Thursday, March 9, 2006

The Strange Fanglomerate

On Wednesday, March 8th, the only sunny day in a million years and my fifty-seventh birthday, Catherine O' Riley enticed me to visit Green Valley, and we stomped and slipped along in a foot of wet snow, through and under groves of manzanita bent low across the trail by recent heavy snows, rounded masses of a hundred pounds or more still hanging on, here and there--and a pleasant sunshine warmed the air, slightly, anyway, and then, down about 3000', we left the snow behind, and I began talking about the Secret Cave and the Strange Fanglomerate, of Ginseng Ravine.

It becomes time to define terms:

1. Ginseng Ravine heads up in the Valley Springs rhyolite ash beds and Mehrten andesitic lahars capping Moody Ridge, and around the perennial springs issuing from the top of the rhyolite ash, grow clumps of the California Ginseng, Aralia californica if I recall, in the Ginseng Family but not itself "ginseng." It does have knotted gnarled roots, which send up brash and ill-smelling six-foot-tall stalks every spring, with large leaves and showy flower-clusters and fruits, in the season. The stalks die back each fall. Ginseng Ravine flows south to meet the North Fork just downstream from the Hotel Site and Joe Steiner's Tunnel (the old George Opel mine), near the center of Green Valley. Ginseng is about two miles long, or less. It is quite steep and has many waterfalls.

2. Fanglomerate. We call consolidated pebbly sediments "conglomerate," but when an alluvial fan somehow becomes consolidated, it is proper to call this sedimentary rock "fanglomerate." I used to call the Strange Fanglomerate, "agglomerate," to distinguish it from ordinary conglomerate, for this strange stuff in Ginseng Ravine is made of very angular, unrounded chunks or "clasts" of serpentine, whereas conglomerate has rounded pebbles and cobbles and boulders, etc. The clasts are supported in a fine-grained grey matrix of sandy sediments.

3. Green Valley. A Gold Rush mining camp on the North Fork, predating Dutch Flat, where two thousand people once lived, and mule trains from Illinoistown brought supplies. Noteworthy for a higher-than-usual incidence of Chinese miners, who were always prominent in the river canyons, anyway; but in Green Valley, they were not only prominent, but persistent, significant Chinese mining continuing down into the 20th century. Green Valley is walled off from the rest of the world by the cliffs of Giant Gap. It is a kind of Yosemite. The rock is mainly serpentine, of the Melones Fault Zone, but there is much glacial outwash. From the I-80 exit at Alta, Casa Loma and Moody Ridge roads are followed to a parking area, where the trail to the North Fork begins, three miles long, about, with over two thousand feet of elevation loss/gain--a strenuous hike.

Very well.

Despite many visits to Green Valley, Catherine had never seen the Secret Cave.

The top of Moody Ridge is about 4100', the North Fork, in Green Valley, 1800', and as I have mentioned, a grove of Ponderosa Pines marks a zone of sweet soil amid the half-poisoned country rock of this part of the canyon, serpentine; and this sweetness derives from glacial outwash sediments, which occasionally buried Green Valley during glacial maxima, over the past million years or two, and in particular, Green Valley seems to have been buried beneath outwash as recently as 12,000 years ago, in the so-called Tioga glaciation.

Although the upper edge of the Ponderosa grove hugs the 2200-foot contour, one does not actually see much outwash until down around 2000'. But one knows, if at all familiar with the response of our native plants to serpentine, that somehow, some way, the serpentine bedrock has been masked, the poison, neutralized; for the Ponderosa Pine, like the Kellogg's Black Oak, will rarely ever grow in serpentine.

Sun-blasted brush and a few Digger Pine line most of the trail. Then, one approaches the Outwash Grove; the tops of the tall pines are still below, but only just below; and at a certain point, a secret trail forks right, back into Ginseng Ravine, to the west, and tho at first it looks like a badly neglected squirrel path, almost immediately it grows into what can only be an old human trail.

I first followed this trail back around 1980 or so. It led to some very strange rock outcrops, serpentine, it seemed, but somehow all fractured and split apart into little chunks or big boulders, and then absolutely welded together. I had never seen the like, and, battering away at it a little here and there, I found it, if anything, tougher than it looked. And it looked plenty tough.

I climbed up on top of one such outcrop, and followed along higher, passing a spring which sprang directly from the frozen matrix of angular serpentine, and then, moving north, I reached a spacious terrace of this strange strange rock, and saw a one-pound coffee can, red with the rust of ages, buried in the ground, almost invisible.

