RECONSTRUCTING THE LATEST PLEISTOCENE AND HOLOCENE GLACIAL HISTORY AT THREE SISTERS AND BROKEN TOP VOLCANOES, OREGON, UNITED STATES

At least three glacial stands occurred since the last glacial maximum (LGM) based on moraines located at Three Sisters and Broken Top volcanoes in the Oregon Cascades. The youngest advances was the Little Ice Age (LIA) glaciation, which reached its maximum extent 150 – 300 yr BP and is defined by large sharp crested and unvegetated moraines adjacent to the modern glaciers. In isolated locations less than 100 m downslope from these moraines, a second set of sparsely vegetated lateral moraines marks the Neoglacial stand of the glaciers between 2.1 ± 0.4 and 7.7 cal ka BP. A third set of post-LGM end moraines are 300 – 900 m downslope of the modern glacier termini postdate 8.1 cal ka BP. Modern equilibrium line altitudes (ELAs) at the Three Sisters and Broken Top are approximately 2500 – 2800 m. During the LIA, the ELAs were 40 – 170 m lower, requiring cooler mean summer temperatures of 0.2 – 0.9°C coupled with winter snowfall increases of 10 – 50 cm snow water equivalent (swe). The post-LGM ELAs were 140 – 310 m lower than modern glaciers requiring mean summer temperatures to have been 0.6 – 1.8°C cooler during the summer with winter snowfall 30 – 110 cm swe greater.