Massive grandeur and sheerness combine to make Llao Rock, on the northwest side of Crater Lake, one of the outstanding features of the crater wall. The top of the rock, 8,046 feet above sea level, stands 1,884 feet above the surface of the lake.

In addition to being an impressive scenic features, Llao Rock is of great significance in the in the interpretation of the events that took place during the building of Mount Mazama, the name given to the peak, the destruction of which resulted in the crater in which Crater Lake is cupped. Llao Rock is a dacite lava flow, one of the most recent flows on the slopes of Mount Mazama. The evidence indicating whether the flow issued from the summit crater of the ancient peak or from a secondary vent on the slope has been destroyed. Regardless of the location of the vent from which the flow issued, there is distinct evidence that the dacite lava in moving down the mountain followed the course of a valley carved by a glacier. A sufficient quantity of lava issued to completely fill and overflow the glacial valley which has been carved to a depth of 500 to 600 feet, the maximum thickness of the flow being approximately 1,200 feet.

The conclusive evidence that the flow buried a glacial valley is as follows: (1) The cross-section of the base of the flow is distinctly U-shaped. Such a cross-section is characteristic of valleys carved by alpine glaciers. (2) Glacial scratches occur on rock surfaces on which the flow rests. The existence of glacial striations at points marked A and B on the sketch of Llao Rock which appears on the following page was determined by Ranger Naturalist Loren F. Miller. (3) Several feet of morainal material (glacial till) occurs immediately below the flow at the point marked B on the sketch.

The question has arisen as to whether the flow occurred while a glacier occupied the valley. Observations indicate that there was no glacier in the valley at the time of the flow. There is distinct evidence that the viscous lava came in contact with the morainal material, pebbles and small boulders being partially embedded in the underneath surface of the flow. Had ice existed in the valley at the time of the flow there would undoubtedly be evidence of caves formed as the ice melted out beneath the flow. Evidence of such caves is entirely lacking.

From the evidence presented it may be concluded that Llao Rock represents a recent lava flow which filled and buried a glacial valley carved to a depth of 500 to 600 feet on the northwest slope of Mount Mazama, and that the flow occurred when the valley was free of ice.