Bedded Sands

"Oh, look at the rocks tumbling down!" is a frequent comment of park visitors as they observe the rocks of the ancient Mazama rolling and bouncing down the caldera wall to a resting place in the waters of the lake.

There are many places along the inside wall of the volcanic caldera where active rocksliding constantly occurs. Near Chaski Bay and along the west section of the caldera by Wizard Island are some of the many places where large fan- shaped deposits consisting of various sized debris can be seen. The large gully-like depression immediately east of the Sinnott Memorial offers an excellent example of this process which is destroying the lake wall, making a very strong factor in its ultimate disappearance.

The walls of Mazama consist of volcanic fragmental and glacial debris, interbedded with well-fractured lava flows. The individual components, particularly of the first two members, are poorly held together and their unsupported ends incline at an angle of fifty or more degrees on the inside of the caldera.

Many factors contribute to the downward movement of the rock debris. Most important is melt-water derived from snow that accumulates to great thicknesses on the rock walls. This gives lubricating and hydraulic action for the removal of fine rock particles that support larger debris. Once a slide is started, more material of all sizes is dislodged along the paths. These tumble and bounce from one rock ledge to another, breaking off the edges, filling crevices crossed enroute, cutting their way through snow fields, and only slopping far below. Winds blowing against the walls also loosen fine particles which are supporting more massive material and help to start rock movement. Scurrying movements of small animals and tremblors caused by the rumbles of vehicles passing on the road, by thunder, or by distant slides are other contributing factors.

As rock slides and other forms of erosion transport material from the caldera wall to the bottom of the lake the caldera wall becomes lower and less steep; rock slides become increasingly less important. At the same time the accumulation of rock debris at the bottom of the caldera tends to fill the depression. Eventually, many thousand of years in the future, the lake will disappear by their combined action. Before the lake entirely disappears, the gradual reduction of the steepness of the caldera wall should result in the cessation of rock sliding as the more usual methods of erosion assume predominance.