Often mistaken for volcanic cones, these hummocks puzzled geologists for over a century until their origins were finally linked to a massive debris avalanche.

For over a century, geologists were baffled by the rolling mounds scattered across Northern California’s Shasta Valley. Travelers along Interstate 5 can glimpse these unusual hills and ridges, often mistaking them for volcanic cones or natural terrain. It wasn’t until the eruption of Mount St. Helens that scientists realized the landscape was carved by the sheer might of what is thought to be one of the largest landslides ever discovered, an ancient collapse so massive it reshaped the region northwest of Mount Shasta.

Shasta Valley, located at the northwest base of Mount Shasta, is home to the towns of Weed, Yreka, Montague and Grenada. The remote area, accessible via Interstate 5, offers a direct route between California and Oregon.

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Phil Dawson, scientist-in-charge of the U.S. Geological Survey’s California Volcano Observatory in Menlo Park, grew up in the shadow of Mount Shasta. His father taught geology at the College of the Siskiyous, and Dawson was immersed early in the region’s dramatic landscapes.

These rolling mounds, visible from the highway, mystified scientists for over a century until the 1980 Mount St. Helens eruption revealed their true volcanic origin.

“That debris flow was recognized to be unusual from the very first time any geologist came........

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