WEATHERING RIND THICKNESS AS RELATIVE-AGE CRITERIA FOR GLACIAL MORAINES IN THE TAYLOR RIVER VALLEY, SOUTHERN SAWATCH RANGE, COLORADO

Weathering rind thickness was used as relative dating criteria to distinguish among six spatially distinct moraines in the Taylor River Valley. These moraines represent the terminal moraine complex of the Taylor River Glaciation (Pinedale equivalent), and the five most prominent of perhaps twelve recessional moraines. Boulder lithologies were sampled according to a consistent sampling protocol. Boulders 30 to 50 cm in diameter were collected from ~20 cm below the surface of moraine crests. Rind thickness was measured with a binocular microscope equipped with an ocular micrometer.

Consistent results were obtained from medium and fine-grained granodiorites. Rind thicknesses on medium-grained granodiorite boulders found on the terminal moraine had a mean value of 10.5 mm (standard error ± 0.5 mm). Mean rind thickness for boulders of the same lithology on the most distal (and presumably oldest) of the recessional moraines was 9.0 ± 0.7 mm. Rind thicknesses on the remaining recessional moraines averaged, distal to proximal, 8.2 ± 1.2, 2.6 ± 0.1, 1.6 ± 0.2, and 1.3 ± 0.4 mm. Thinner rinds were developed on fine-grained granodiorite boulders. However, a somewhat similar trend of decreasing rind thickness with presumed decreasing age of deposits was observed. Mean rind thickness for this lithology on the terminal moraine was 3.6 ± 0.4 mm. On the recessional moraines, mean thicknesses measured 2.8 ± 0.3, 1.8 ± 0.2, 1.1 ± 0.1, 0.8 ± 0.1, and 1.3 ± 0.1 mm, distal to proximal respectively.

Rind thicknesses measured in the study area are consistent with those for Pinedale equivalent moraines elsewhere in the Rocky Mountain region. Cosmogenic ¹⁰Be and ³⁶Cl dates (in progress) from moraines in the Taylor River Valley will allow calibration of the weathering rind curve.