A miner's cache of gold, no doubt! Those crafty, crusty old fellows, always worried someone would steal their poke! Ergo, bury it somewhere, hide it in a hollow oak, put it in a coffee can, and set it below ground level, and scatter rocks and leaves over it ... .

I cleared the debris away, and saw that the "coffee can" extended indefinitely deep into the solid rock; an impossibility, of course, but, there it was! What--the gold had fallen through? What kind of inept miner?! ...

I strode to the edge of the level cliff-top, said cliff facing directly into Ginseng Ravine. The strange fractured serpentine-mass fell away sheer about twenty feet, and below, very steep slopes plunged towards waterfalls and cascades, dimly seen through a curtain of Canyon Live Oak, Bay Laurel, and young Douglas Fir.

I found a way down around the end of the cliff, and circled back to a point below my "coffee can." And there was the Secret Cave, with various old junk lying about, including a ruined table, built with square nails, and then repaired with round nails; and I saw at once there had been two different times of occupation, neglecting possible Indian uses, and that the latter time of the two was likely the Depression. An old Prince Albert tobacco can was in the cave, containing buttons, and a fragment of a 19th-century saw blade, etc.

A trail led from the Secret Cave south a few yards to the very spring I had seen from above; and an old galvanized one-inch pipe was easily cleared of obstructions, and water flowed through the pipe again, clear and cold.

Western Azalea crowded the spring.

And the "coffee can"? It was, of course, a stove-pipe; the miner had managed to punch through the cave ceiling, following a natural crack in the Strange Fanglomerate, fully ten feet up to the surface of the terrace.

For, yes, the Secret Cave is *in* the Strange Fanglomerate. It is a west-facing cave, and a wild and beautiful place.

Catherine and I explored cave and spring, then dropped to the waterfalls below, where there are other impressive exposures of this same fanglomerate.

The fanglomerate, let us say, is 800 feet above the North Fork, or about 2600' in elevation. It has several distinct "strata" ten to twenty feet thick, all sloping south towards the North Fork; these several strata may be the signatures of several intense episodes of erosion, on the canyon walls above.

I suppose this fanglomerate is 750,000 years old and dates to the Sherwin Glaciation, to which date and glaciation I also assign the Hayden Hill Terrace, the highest glacial outwash terrace I know of in Green Valley, 600 feet above the river.

My reasoning is somewhat as follows.

In Sherwin times, a glacial outwash plain developed in Green Valley, its surface at the 1800+600 = 2400' contour, while the bedrock floor of the North Fork was then at 2200'. The various ravines on the canyon walls now discharged their sediments, not into the North Fork itself, most likely, but onto the outwash plain. Hence alluvial fans built up; we might also imagine that, during the Sherwin, temperatures were enough colder that frost-wedging and overall erosion of the canyon walls occurred at a much higher rate than at present; so that, like the other ravines, Ginseng Ravine carried more sediment than usual. And most of that "sediment" was angular chunks of serpentine: a constant rubble of angular serpentine slid down into Ginseng Ravine. And, so far as possible, the waters of Ginseng Ravine moved that mass of raw rocky angular seds down towards the outwash plain, at 2400'.

But there all gradient was lost and the rocky seds stopped, and backed up into Ginseng Ravine itself, above the outwash plain, forming a kind of narrow, ravine-walled alluvial fan, about two hundred feet thick and five hundred to a thousand feet long. The top of the alluvial fan pitched steeply down towards Green Valley.

There is an unknown (to me) cementing agent present in ground water in serpentine bedrock, and one of the mysteries of Green Valley is the cemented glacial outwash, seen mostly quite near the North Fork, and always directly in contact with serpentine bedrock.

Whatever this cementing agent is, it is tremendously strong. These cemented outwash seds are true conglomerates, even though some may be quite young, less than 20,000 years old, say.

OK. Clearly the fanglomerate was cemented by the same mystery mineral. Perhaps it is a type of iron arising in a chemically "reducing" (oxygen-starved) environment. It does not turn red, tho, when exposed to air for any length of time, up to thousands of years. So I wonder whether the cementing agent could really be any type of iron.

At any rate. My model is: the Green Valley outwash plain stood at 2400' elevation, 750,000 years ago, and the ravines delivered so much rocky serpentine debris to the outwash floodplain that it backed up into each ravine, forming a steeply-pitching alluvial "fan." The parts of each fan closest to the underlying bedrock, suffused with ground water for thousands of years, became cemented. But the other parts--the "core" of the fan, let's say, and its uppermost layers--were not cemented, so, after the Sherwin ended, and temperatures moderated, and not so much serpentine was being frozen off the slopes above, the waters of Ginseng Ravine cut the unconsolidated, uncemented part of the alluvial fan away, and left the cemented part.

