On the west rim of Crater Lake, approximately 600 feet south of the Devil's Backbone and 200 feet below the rim of the crater, there is a deposit of varved clays which resemble the typical varved clays associated with glacial deposits.

These clays are limited in extent, the greatest exposure being nearly six feet in thickness and thirty feet wide. Beyond, a mantle of brush obliterates all trace of the clays. Farther south of the exposure a lava flow rises above the elevation of the clay deposit. No trace of the clays can be found either at that point or farther south.

The thick growth of brush is due to moisture issuing at the top of the clay. Since the material above is definitely glacial in that it consists of typical morainal debris, it accordingly is more porous than the clay. The spring which supplies water for the North Entrance Ranger Station issues at the top of the clay deposit. In general the structure of the crater wall favors a westward drainage away from the lake; however, there may have been a small basin in which the clay was deposited which causes drainage toward the lake. Such a feature would be only local in character.

In both directions, north and south of the exposed varved clays, toward the Devil's Backbone and Hillman Peak respectively, the glacial deposits are more sandy, although a few irregular sand clay layers may be traced through the deposits. In several places, the most notable of which occurs along the trail to the spring, there are deposits of well stratified, water deposited sand. These glacial and fluvio-glacial deposits are further evidence that the entire western slope of Mount Mazama was covered by ice having a thickness of several hundred or perhaps a thousand feet in places. Morainal deposits to the southwest of Hillman Peak certainly indicate extensive glaciation on the west slope of the ancient peak. Only Watchman and Hillman, the two peaks on the west rim of Crater Lake may have projected above the ice sheet.

The varved clays rest on a series of lava flows. No glacial striae were found on the surface of the flow directly beneath the clays. The banding in the clays is uniform and regular, the varves being evenly spaced. The varved character of the deposit indicates definitely deposition of fine sediment in a body of quiet standing water. Such a body of water may have existed during an inter-glacial stage, the water being held in place by morainal material and the ice. During an advance of the ice they clays, if frozen, may have remained undisturbed by the overriding ice. There is also the possibility that the deposit represents deposition in a dammed subglacial stream beneath stagnant ice, in which case the glacial debris above the clays settled out of the ice as it melted.

The section including the varved clays, the glacial drift above and the underlying lava flows, was exposed when the peak of Mount Mazama was destroyed.