Great Basin Naturalist


Patterned ground consisting of Mima-type earth mounds and associated sorted stone circles and nets is widespread on the Columbia Plateau of western North America. Studies of the geometric relationships of mounds and stone nets to slope aspect and steepness were carried out at the Lawrence Memorial Grassland Preserve, north central Oregon in June 1987. Mound and moundfield characteristics were sampled on randomly chosen 1-ha plots on slopes of different aspect and steepness. Mounds were largest, most circular and symmetrical in form, and most fully encircled by beds of size-sorted stones on level sites. On slopes of increasing steepness, mounds decreased in size, showed increasing asymmetry and downslope elongation, and became connected into lines oriented up- and downslope. Encircling stone beds became more weakly developed or disappeared on upslope and downslope sides of the mounds, and the lateral beds developed downslope extensions that eventually merged with those of adjacent upslope and downslope mounds. These patterns are interpreted as reflecting changes in the manner of soil translocation by northern pocket gophers, Thomomys talpoides, due to their responses to tunneling on slopes and to the modification of the flow of water across the slope because of the presence of mounds.