Monday, April 24, 2006

Mist Walking

Yesterday the clouds hung thick over and within the great canyon, and a slow brightening evoked the concept of a sun. It was time to take a walk, and from near the head of the Green Valley Trail, I dropped away east into the hidden valley of the Nary Red Channel.

This Nary Red Channel is a tributary of the Eocene-age Dutch Flat Channel, itself a tributary of the Eocene South Yuba Channel; but the Nary Red is much in the news right now, as last Friday a sinkhole opened up over in Alta, and swallowed a young man alive.

This sinkhole is without any doubt related to some old tunnel, perhaps fifty feet below the house, its outlet in Little Bear River long collapsed and hidden from view. There are dozens of such tunnels in the greater Dutch Flat area.

The experts clustered around the sinkhole should ask me what's what. The place to look for the outlet of the collapsed tunnel is away down the Little Bear, where bedrock starts to crop out. That's where the miners of days gone by would have started their drift or drain tunnel (it might have been either one which triggered the sinkhole). For, the basic scheme was, drive in horizontally through the bedrock, under the channel, then make an "upraise" and tap the gravels from below, with the help of gravity.

It is also possible that a vertical shaft, opening to a drift or drain tunnel at depth, was bulldozered over a few decades ago, when whoever owned it, decided to put up the property for sale. The shaft may have looked like a broad shallow hole in the ground, with some garbage, old washing machines etc., embedded in it. For it was a commonplace to dump garbage into mine shafts, in this area. The owner or bulldozer operator might have thought, "I'll just fill in this hole, bury the garbage."

Fast forward a few years, the lot has sold, a house is built. Now, wait; wait for a very very rainy rainy season. The old tunnel below always has water in it, and someone near the collapsed entrance, a couple hundred yards down the Little Bear, might notice a year-around spring issuing from a hollow in the bank. But when the water table becomes over-charged, a lot of water is in the tunnel. It fills the tunnel, and pressure builds at the collapsed outlet; water begins to break through, and finnaly succeeds. The tunnel drains, and as it does, a portion of its roof collapses. Over a period of weeks, more and more of the roof (soft Eocene river seds) falls into the tunnel and is washed away.

The exact courses of the sources of the Nary Red are not known or knowable, as our huge modern canyons have been incised deeply into the landscape, erasing most of the various older river channels in the process. For instance, the North Fork canyon cuts off the Nary Red where I was walking, yesterday, and one has the channel exposed more or less in a pure cross section.

One can follow right along the base of the Nary Red, which was incised into the serpentine of the Melones Fault Zone. There does not seem to have been a deep accumulation of Eocene channel gravels, here, for weakly welded rhyolite tuff beds occupy some of the deepest parts of the channel, not far above bedrock. Like, maybe less than fifty feet. But the Nary red is notable for the thickness of its "intervolcanic" channel sediments, intermixed rhyolite ash and hard cobbles of chert and quartz robbed from some Eocene channel upstream and to the east. These intervolcanic gravels can be seen in the railroad cuts a quarter-mile east of the Casa Loma Road crossing.

When one is directly across the North Fork canyon, one can look back at the Nary Red and see the serpentine bedrock sloping from east and west into the axis of the channel.

I was at that point yesterday, where Indians once flaked their arrowheads, as one sees in the fragments of chert exposed wherever pocket gophers have churned up the meadowy turf. The place is a paradise of giant pines and ancient oaks. All through the woods one sees collapsed shafts, mining ditches, collapsed tunnels, and so on. One of the open shafts is full to the surface with ground water, and bears use it as a swimming hole.

Near the deepest part of the channel are huge chunks of the rhyolite tuff, ten feet through, which are identical to the quite anomalous tuff boulders down in Green Valley, just east of Moonshine. I suppose that one of the two Tahoe glaciations sent enough ice down Canyon Creek that a tongue split away and scoured tuff beds out the Nary Red Channel. Certainly the more recent Tioga glaciation did not reach this far down Canyon Creek, and may not have even entered its headwaters. Thus the tuff boulders in Green Valley have been there for perhaps 65,000 years, or maybe 120,000 years.

There are also some Tahoe-age (?) glacial outwash deposits in Canyon Creek, scattered all the way down to the Oxbow, near the Old Wagon Road. And high on Moody Ridge, 400 feet above Canyon Creek, back in 1976, I found one granite glacial erratic, a boulder some ten feet long and four feet thick.

I think these are all local vestiges of the Tahoe, either Tahoe I or Tahoe II. One cannot speak with much assurance of these vestiges. It would be helpful to identify moraines, if any such still exist. None are obvious, I have looked.

I dropped over the edge into a lovely mixture of serpentine cliffs and steep, grassy, flowery glades. Rather than drying and warming and clearinging, a light rain began to fall. I passed one of the tunnels, out of view above, but I could see the steep bench-cuts which must have once supported some kind of sluice boxes or undercurrents. Their steepness is puzzling and reminds me of the Giant Trails over by Iowa Hill, leading away from Town Ravine. These too I think supported sluice boxes of some sort. It could be that they were just splitting the tailings stream, and diverting some portion of it down to the North Fork, where it would be run through ordinary sluice boxes.

A descending traverse led me to MoonShine ravine and a pretty waterfall, forty feet high. I recalled that somewhere below me was an old tunnel, and continued to descend. Eventually I spotted the "secret" trail down from the railroad tracks; marijuana growers had occupied the tunnel, back in the 1980s, and I have yet to finish carrying their garbage up and out.

The tunnel proved elusive--I had not been there in ten years--but I found it at last, and after a pause to look around, I crossed a small tributary of Moonshine and began a slow slog up the grassy cliffs.

It was tempting to drop right on down into Green Valley--I was a thousand feet below the canyon rim, almost--but the rain had slowly intensified, and I was getting wet. So I wound my slow way up the flowered cliffs, and soon enough, was home and changing out of wet clothes.

Such were a couple pleasant hours walking in the mist.

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