An Assessment of Prairie Mound Origin Theories
at University of Arkansas Experimental Farms

Abstract

            Prairie mounds are common geomorphic features in floodplains throughout the western United States and Canada, but their origin is still disputed after more than a century of study.  Most studies date prairie mound formation to the Early or Middle Holocene, and with a few possible exceptions, no prairie mounds appear to be forming today.  This makes them good candidates for proxy indicators of past environments, because they were formed under conditions no longer present.  Each theory of prairie mound origin holds different implications for past environments.  This study examines soil, sediment, and topographic data from two prairie mounds at the University of Arkansas Experimental Farms to evaluate aeolian, seismic, fossorial rodent, and fluvial origin hypotheses.  The evidence from these mounds argues strongly against an aeolian or fluvial deposition origin, weakly for a fluvial or aeolian erosion origin, and is equivocal in respect to seismic and fossorial rodent origins. 

Introduction

            Prairie mounds, also known as pimple mounds, "hog-wallows", and Mima mounds (after the type locality at Mima Prairie in Washington State), are poorly understood geomorphic features whose origin has been debated for more than a century.  Most theories of mound origin fall into the categories of seismic shaking, dune formation, periglacial processes, fluvial erosion and/or deposition, and fossorial rodent burrowing.  Studies of prairie mounds have revealed a diversity of internal structures from region to region, and it is possible that they were formed by different means in different places.  They are usually situated in floodplains and are often reported to have a hard or impenetrable horizon shallowly buried beneath them.  Prairie mounds are generally considered to have formed during the Early or Middle Holocene (Washburn, 1988; McFaul, 1979).  There are no reports that any mounds are forming today, with the possible exception of a mound field in southwest Arkansas (Larry Ward, personal communication).  This mound field is not apparent on any but the latest of aerial photographs.  The site has not yet been visited for the purpose of mound studies, however, and they may simply have been obscured in the earlier photographs.

            The relict nature of prairie mounds makes study of their origin difficult, but it also affords an opportunity for study of past geomorphic processes.  Prairie mounds were formed by processes no longer operating or operating under conditions different from today, and therefore they may serve as proxy environment indicators.  If the mounds were formed through aeolian processes, for example, they would reveal something of past prevailing wind patterns and ground cover.  If they were formed through seismic shaking, dating their formation would help reconstruct the seismic history of the regions in which they occur.  If they were formed through fluvial processes, they would reveal something of the history of floodplains and ground cover.  If they were formed by the burrowing activity of rodents, they would indicate the range of the animals that made them, and raise the interesting question of why few or none appear to be forming today.

            In this study I examine the evidence gathered from two prairie mounds at the University of Arkansas Experimental Farms in Fayetteville, Arkansas, and assess the hypotheses of aeolian, seismic, fossorial rodent, and fluvial origins.  I do not include the periglacial hypothesis because the known climatic history of northwest Arkansas precludes such an origin.  I primarily examine the origin hypotheses in light of evidence gathered from this study.  For further discussion of origin hypotheses involving other lines of evidence see Butler (1995) or Washburn (1986).

            The mounds I chose for study are located at the University of Arkansas Experimental Farms near the eastern edge of a mound field that stretches at least three kilometers northeast to southwest and at least two kilometers northwest to southeast (Fig. 1).  Construction has obliterated many of the mounds, but 1941 aerial photographs reveal mound densities of about 6 to 8 per hectare.  All of the mounds appear to be situated within a floodplain landform.

Figure 1.  Prairie mounds clearly expressed in 1941 aerial photograph.  Study area is near center of image.  Click image for larger view.