The Auriferous Gravels

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Ten years after the hike up to Three Lakes, I met the woman I was to marry, and a year after that, in 1961, we were a family- her children from her first marriage had become mine- and living for the summer in Meadow Valley, just below Spanish Peak. I was starting my field work as a graduate student in Geology on the Bucks Batholith.
We rented a small cabin on the valley floor from an elderly couple who had lived there all their lives. In his nineties then, Andy Jacks had worked in the gold mine on the top of Spanish Peak as a youth. I note on the most recent map I have that the name Jacks, like Bucks, has been perpetuated in the landscape-Jacks Meadow Pond, Jacks Meadow Creek. I guess we lived in Jacks Meadow cabin.
The mine was in the auriferous gravels that lay under a cap of volcanic flow rocks at the summit. It wasn’t a big mine, and had long since played out.    My wife, whose first husband was the son of a similarly aged Sierra gold prospector, was a little puzzled by my studied disinterest in finding gold in the course of my field work. “The old timers got it all” I kept telling her. It was hard enough to keep your mind on the bedrock geology, obscured as it was by various biohazards (snakes, ticks, poison oak, etc.) without adding the further distraction of gold fever.
The gravels were significant to me, however, in my interpretation of the bedrock geology, and they will take us back another step in time, much longer than the glaciers, but still far short of the full depth of time we will explore.
Four miles south of the deposits on Spanish Peak, and 1,000 feet lower in elevation, on a ridge just south of Bucks Creek, in a place visited only by geologist, and old time prospectors undoubtedly, there is another patch of the gravels. Pebbles, cobbles, small boulder rounded in some ancient stream now incongruously perched along a ridge line, and being daily eroded away. A little further to the east, and still lower in elevation, I discovered a prospect- a shaft cut down through some volcanic rock into the gravels below. It looked recent, but not very. Gold prospecting was a seasonal occupation for many people in the Sierra at the time. Not that anyone ever talked about it much.

The gold-bearing (auriferous) gravels were deposited in streams that wandered, stream-wise, across a low-lying flood plain. As such, the drop in elevation down from the deposits on Spanish Peak, to the ridge and thence to the prospect, which amounted to close to 2000 feet, seemed too much to be accounted for a just due to the natural gradient of the ancient river.  Rather, it seemed to me, it was best explained as due to fault movement. This interpretation fits in with the general interpretation of the history of the Sierra Nevada Mountains in the most recent era of geologic time- called “Cenozoic”.   The story goes like this:
At the dawn of Cenozoic time (literally called “Eocene”), there were no lofty mountains here. There was a low lying flood plain, in whose streams the gold-bearing gravels were deposited. The gold first took form as a metal in veins in the bedrock at a time long before the flood plain, the glaciers, and the people. And at a place miles underground. The persistent process of erosion and uplift exposed the veins at the surface in Eocene times, even as it does so today. Some of the gold found its way into eddies and pockets in those Eocene streams. Volcanism covered those streams, and uplift by faulting promoted the erosion we still see going on. Most of the old flood plain was carried away, but remnants remain behind in the high, flat surface one sees projecting gently upward from the floor of the Great Valley to the Sierran Crest.
It was brought to its present configuration by faulting and uplift.  The uplift was like a hinged door, the hinge lying along where the Great Valley is today, and the uplifted edge is composed of the faults that define the eastern front of the Sierra range, that spectacular wall of bedrock that towers above Lone Pine and Bishop.
In the south, the faulting is concentrated in a narrow zone that now defines the front of the range. Northward, the faulting splays out into a series of faults fanning westward. Between the different lines of faulting, there are down-dropped blocks that have names like Lake Tahoe, Sierra Valley, American Valley, Meadow Valley. Further north, the whole complex is truncated abruptly and obscured by masses of volcanic rock coming down from the north.
It is the remnant of this flood plain that the eye follows around a full circle from the top of Bald Eagle Mountain. It is into this flood plain that the present gorge of the North Fork of the Feather River, and all the rivers of the Sierra, is cut. It was on this flood plain the glacial ice accumulated and started its flow.
The mine at Spanish Peak was developed in a remnant of an Eocene stream channel. You could probably pan some small trace of it out of Spanish Creek as it flows out through Meadow Valley to join the North Fork of the Feather River at Quincy. Or downstream at Rich Bar, or from behind the power dam at Cresta.  Where you would look for it depends and what story you believe about how it got there. And that story, like any history, provokes two questions:
In what ways is it wrong?
What happened before?

Next: The Mature Years


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