Prairie mounds at Oregon’s Zumwalt Prairie

© Michael Durham/Minden Picture

Little hills on the prairie. Mysterious prairie mounds abound

Collections of these dome-like hills are common in landscapes throughout the United States. Depending on your region, you might know them as Mima mounds, hogwallow mounds, or even pimple mounds–and their origin isn’t always clear. Theories range from seismic activity to gophers—and even just an accumulation of sediment. The prairie mounds on our homepage today are part of Oregon’s Zumwalt Prairie, a protected grassland area in northeast Oregon. Encompassing some 330,000 acres, it’s of one of the largest remaining tracts of bunchgrass prairie in North America. Once part of an extensive grassland in the region, this portion has remained preserved due to its high elevation, which made farming difficult.