Monday, July 10, 2006

In Search of Old Trails and Pliocene Basalt

Sunday morning I whisked over to Grass Valley to meet geologist David Lawler for explorations up in Nevada County, near the popular Grouse Ridge hiking area. We had two objectives: to find and follow a certain old trail running up Canyon Creek (not Gold Run's Canyon Creek, but a major tributary of the South Yuba, joining the latter just above the town of Washington), and to visit one or more of the little patches of "Pliocene Basalt" as depicted by the USGS's Waldemar Lindgren in the ca. 1900 "Colfax Folio" geologic map.

So we piled into Dave's rattletrap old Jeep and zoomed slowly up Highway 20 above Nevada City. This is an excellent drive to get a look at the Tertiary (or "young") volcanics which cap so many of our ridges in this part of the Sierra. The roadcuts expose much in the way of the andesitic lahars or mudflows of the "Mehrten" formation, and occasionally one also sees the cream or white older rhyolite ash of the "Valley Springs" formation.

Highway 20 eventually descends into Bear Valley, at the headwaters of Bear River, where again and again the South Yuba icefield spilled over the dividing ridge into the Bear, wearing it down to near non-existence, and by the looks of things, a moraine-dammed lake formed, subsequently silting in the become the wet meadow we see today.

Here two branches or "feeders" of the Henness Pass Road met, one from Dutch Flat, the other from Nevada City, and continued north as one road across the South Yuba and on towards Bowman Lake and points east. This road was known as the Pacific Turnpike and was built by combined Irish and Chinese labor in 1863, opening for travel in 1864, when stagecoaches and freight wagons made great use of it. It led to Virginia City, Nevada.

We climbed out of the South Yuba on the Pacific Turnpike, passed the road right to Grouse Ridge, and soon we were paralleling Canyon Creek, a thousand feet below, and approaching the Windy Point Cliffs, we began scanning the roadside carefully for our lost trail. We passed one logging road dropping away west and south, then another, and then saw the duck some intrepid hiker had placed at the unsigned trailhead, and parked nearby.

This was a good match for our maps, but nary a blaze could be seen. The "trail" was a rough piece of road, hashed up into a welter of loose rocks by bulldozers and impassable for cars or Jeeps, and it led us down to the line of a canal, now defunct, which once issued from Bowman Lake, a mile to our north.

Here a broad bench cut ran almost level, still several hundred feet above Canyon Creek, which wound through masses of glaciated granite below us to the west. Rather than a ditch, this section had been flumed, and in its latest incarnation, which we guessed to be from the 1960s, PG&E had used a semi-circular metal flume, supported by gigantic timbers. I saw many a six-by-eighteen, and one does not see six-by-eighteens often. Apparently, PG&E had been much bothered by snow avalanches.

We could not tell if we were on "the trail," and if we were on it, where it left the line of the canal and dropped down to Canyon Creek. Scouting north, we found a tunnel, over eight feet square, blasted into the solid granite, where it seemed that once the flume had been hung from the very cliffs, but at last PG&E could not tolerate the avalanches, and drove the tunnel. We walked fifty yards into the thing and saw no hint of light ahead; it must have been a long tunnel.

Everywhere below the defunct canal were timbers and sections of metal flume, scattered by the many avalanches over the years. PG&E had even used complicated arrays of railroad track, welded together, to attempt to hold the flume to the cliffs, railroad track bent into special curves fitted to the ins and outs of the cliff itself. Now all that too lay in disarray.

We determined to drop directly down a steep bouldery gully to Canyon Creek, and to scout down there for our lost trail. We had to skirt the base of the cliffs to reach the gully (the one good rocky avenue in a sea of brush), and at a certain point, an old cable hung down the cliff, inviting one to grab it for support in a difficult area. Dave so grabbed, and instantly a boulder cut loose from somewhere above, and thudded to the ground a few feet away.

A near thing.

As we reached the base of the steep slopes, where timber grew in thick glacial till, we ran into a welter of skid trails which boded anything but good for our chances of finding the old trail.

As it turned out, we were too far north, and should have followed the defunct canal south. Howsoever, we enjoyed a nice reconnaissance of Canyon Creek as it wound past glaciated granite domes, in a series of waterfalls and gorgeous deep pools. We had lunch near two of these waterfalls and pools, and admired the granite, which was sort of porphyritic, a crystal mush as all granite is, but this particular mush dotted with raisin-sized raisins of white feldspar, which were left as a billion tiny eminences rising ever so slightly above the glaciated surface.

We scouted north, following a bulldozer skid trail which, of course, might well be the line of the historic trail, since that is apparently how Tahoe National Forest mismanages its own trail system, i.e., TNF allows the trails to be obliterated by timber harvests. When I saw how close Canyon Creek pinched in towards the base of the windy Point Cliffs, I realized we were too far north, and we made an about-face and marched south, scanning what trees remained for old Forest Service blazes, without any luck. We had some nice strolls across large expanses of glaciated granite (this "granite" may be part of that pluton known as the "Bowman Lake Pluton"), but the afternoon was wearing on, and we wished to visit some basalt to the north, so we began climbing up and away from Canyon Creek as we made distance south, hoping to strike the line of the old trail.

