| The Swift Creek volcanic assemblage consists chiefly of deposits of pyroclastic flows, lahars, and alluvium which are interbedded with tephra. These deposits and the glacial drift of late Pleistocene age associated with them provide the best available record of geologic events at Mount St. Helens during the period between about 36,000 to 12,000 years ago. The assemblage was formed during intermittent episodes of andesitic and dacitic volcanism and includes some of the oldest known volcanic deposits from Mount St. Helens. -- Hyde, 1975 |
