A glacier dropped some of its icy load (moraine deposit) thousands of years ago, near Dorrington, forming summit level ridge. On New Year's Day in 1997 these unconsolidated moraine deposits soaked up all the rain and snow-melt water possible during an unusually wet winter. The rain kept falling, but the mountain could no longer absorb more water.
Then gravity took over.
An estimated 20,000 cubic yards of material slipped out of the ridge and flowed downhill. The flowing ooze crossed the highway and swept the entire forest within its 100' - 500' wide path. Two miles later an estimated 250,000 cubic yards crashed into the raging North Fork of the Stanislaus River. Up to 23" of soil had been stripped to bedrock. The 250' long bridge at Sourgrass was ripped from its footing and flipped onto the opposite shore in twisted pieces. The upper mile of the slide is within clear view of the Highway, 3 ½ miles east of Dorrington.
Take Ebbetts Pass National Scenic Byway or Hwy 4, to Dorrington, just east of Arnold. Then travel south on Board's Crossing-Sourgrass Road for about 5 miles until you get to the river.