SURFICIAL DEPOSITS SHOWN ON THE GEOLOGIC MAP
The age and extent of the glaciers responsible for the old drift are not known, but glaciers evidently covered this part of the Cascade Range very early in the development of Mount Rainier, perhaps even before the volcano first appeared. The old drift consists mostly of glacial deposits over lying bedrock formations that are older than Mount Rainier and underlying lava flows from the volcano. The lava flows are of a rock known as andesite,1 and they form many of the ridges that radiate from the base of the volcano, such as Mazama and Rampart Ridges and Burroughs Mountain. Each old drift deposit shown on the geologic map contains compact till, but some also include mudflows and rubbles of volcanic origin.
Old drift exposed in a roadcut upslope from Narada Falls lies on a surface of glacially smoothed and scratched bedrock (fig. 4, locality 1). The till at this locality is overlain by a pumice-bearing deposit of sand and fine gravel that is interbedded with mudflows. This deposit, together with the till, is about 100 feet thick and is overlain by a lava flow from Mount Rainier that forms Mazama Ridge.
Old drift at the west end of Burroughs Mountain (fig. 4, locality 2) consists of about 800 feet of compact bouldery gray till and beds of sand and fine gravel. It is overlain, consecutively, by a deposit of loose gray and red volcanic rubble about 300 feet thick and by a lava flow from Mount Rainier. In Glacier Basin (fig. 4, locality 3) old till lies on top of compact mudflows and hard bedrock (fig. 5) . The till is very compact and bouldery (fig. 6) and is several hundred feet thick. Banked against it is an old lava flow from Mount Rainier, and both the till and the flow are over-lain, in turn, by a red volcanic rubble and a younger lava flow from Mount Rainier. The red rubble probably is part of the deposit that overlies the old drift at the west end of Burroughs Mountain. The old mudflows, the till, and the red rubble have been intruded by a wall-like andesite dike that forms a prominent ridge along the north side of Glacier Basin.
Last Updated: 01-Mar-2005