Younger Dryas readvance in Squamish river valley, southern Coast mountains, British Columbia

Stratigraphy and landforms in the Brohm Creek basin and lowermost Mamquam River valley provide evidence for a readvance of a valley glacier in the southern Coast Mountains of British Columbia during the Younger Dryas interval. At Brohm Creek, till containing wood 10,500 and 10,700 14C yr old (ca 12,500–12,900 cal yr old), overlies an ice-contact terrace that is correlative with similar terraces at the mouth of Mamquam River, 11 km to the south and 900 m lower in elevation. A line linking the two sites has a slope of 5°, which is similar to slopes of termini of present-day valley glaciers in the southern Coast Mountains. We used these observations to reconstruct the Younger Dryas glacier in Squamish valley at the head of Howe Sound. The glacier deflected landslide debris, derived from steep slopes at the head of Cheekye River basin on the west flank of Mt. Garibaldi, southward to an ice-marginal lake at the mouth of Mamquam River. Foreset-bedded sediments in the core of a large fan at the mouth of Cheekye River indicate the glacier terminus had retreated up Squamish valley beyond Cheekye and Mamquam rivers by 10,200 14C yr BP (11,900 cal yr old). Radiocarbon ages on the oldest postglacial sediments in a kettle on the landslide debris show that stagnant ice in the study area had melted before 10,000 14C yr BP (11,500 cal yr BP). The readvance of the Squamish valley glacier during the Younger Dryas Chronozone was probably driven by a regional climate reversal.