Volcanic rocks from an ancestral Mount Shasta exposed in this road cut along U.S. Interstate 5 are from a large hummock of a landslide deposit. The landslide extends more than 45 km NNE from the summit of the volcano; this location is about 18 km due west from the summit (in distance).
These rocks are primarily pyroclastic-flow deposits erupted by Mount Shasta onto its cone and then later transported here by an enormous landslide sometime between 380,000 and 300,000 years ago. The various layers are displaced along internal faults that likely developed as the landslide swept down the volcano. The origin of these volcanic layers confounded geologists for many years. Were they the products of individual small eruptions, deposits left behind by a glacier, or erosional remnants of older volcanic rocks from a different volcano? It took the landslide at Mount St. Helens to solve the mystery.
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