Of all the ancient gravel channels in the gold country that proved so productive two, one in the north and the other in the south, could well have been the richest and the most difficult to mine of any because they were buried under ancient lava flows hundreds of feet deep. The Tuolumne Table Mountain extends across the county like a gigantic black wall, with basalt sides nearly perpendicular and a bare flat top. It was formed 9 million years ago by and a now extinct volcano. The Little Walker Caldera is all that remains of this volcano and can still be seen east of the Sonora Pass. One of the most impressive geological formations in Tuolumne County, the lava flow runs southwest along the modern Stanislaus River from the pass to Knight’s Ferry with a width of up to a quarter mile and rises over a thousand feet in places.
After the lava cooled and hardened the softer earth around it slowly eroded over millions of years leaving not only the hard basalt exposed, but in some places, especially along the northern side, the ancient gravel bed lay exposed as well. And it was from this side that most of the many tunnels that were dug into the rich gold lodes below were started. Any shaft from the top of the mountain had to penetrate 140 feet of basalt, another hundred feet of volcanic sand, fifty feet of mixed sand and clay and finally the twenty-five feet of the ancient gravel channel where the gold would be found at it’s base. Hundreds of millions of dollars in gold was removed from under the lava flow through the years.
The Butte Table Mountain starts near Lassen Peak and runs south to the north bank of the Feather River opposite Oroville where there are two distinct ridges, North and South Table Mountain. In the far north the lava flow is covered by other formations but in Butte County the original lava flow was bigger than in the south and had several branches. And, like in the Southern mines, the soft earth eventually eroded away exposing the gravel bed into which the tunnels were dug. In the town of Cherokee the basalt had been denuded from the flow for some four miles allowing easier access to the gold bearing gravel. Sebastapol in Morris Ravine and nearby Oregon City were two other mining sites that benefited from the wealth in this ancient gravel channel.
John Putnam is the author of Hangtown Creek, a thrilling saga of the early California gold rush.