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Small hills, called hummocks, indicate large eruption from Shasta

Hundreds of small hills, called hummocks, extend from the base of Mount Shasta at least 45 km (27 mi) NNE across the floor of Shasta Valley. The hummocky landslide deposit was emplaced between 380,000 and 300,000 years ago during a massive debris avalanche that is not believed to be associated with volcanic activity.

The hummocks are part of an exceptionally large landslide that covers an area of about 675 km2 (260 mi2 and has a volume of at least 45 km3 (17 mi2). They consist of massive blocks of lava flows and layers of unconsolidated deposits of pyroclastic flows, lahars, tephra, and sediment, which were transported from Mount Shasta intact within the landslide. The flat areas between the hummocks are underlain by thoroughly mixed and shattered volcanic rocks of the landslide.

Boulders of volcanic rock from Mount Shasta are scattered along the west side of Shasta Valley (out of view toward left) at heights of as much as 100 m (238 ft) above the adjacent surface of the landslide deposit. The boulders represent a lag that was formed after the main body of the landslide came to rest, when much of the still-fluid part (called the matrix facies) drained away and flowed out of the Shasta Valley. The boulders were left behind as the still-moving part of the landslide drained away downslope.

More information about this deposit and the debris avalanche that formed it is available on the Debris Avalanche Hazards web page.