Abstract

Fifteen channel samples and six oriented samples were collected from the flood gravel left during the great flood of March 1938, in San Gabriel Canyon, California. The samples extended along 7 miles of the canyon, from above Dam No. 1 to Sycamore Flat, near the headwaters. The gravel was analyzed for size, shape, roundness, lithology, and fabric. Size, degree of sorting, shape, and lithology showed mainly nonsystematic variations along the valley; roundness and fabric showed systematic changes as a function of distance along the valley. Pebble imbrication changed from a downstream direction near the head of the canyon to typical upstream imbrication farther downstream.

The problem of evaluating the analytic data was complicated by the presence of several log jams along the valley, which imposed some limitations on the sampling process. The nonsystematic fluctuations of some of the sedimentary characteristics are attributed to similarity of depositional conditions initiated by the log jams.

An important part of the study is the development of a theory of pebble rounding, on the assumption that the rate of rounding is proportional to the difference between the initial roundness and the final roundness value which is approached by the pebbles in the given environment. The analysis yields a function of the type P = Po (1e–kx), where P is the roundness at any point along the stream, Po is the final roundness value approached by the pebbles, k is a constant, and x is the distance along the stream. The observed data agree fairly well with the theory.

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