Mary Lake is a small, subalpine lake located proximal (<3 km) to the Continental Divide near Lake O'Hara in Yoho National Park, British Columbia. A 1.25 m core recovered from surficial bottom sediments was found to contain both Bridge River tephra (2350 years BP) and Mazama tephra (6800 years BP). Identification of the tephras was initially based on distinctive colour, texture, phenocryst assemblages, and glass shard habits and was confirmed by electron microprobe analysis of the constituent glass shards. The microprobe technique employed backscattered and secondary electron imagery and both energy dispersive and wavelength dispersive analyses.Mean annual sedimentation rates calculated from tephra depths in the Mary Lake core were found to be of the order of 0.15 mm/year for the post-Bridge River time period and 0.02 mm/year for the Mazama – Bridge River interval. The preservation of Mazama tephra in Mary Lake lacustrine sediments indicates that deglaciation had proceeded upvalley from the Mary Lake basin prior to 6800 years BP, and consequently, moraine systems exposed downvalley are pre-Mazama in age.