Monday, January 30, 2006

Visit to Green Valley

I have written so many times about Green Valley, that I hesitate to repeat myself, in order to develop context for remarks about the geologic history there.

Briefly, then, Green Valley is a wider-than-usual reach of the North Fork canyon, where it crosses, at about a right angle, the weak serpentine of the Melones Fault Zone. To the west, the soaring cliffs and pinnacles of Giant Gap (a different and stronger kind of rock, there) seem to shut off Green Valley from the world at large: how often the storms stall over the high country, while lower in the foothills, the sun has breached the clouds: so brooding showers and storm-wrack hover over Green Valley, while in Giant Gap, the fog glows gold and shafts of light from a westering sun burnish the dark cliffs.

It happens that Pleistocene-age sediments, glacial outwash deposits, are abundant in Green Valley, and were vigorously mined. The sediment load of the North Fork would climb well above the river's ability to transport it, during times of glacial maxima, of which there have been many, even within the last hundred thousand years.


Sediment cores from Owens Lake, at the south end of the Owens Valley, east of the High Sierra, which during glacial maxima, overflowed south through a systems of rivers and lakes, into Death Valley--sediment cores reveal no fewer than sixteen episodes of glacial maxima, within the last 52,000 years.

Yet we only have a name for the last few maxima, from about 25,000 to 10,000 years ago: we call it the Tioga glaciation; and we treat these several maxima as one episode.

Many of the outwash deposits in Green Valley are pre-Tioga, but none have been dated with certainty. Some might be what are called Tahoe I, others, Tahoe II, in age, respectively, about 120,000 and 60,000 years ago. Some might be Sherwin in age, about 750,000 years.

It so happens, too, that many relict channels exist to either side of the present river. Some of these were worked by ground sluicing, others by full-on hydraulic mining, and some could only be reached by tunnels. For in these channels is gold.

Imagine that during a glacial episode, the North Fork's sediment load is so extreme that the canyon fills to a depth of 200 feet above the "natural" river level, with boulders and sand etc. washed down from the North Fork glacier, miles up the canyon.

A flood plain exists within Green Valley, a mile long and half a mile wide. The North Fork meanders back and forth across this plain. At any one time, it follows some particular sinuous course. If we had a time-lapse movie spanning a thousand years, or better yet, a few thousand years, we'd see the course of the river, over the Green Valley glacial outwash floodplain, whip back and forth like a snake.

From study of existing temperate-latitude glaciers we deduce that the North Fork carried much more water, then, than it does today. Yet even those raging flows of glacial meltwater could not strip the sediments out of Green Valley.

However, it is natural to imagine that, while the sediments might have been 200 feet deep over the previous bed of the river, to either side, the bedrock floor of Green Valley would rise, until it intersected the floodplain surface, along the northern and southern edges of the plain.

So towards these edges the sediments were not "200 feet" deep; they were, let's say, less than fifty feet deep.

Under such conditions the raging flood-like flows of meltwater might well transiently mobilize the floodplain sediment, near the river, all the way down to bedrock; and if that mass of boulders and sand moves across that bedrock, it carves a channel.

So, considering that over the long term, the course of the river whips back and forth across the floodplain like a snake, we should expect to find many relict channels in Green Valley.

And we do.

We also find many "strath terraces." These are level or gently-sloping areas of bedrock standing fifty feet or more above the present level of the river. For, consider this subtlety of the snake-like, whipping-back-and-forth, glacial-maximum North Fork: imagine again that the extreme flood-stage-like flows have mobilized the sediment column of boulders, sand, cobbles, mud, whatever, down to bedrock.

If the transient channel of the river on the floodplain, mobilizing these sediments, *is itself moving laterally at the same time*, the sediments do not act to carve a distinct bedrock channel, but instead merely plane off the high spots in the bedrock, which project up into the maelstrom of boulders and sand.

Now suppose that the glaciers melted away, so the sediment load returned to modern levels, and the North Fork has rapidly incised through this Green Valley glacial outwash floodplain, and re-occupied, perhaps, its pre-glacial bedrock channel.

This "rapid" incision has likely taken a thousand years or more. During that time the floodplain has been lowered vertically and has shrunk horizontally. The river continues to whip back and forth across its narrower and narrower width. It is a freak of fate if any portion of the original floodplain surface remains intact. But it does happen. And as the river whips back and forth, it strips the outwash down, and exposes strath terraces as it does so.

In some places these strath terraces are quite easy to see. Elsewhere they remain buried under a thin veneer of outwash sediments.

If one were to try to characterize the shape of the bedrock underlying the often-thin veneer of glacial outwash sediments in Green Valley; that is, if one were to draw an idealized cross-section, cutting the canyon walls, and the North Fork, at right angles, by a vertical plane, striking north-south, one would at first glance see that the canyon is symmetrical about the axis of the river: it is about the same to the north as to the south. At second glance one would notice that, above the line of the glacial outwash floodplain, the bedrock canyon walls are steeper, but below that line, the bedrock is at a relative shallow angle.

