 Little Walker Caldera last produced a major eruption about 9 million years ago.Andesitic lava and tuff from the series of three eruptive events are found several hundred feet thick as far away as Antelope Valley, Mono County, 20 miles north, Bridgeport Valley to the south, and Sonora 60 miles to the west.The lava flows near Sonora are readily visible from California State Route 108 and followed the gold laden stream beds in the area.