And to this very day the weakling waters of Ginseng Ravine have not succeeded in cutting all the way through the fanglomerate, down to the pre-Sherwin bedrock floor of the ravine.

It is certainly possible that these fanglomerates, found in every ravine in Green Valley, date from one of the younger glaciations, Tahoe I, say, 130,000 years ago, or even Tahoe II, 65,000 years ago; we have no age control whatsoever on these fanglomerates, except, perhaps, some educated guesswork.

For instance, one of the strongest arguments that the Strange Fanglomerate is *not* Sherwin in age, and *must* be younger, is that the cemented fanglomerate filled the pre-existing bed of Ginseng Ravine. What fanglomerate remains in the creekbed is not thick, not deep; the bedrock is only inches or feet below. In fact, parenthetically, the extreme toughness of the mysterious cementing agent is betokened by the fact that, at the creek itself, the fanglomerate has been planed down smooth, each individual clast of angular serpentine reduced to make one, common, smooth, nearly plane surface. This could only occur if the fanglomerate was very well cemented.

But my main point here is that the existing creekbed is almost coincident with the pre-fanglomerate bedrock floor of Ginseng Ravine; another few thousand years ought to finish the job of cutting through the fanglomerate altogether.

So. Think about Ginseng Ravine *before* the alluvial fan was emplaced. It had incised slowly into the canyon wall. At some instant in time, a portion of the floor of the ravine was buried under alluvium. Suppose that instant was exactly 750,000 years ago. Then, whatever else we might know about the rate of incision of Ginseng Ravine into the canyon wall, we can say, "Ginseng Ravine cut this deep, right here, 750,000 years ago," and we would be pointing to where Catherine and I stood the other day, beside waterfalls flowing over polished fanglomerate, staring at other smaller waterfalls spilling over cliffs of fanglomerate across the way, on the east wall of the ravine.

So. Since this special spot, the base of the fanglomerate, coincides with the *present* floor of Ginseng Ravine, we are left concluding that not much incision can have occurred since the fanglomerate was emplaced.

Yet the longer the time, the more the incision.

Hence we must lean towards less time, not more, a younger fanglomerate, not an older.

Yet--yet--the fanglomerate must have armored the floor of Ginseng Ravine in millennia past, just as it armors it still, today, and that may have slowed apparent incision: Ginseng had to expend its energies cutting through the fanglomerate, before any true deepening of the ravine could proceed.

Do you see what I mean? There are conflicting impulses, it seems; on the one hand, we wish to think that not much time could have passed, because the base of the fanglomerate is close to the pre-fanglomerate bedrock floor of the ravine; on the other hand, we wish to think much time may have passed, for the fanglomerate armored the creek, and retarded incision.

One always also wishes to make the fanglomerate old, on account of its remarkable toughness; but this is a false trail; it is just amazingly well-cemented for an essentially young rock. The cemented outwash down at river level is much the same: bizarrely tough and resistant to erosion, yet quite young.

So, I cannot really say that the Strange Fanglomerate is Sherwin in age, or Tahoe I, or Tahoe II; it is about certain it cannot be Tioga in age, but we are left having to say, greater than fifty thousand years old, less than a million. Not very good!

As described in previous emails of years past, this serpentine fanglomerate is not only found in all the major ravines in Green Valley, but also in the Bear River canyon, to the north; so I suspect, extrapolating, we may have half a dozen or more major canyons here in the Sierra, which will be found to contain vestiges of this special cemented fanglomerate. For many canyons cut the Melones serpentine.

I call the fanglomerate strange; in the fullness of time, I might see that it is ordinary.

Catherine and I found a different old trail leading up and out of Ginseng and back to the main Green Valley Trail, which we immediately left and wandered out into heavy manzanita to the east, following an old trail-bed, from a century or more past, which switches back and forth on an easier grade than the present trail, and starts to aim towards the Hotel, more directly; but then we lost the darn thing, and while scouting widely back and forth, unsuccessfully, continued down to the High Ditch, which spans Green Valley from east to west at about 2000 feet in elevation.

We explored east along this fine old mining ditch until we reached surprisingly full and boisterous Moonshine Ravine, and then it was time to turn tail and run for cover, I mean, I had to pick up my daughter from her school bus; so, quickly, under increasingly cloudy skies, we slogged on up and up and out, reaching the top just before a light rain set in.

Another great day in the great canyon. Thank you, Catherine, for saving me from my chores.

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