Pleistocene glaciation in the Trinity Alps of northern California

Abstract

The glacial sequence of the Trinity Alps has good development, significant location, and value as a reference datum for other Pleistocene events. The principal glacial episodes identified are Late (Morris Meadow), Middle (Rush Creek), Early (Alpine Lake) and Ancient (Swift Creek). The 3 youngest are judged Wisconsin, the oldest pre-Wisconsin. Other evidence suggests possibly still older glaciations, and youthful moraines at high elevations may be Neoglacial (post-Hypsithermal). During the Late (morris Meadow) glaciation, the Trinity Alps contained at least 30 valley glaciers. Maximum lengths and lowest elevations attained are: Late, 7.8 mi. and 3,350 ft.; Middle, 9.3 mi. and 2,800 ft.; Early, 11.3 mi. and 2,500 ft.; Ancient, 13.7 mi. and 2,450 ft. Length was more variable than terminal elevation. Where the glacial detritus was derived largely from serpentine, numerous debris flows extended down-valley from glacier snouts. Deposits of flow debris, almost indistinguishable from till, complicate interpretation of glacial relations on lower Deer and Swift creeks. The rock floors of stream terraces, well exposed in abandoned placer workings along Canyon Creek, lie at 10, 20, 40, 90, 160, and 300 ft. above the stream. Direct tracing to terminal-moraine positions suggests that the 10-, 20-, and 40-ft, rock-terrace levels are chronologically, but perhaps not genetically, related to the Late, Middle, and Early phases of Wisconsin glaciation. The 90-ft. terrace may be a time-equivalent of the Ancient glaciation (pre-Wisconsin), and the 160- and 300-ft. terrace could be chronologically related to still earlier Pleistocene glaciations. In the high country, carapaces of ice formed on sloping valley walls above the level of the ice streams partly filling the valleys. Two vestigial glacierets remain at elevations of 8,200-8,400 ft. on the N. side of the Thompson Peak ridge. Wisconsin orographic snowline lay at 6,500 ft., climatic snowline at about 8,000 ft. Corresponding figures for the present are judged to be about 7,500 and 9,000 ft. Wisconsin orographic snowline rose inland 18 ft. per mi., judging from data obtained on Mt. Lassen, 85 mi. to the ESE.

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