Tuesday, February 7, 2006

Visit to Canyon Creek

Late this morning, on impulse, I threw some things in my pack and drove to the Canyon Creek Trail in the Gold Run Diggings. The little bridge across the creek is gone, ripped away in the flood event of the beginning of the year. It should be replaced, and The Inimitable Julie, as well as Catherine O'Riley, have advocated immediate action.

So, one of the things in my pack was my 25-foot tape measure. I recalled the old bridge as about twelve feet long; but my recollections have become suspect, and it seemed good to measure the chasm once again.

When building a bridge one seeks a narrow crossing-point. Then the bridge will be as short as possible. Hence, in terrain like this, one seeks an "inner gorge," a place of exposed bedrock where the channel has narrowed.

This is just the case at the Canyon Creek bridge site. Except during floods, the creek is confined within rock walls six feet apart.

However, the old bridge, built in 1998, was not twelve, but a little less than fourteen feet long, I determined once I arrived. It had been made using two 2X6 joists, with 2X6 blocking a foot long nailed between them every 32 inches, and a deck of three 2X6s laid lengthwise, parallel to the joists, attached to both blocking and joists using deck screws. One end was trapped rigidly within a gap in the bedrock, the other end rested on sloping smooth rock; gravity held it in place; had it been anchored, there is a chance it might have survived this last flood. For it was a sturdy little thing. No railing; that lack spooked a number of people.

It was a glorious day, incredibly warm. Canyon Creek was loud, boisterous, surging clear green water with long stretches of frothing white rapids. I had hoped to jump the creek and continue down the trail, but no, it is still too high.

So I walked around and looked at the bridge site from many angles. Of course one wants a bridge which stands high enough not to be ripped out every ten years. But the higher one places the bridge deck, the longer the bridge becomes.

I determined that by raising the level of the bridge about five to six feet, its length would grow to slightly over 20 feet.

Some homemade, arched, glue-laminated beams might serve. Then, blocking as before, and a bridge deck as before. But now the bridge would stand fifteen feet above the creek, not eight feet, and a railing would become essential, and the bridge deck must be widened to at least two feet.

I saw that bobcats, or a bobcat, had been frequenting the area, pooping here there and everywhere on the water-carved and polished bedrock. Only the most prominent and conspicuous spots will do.

The rock here seems to be some metamorphosed volcanic mudflows and ashflows (perhaps; it would take a real petrologist with a microscope to be sure). It is mapped as part of the late Paleozoic Calaveras Complex. The original strata are now tipped up near vertical. The rock seems quite siliceous and takes a high polish; it is various light colors, greys and tans, and makes a kind of fairyland of sculptured rock blades, rounded under the grinding impacts of a thousand floods. There are gigantic bolts and iron pins sprouting from the rocks, which once anchored the broad sluice boxes of the Canyon Creek Placer Mine, in the 1870s and early 1880s.

Broad? Eight feet broad. And the "undercurrents," a special type of sluice box used much in Canyon Creek, were fifteen or twenty feet wide.

The wooden sluice boxes were lined with narrow-gauge railroad track running lengthwise, and many pieces of this "sluice iron" are now embedded in the boulder-bars along the creek. Some have been bent into giant horseshoes six feet across, by some rending flood of years gone by. Some have been worn into slender threads of iron--or could it be steel?--during their time in the sluice boxes.

So the sluice iron itself is rather sculptural in character. Only rarely are the pieces of worn railroad track straight. Only rarely can one tell that they are in fact pieces of railroad track.

I have always thought that a bridge, or at least its structural members, might be made from this sluice iron. I have all kinds of ideas about making trusses and so on. A welding torch, a drill, some half-inch bolts and so on, and voila!

But my "ideas" usually involve long pieces of almost-straight sluice iron, which I would bend into slight curves or tight zig-zags (then the curves would bound the zig-zag, welded or bolted at points of contact). Twenty feet long, say. And there really aren't any of those. There are quite a few, though, over ten feet in length.

After an hour or so I strolled back up the trail. What a beautiful creek, what a beautiful day.

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