The deposits of an exceptionally large debris avalanche extend from the base of Mount Shasta volcano northward across the floor of Shasta Valley in northern California. The debris-avalanche deposits covers an area of about 675 km2 (260 mi2, and their estimated volume is at least 45 km3 (10.8 mi3. Dating methods suggest that the debris avalanche occurred between about 300,000 and 380,000 years ago.
Flat areas that slope generally northward at about 5 meters per kilometer (1 foot per mile) separate hundreds of mounds, hills, and ridges, formed by the avalanche deposits. The hills and ridges are formed by the block facies of the deposits, which includes masses of andesite lava tens to hundreds of meters across as well as stratigraphic successions of unconsolidated deposits of pyroclastic flows, lahars, air-fall tephra, and alluvium, which were carried intact within the debris avalanche. The northern terminus of the block facies is near Montague, at a distance of about 49 km (30.4 mi) from the present summit of the volcano. The flat areas between hills and ridges are made up of the matrix facies, which is an unsorted and unstratified mudflowlike deposit of sand, silt, clay, and rock fragments derived chiefly from the volcano.
Boulders of volcanic rock from Mount Shasta are scattered along the west side of Shasta Valley, and in the part of Shasta Valley that lies north of Montague, at heights of as much as 100 m (328 ft) above the adjacent surface of the debris- avalanche deposits. The boulders represent a lag that was formed after the main body of the avalanche came to rest, when much of the still-fluid matrix facies drained away and flowed out of Shasta Valley down the Shasta River valley and into the Klamath River.
The debris avalanche probably originated in a quick succession of huge landslides of water-saturated rock on the northwest flank of ancestral Mount Shasta, each of which cut progressively deeper into the volcano. There is no evidence of volcanic activity occurring at the same time of the avalanche and the reason for it is unknown.