Geological Survey Bulletin 1288 Surficial Geology of Mount Rainier National Park Washington

The surficial deposits of Mount Rainier National Park record a complex series of events that have occurred within the recent geological past. Glaciers formed the oldest deposits very early in the history of Mount Rainier, perhaps even before the volcano as we know it today began to develop. Huge glaciers again blanketed almost the entire park twice during the last 40,000—50,000 years. The last of these major glaciations ended only about 10,000 years ago, but within just the last few thousand years the glaciers of the park have experienced renewed growth at least twice.

Mount Rainier has erupted repeatedly since the end of the last major glaciation, but most of this activity has been on a feeble scale in comparison with the cataclysmic eruptions that built the main cone. The production of new lava and pumice, some as recently as the mid-1800's, has not kept pace with the piecemeal destruction of the volcano by glacial erosion and repeated landslides. Some of these slides have had wide-reaching effects, for at least two mudflows generated by them were large enough to reach far down valleys and to bury parts of the adjacent Puget Sound low land.

Although all the surficial deposits were created by geologic events of the past, when viewed in the immense perspective of geologic time—measured in hundreds of millions of years—these events occurred only yesterday or early this morning; in all probability they will recur tomorrow.