But all we struck were more skid trails, logging roads, and log decks. Eventually we decided to just flail up the steep slopes and find a way to the road and the jeep. As we reached the steeps, I remarked, "You know, we should be in the right area, we could find the damn trail even now," and about ten seconds later I saw my first "small i" Forest Service blaze of the day. We had found the trail.

Following it up was a little troublesome, for almost no one has walked it for many years, and no wonder, as long stretches of it have been erased by logging. The trail is about buried beneath woody debris from the conifers along its line. We found more blazes, however, and soon had reached that same defunct canal. Here we lost the trail again, and scouting higher, found no trace, but reached the good old Pacific Turnpike, and walked a quarter-mile north to Dave's Jeep.

We deduced that the ducked bulldozer road we had followed down to the defunct canal does indeed form part of the old trail, and that once one reaches the canal, one has to follow it south a couple hundred yards or so, before the trail continues its descent to Canyon Creek. We had followed it north.

The road worsens as one approaches Bowman Lake, an old hydraulic mining reservoir, as were Lake Spaulding and Fordyce and Lake Valley and many another, now used by PG&E for water storage and power generation. Its dams have been raised since the olden days. When the Pacific Turnpike was built, the lake was, I believe, still a lush meadow a couple miles long, surrounded by high wild mountains of mixed granite and metamorphic rock, with precious little forest, the glaciers having had their way with things. Here and there an alcove in the cliffs preserved some glacial till, and some real timber flourished.

About midway east along the north side of the lake, on the Turnpike, a very rough road forks away north to McMurray Lake and Weaver Lake. One of my basalt locations was at the outlet of Weaver Lake. I wished to gather a sample for Dr. Brian Cousens of Ottawa, Canada, who has been engaged for years in the study of the youngest of the young volcanics in this part of the Sierra, mainly around the Squaw Valley Eruptive Center. There are a number of tiny relicts of basalt scattered in the middle elevations, well away from the various eruptive centers long the present Sierra crest, and these youngest-of-all Tertiary volcanics should help us unravel the history of the incision of our modern canyons. These Pliocene basalts are found only on the ridges dividing our present canyons. I have gathered samples for Brian from Sawtooth Ridge, between the North Fork American and the North Fork of the North Fork American, and from Lowell Hill Ridge, between the Bear and Steephollow, and from various other localities.

So, I find good unweathered chunks of basalt, and mail them off to Brian.

As Dave and I reached the north end of the lake, which drains north to the Middle Yuba River, a couple miles away, we saw part of the basalt, a rubbly mound rising almost a hundred feet above the road. We gathered our gear and climbed to the summit, where one can find blocky basalt cut smooth by the glaciers, with striae visible. Soon we realized that the flow was rather extensive, and scouted north along its almost flat surface, where it fell away in increasingly steep cliffs.

Geologists are alert to the difference between the lee, or down-ice, sides of rock outcrops, and the stoss, or up-ice, sides. Glaciers will pluck away rocks from the lee side, leaving cliffs, while the up-ice, stoss side, will show a gently curved and smoothed surface. The Weaver Lake Basalt exhibits this pattern to an alarming degree. We found ourselves skirting the edge of a rather monstrous and strangely steep cliff.

To the north, another part of the same flow had preserved more of its original thickness, being away from the axis of deepest ice. Pyramid Peak, as it is named on the map, exposes perhaps as much as five hundred feet of this same basalt.

The basalt itself was fine-grained and quite dark in fresh exposures, weathering to a light gray. It exhibits a weak columnar structure, but nowhere did I see good hexagonal columns, and all in all it has much more of a blocky character, than columnar. It is perhaps a half mile by a quarter mile in lateral extent, by about one hundred feet thick, there at the north end of Weaver Lake, and is almost bisected by the outlet of Weaver.

Dave and I slowly approached the ravine cut into the basalt where the Weaver waters escape to the north. We began to realize that the lee-side steepness of the cliffs was more than a little strange, it was a lot strange; for directly beneath the basalt was a 20-foot-thick section of rhyolite ash, and beneath that, a couple hundred feet of indeterminate soft sediments, likely also related to the rhyolite ash. And all this was much, much, much weaker than the basalt, and, facing north as it did, at an elevation not far short of 6000', was exposed to severe frost and thus to frost-sapping, an important agent of erosion, but here carried to extremes. For our basalt cliffs were not just steep, they were overhanging. At times while we had wandered the edge of the cliff, we had been standing on these massive overhangs without realizing it.

The creek itself drops over part of the overhanging section, and makes a pure out-and-out waterfall of at least one hundred feet. This would make for quite spectacle at higher flows. Yesterday, a modest amount of water simply dropped away into empty space. It's quite scary to approach the edge, there, but one tiny pine allowed us to lean over and look straight down the falls. Wow.

We were much impressed with this overhanging topography, and felt that luck had blessed us once again, for all we had imagined was scrambling up some knoll of basalt, and collecting a few samples with Dave's rock-pick.

Whereas it turned out we had stumbled upon one of the more interesting and unusual places in this part of the Sierra. I can't think of another waterfall like this one. Weaver Lake is beautiful, Pyramid Peak is bold and beautiful, but the Weaver Lake Basalt and its overhanging cliffs are, well, amazing, scary, and also beautiful.

At last it only remained to make the long drive back to Grass Valley, where we arrived about eight in the evening, sunburned, dusty, a little sore, but quite satisfied with our explorations.

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