Then when we examine this region of the profile--the shallow-angled bedrock beneath the outwash plain--we see that, in detail, there is a succession of sloping or level strath terraces and channels, until, flanking the river to either side, there is a lowest last symmetrical pair of strath terraces, about fifty feet above river level.

These are the most recent, Tioga-age, "recessional" strath terraces, cut, one expects, as the floodplain shrank to a narrow band, just before the North Fork re-occupied its pre-glacial channel.

That the North Fork has indeed re-occupied its own pre-Tioga bedrock channel is evidenced by outcrops of a very peculiar type of conglomerate found in Green Valley, always where the outwash sediments were in direct contact with the serpentine bedrock. Some cementing agent, perhaps magnesium, bound the bouldery mass into an amazingly solid rock. These masses of very young conglomerate are found, sporadically, at river level or sometimes well above river level, from the east end of Green Valley, to the west end. And there is one unequivocally true inference we may draw from the river-level position of these conglomerates, which, incidentally, were often so very rich in gold it was like a raisin pudding, except the "raisins" were nuggets of gold--the one almost idiotically true statement we can make about these mysterious conglomerates, is that the North Fork must have already incised to their level, if not below their level, at the time of their deposition.

They are absolutely typical in composition, these conglomerates, so far as the boulders and rocks which comprise most of their volume: they are boulders of granite, of Shoo Fly Complex metasediments, of the Sailor Canyon Formation and Tuttle Lake Formation and a few other metamorphic rock formations, of the North Fork basin upstream from Green Valley.

And add to these a certain proportion of often angular chunks of serpentine from local sources.

At any rate: the idiotically true inference--we were not there to witness this, first hand, hence we infer--is that the North Fork must have cut into the serpentine bedrock to at least the depth of the conglomerate deposits, before they were deposited.

Else how could they exist?

So if we find them at river level, and see them extending under water in places, we know that at the time they were deposited, the river's bedrock channel must have been at much the same level as it is now.

We can safely dismiss the notion that the conglomerates are brand new. Today and for the last ten thousand years they have been steadily eroded away, not deposited. Everything about them shows that.

No, they are old, but on the other hand, they can't be all *that* old. We know that the North Fork has cut about 2500 feet down in 4 million years. This works out to an average rate of incision of more than six inches per one thousand years. This helps us set an upper bound for the age of the conglomerate; but the issues surrounding this are too complex to go into very much, here.

Nowhere do we see the serpentine bedrock floor of the current channel in Green Valley. It is always masked by bouldery sediments. From conversations with gold dredgers in Green Valley, back in the 1970s, I learned that the actual bedrock floor of the channel is sometimes more than fifty feet below the river's surface.

But let us say that it is in fact "fifty feet" below the water level. So if we were to take the "six inches per thousand years" at face value, and were to ask, 'How long ago was the bedrock floor of the channel, where the water surface is, now?', we would deduce at once that the answer would be, 100,000 years.

My own instinct is that the current bedrock channel of the North Fork, in Green Valley, is much the same as it was in Tahoe II times, 65,000 years ago. Between Tahoe II and Tioga glaciations, a warmer and drier interval allowed the Tahoe II outwash plain to be eroded away, and the North Fork established a course through the serpentine bedrock.

Then along came Tioga glaciation, and the outwash plain built up again. The Tahoe II bedrock channel was just exactly where the floodplain was "200 feet" deep. Hence there the sediment column could *not* be mobilized, all the way to bedrock, despite the high, flood-like flows of meltwater. Hence the deepest sediments, near the serpentine bedrock channel floor and walls, were held fixed for many thousands of years, during which time, some kind of serpentine "tea" permeated these deeply buried sediments, and they became cemented into conglomerate.

Then the Tioga ended, the outwash plain was dissected and largely washed away, and these deeply-buried conglomerates were at last exposed.

They are quite curious and can be found in other canyons crossed by the Melones Fault Zone serpentine, as well. Someone should do their doctoral thesis in sedimentology about these very young, yet very tough, gold-bearing conglomerates. Date them, study them, map them.

I set out rather late today, around one in the afternoon, to see what the North Fork looked like, after the flood even of a month ago. I found that the lack of water bars on the Green Valley Trail had led to some moderate erosion along long reaches of the trail, and that several large Digger Pines had fallen on or near the trail, and need to be cut away.

The 1800-foot contour crosses the North Fork in Green Valley. The uplands nearby run about 4100 to 4400 feet in elevation. At about the 2200-foot contour, Ponderosa Pines begin to dominate the forest, along with Douglas Fir; this change in forest type represents the transition from the serpentine bedrock into the glacial outwash deposits; for serpentine is poisonous to Ponderosa Pines and Black Oaks.

