Shasta River Basin Geology

Mount Shasta dominates the landscape almost everywhere in the Shasta River watershed. At 14,162 feet high, the mountain looms far above everything else. Several glaciers exist on the mountain, and glaciers descended the slopes of the mountain to an altitude of 2,800 feet during the Pleistocene epoch . Glacial moraines deposited during that time can be seen in the southern part of the valley. Today the glaciers continue to deposit materials on the lower northwest flank of Mt. Shasta. 

A geologist recently discovered that the hillocks covering much of the central valley floor were deposited in a huge debris-avalanche (Crandell, 1989). Unknown factors, possibly the rapid melting of glaciers or of a frozen crater lake, supersaturated the conglomerate of sand, gravel, old lava and ash that formed the mountain, causing it to liquefy and flow outwards to the north, completely filling the area we now know as the Shasta Valley, and flowing into the Klamath River.  Large chunks of consolidated material from this debris flow remain today as rounded hills interspersed with depressions. The depressions fill with water in the winter, and provide wetland habitat. These formations can be seen along Interstate 5 between Weed and Grenada.