PARTICLES IN HARVESTER ANT MOUNDS AS AN AID TO MAPPING BEDROCK GEOLOGY IN ARID REGIONS

In the process of mapping bedrock geology in the arid southwest, most field geologists note that ant mounds developed on residual soil and regolith commonly contain particles that appear to be lithologically related to the underlying bedrock. Toward this end, we designed a study to evaluate the correspondence between the texture and mineralogy of particles contained within harvester ant mounds and the lithology of the underlying bedrock. For this study, particles occurring in conical-shaped harvester ant mounds in the vicinity of Canon City, Colorado were evaluated. The architect for most of the sampled ant mounds was _Pogonomyrmex occidentalis_. Based on established bedrock geology maps, at least two mounds were sampled from soils developed on each of seven different bedrock types. The bedrock lithologies include mica schist, pegmatite, quartz sandstone, arkose, dolomite, limestone, and shaley limestone. The rock units range in age from Precambrian to Tertiary. Approximately 300 g. of sediment (and a few ants) were sampled from each mound. Some outcrops of bedrock adjacent to the ant mounds were also sampled for comparison purposes.

Particle-size summary statistics for the mounds were estimated with the method of moments (through sieve analysis). Grain mounts of particles were inspected with a binocular microscope and estimates made of particle shape and roundness. In order to further evaluate mineralogy of the samples, petrographic thin sections were prepared from grain mounts and from hand samples of the bedrock. Direct measurement of particle sizes under the microscope and tabulation of corresponding particle weights are in progress.

Results to date suggest that particle lithologies occurring in harvester ant mounds are an excellent guide to the lithology (and geologic formation) of the underlying bedrock. The most surprising outcome is that the particle size and sorting in the ant mounds is remarkably uniform, regardless of the bedrock type or geographic location. Frequency distributions for each of the mounds are slightly positively skewed; most of the mounds have a grain size mode of 1.49 phi units (about 2.8 mm). The particles in the mounds are very-well-to-well sorted (ranging from about 0.33 to 0.36 phi units). Harvester ant mounds may very well contain the best-sorted granule-size particles in nature.