a four-stage conceptual model. Mounds were interpreted as periglacially produced clay domes formed in polygonal ground and stone perimeters as loose gravity accumulations in unexplained shallow trenches at dome peripheries. The model was widely cited to account for similar stone-bordered prairie mounds and rubbly soil intermounds in the Pacific Northwest. Our observations and measurements indicate, however, that these mounded landscapes are more complex, and that a polygenetic origin best explains them. We suggest that combined bioturbation, seasonal frost action, and erosion processes, with occasional eolian inputs, best account for the mounds, their well sorted stone borders, and the poorly sorted rubbly soil intermound pavements. We propose a transitional, eight-stage conceptual model to explain this complex landscape. The model may generally explain the origin of other similar strongly bioturbated, cold winter-impacted, erosion-prone mounded tracts in the Pacific Northwest.