 The eruption originated from a vent on the northeast flank of Plinth Peak, the highest and one of four overlapping stratovolcanoes which together form the Mount Meager massif.This activity produced a diverse sequence of volcanic deposits, well exposed in bluffs along the long Lillooet River, which are grouped as part of the Pebble Creek Formation.The explosive power associated with this Plinian eruption sent an ash column estimated to have risen at least above Meager, indicating it entered the second major layer of the Earth's atmosphere.