The Distribution and Origin of Mima Mound Grasslands in San Diego County, California

Abstract

In North America, Mima—type earth mounds are found west of the Mississippi River from southern Canada to northern Mexico. They occur on poorly drained soils with a shallow basement layer or permanent water table. In San Diego County, California, Mima mounds are found at the edge of coastal salt marshes above a shallow water table, on coastal marine terraces having a cemented, clay—capped hardpan, on foothill slopes and stream—cut terraces over shallow bedrock, and in cismontane valleys, mountain meadows, and the margins of desert—edge marshes where a dense subsurface clay horizon creates poor drainage. Major hypothesis of mound orgin involve (1) frost—sorting, (2) erosion, (3) wind disposition, and (4) fossorial rodent actitivy. At Miramar Mounds National Landmark in San Diego, soil samples (1980 cm3) were collected from the tops, edges, and neighboring basin centers of 10 mounds for analysis of small rock content. Soil plugs containing metal markets were inserted in pocket gopher tunnels at mound edges, and the movement of these markers traced with a metal detector. Mounds on a 0.58—ha plot were mapped and their dimensions measured. Contray to all but the fossorial rodent hypothesis, gravel and small pebbles that such rodents are able to move were concentrated in mound soils. Clearing of the experimental soil plugs by pocket gophers was accompanied by a significant moundward translocation of mined soil. The spacing of mounds tended towards uniformity, but intermound distance increased with increased significantly with increase in mound size. The distribution of mound fields in San Diego County corresponds closely to the distribution of original valley grassland inferred for a 10—yr fire cycle. It is hypothesized that Mima mounds are an indicator of original grassland environments.