The fork between the East and West trails is met not long after reaching the Outwash Ponderosas. I took the East Trail and dropped past Joe Steiner's grave down to the Hotel Site, where a brief rain shower pattered down, and then followed along east to a large mass of white rock rising beside the river. A deep pool is formed there, and, as with many such pools, a large mass of boulders and sand has formed just downstream, where the current slows.

All these near-river-level gravel bars had been reshaped by the flood event of a month ago. They were objects of almost mathematical perfection. If a boulder has any degree of "flatness" to it, and many of our metamorphic rocks will make fine, flat boulders, the river will leave them shingled, the upstream boulders overlapping the downstream boulders, and the flat surfaces tilted so as to face upstream.

In January 1997 a flood event had scoured all small trees and many large trees from the sides of the channel, and from the gravel bars. It was interesting to watch the gravel bars become populated with willows and alders, over the past nine years. Now they have been partly ripped out again, and those which remain are now leaning downstream, and have had their bark scoured away, and seem ghostly skeletons. Everything is raw and primeval, as it was after the January 1997 event. The shingling of the boulders was quite striking and I wondered whether, in the course of a few months even, if animals and people walking these boulder bars will tip one boulder this way, another that way, and the shingling effect will be softened.

So. I was at White Rocks, just upstream from the Hotel Site. The waters of Moonshine Ravine flared out into a powerful waterfall, bursting from a cliff a few yards away, through a drain tunnel from one of the old mines. Casa Loma Ravine enters the North Fork right at the base of White Rocks.

These rocks are some kind of wild card embedded in the serpentine; they may represent a patch of serpentine which was replaced by siliceous rock, molecule by molecule, after the manner or petrified wood. Various boulders from this outcrop are scattered downstream to a distance of hundreds of yards and more; they are often boulders so large as to be well beyond the present North Fork's ability to move them; yet move they did, while within the matrix of bouldery glacial outwash.

The White Rocks are coarsely granular in texture, mimicking granite, but are composed entirely by light-colored minerals, which I take to be predominantly quartz.

Here and there along the margins of the shingled boulder bars were patches of grey sand. I saw the footprints of deer and bobcats and foxes and one small bear.

I wandered about, examining the imprint of the flood. Moving down the river, I came to an area below the Hotel Site, with its fine and quite massive dry-laid stone wall, made from boulders of glacial outwash perched on the hotel's strath terrace. The wall merely expanded the terrace.

Large springs issue forth all along the edge of the terrace. I know of a certainty that several relict channels exits upslope. These become supercharged with groundwater during major rain events and take weeks and months to drain down, and actually, never do drain down, entirely; there are perennial springs, major perennial springs, near the hotel site, for instance.

Such springs form indications that a buried river channel exists. One sees the same pattern in the Tertiary gravels, high on the divides between the canyons: again and again, if a channel nears the existing ground surface, one will see springs. In fact, there is reason to believe that the miners would drive tunnels into the mountainsides, when they found such springs, because they are so closely linked to the ancient Eocene channels.

The same strategy may have been followed in Green Valley. One of the principal tunnels driven through the serpentine into a relict channel is yards west of the Hotel Site and its springs.

The whole day had been cloudy, but around three in the afternoon, or four, the sky west began to glow, and very briefly something like sunlight entered Green Valley. Then the clouds knitted back together and darkness increased.

I continued west and downstream until a cliff stopped further progress, and climbed a couple hundred feet into the Outwash Ponderosa Forest, here dominated by Douglas Fir. I struck a major (but overgrown) trail through the woods which had often caught my eye in years past, but which I have never followed from end to end. Ignoring it, I made my way north and entered Ginseng Ravine, picked up the high mining ditch, and took the Low West Trail steeply up to the West Trail and the fork I had passed a few hours before.

Climbing above the Outwash Ponderosa Forest, I saw that a large flat-bottomed mass of fog hung over the south canyon rim in Giant Gap, the base being at about the 3900' contour.

Such fog masses are quite common during and right after storms in the North Fork. When they have flat bases, it shows that the air has stratified, and that not much wind can be mixing the atmosphere; for the flat base represents the dew point, the air below being too warm to form fog, but the air above, cold enough.

For any given humidity the dew point is found at a certain temperature, and conversely, for any given temperature, the dew point is reached at a certain humidity.

That is, the dew point represents saturation of the gaseous water in the atmosphere: it is forced to condense into droplets, ergo, fog, clouds, and so on.

This fog bank was of what I call the "lee slope" type.

When a wind passes over a ridge at right angles, it will often create a partial vacuum on the lee side, and air rushes up from below to fill this partial vacuum (like an airplane wing). This "air from below" is warmer, and therefore moister, than the air it is replacing. If conditions are right, fog will form as the warmer air uplifted on the lee slope crosses the dew point.

Here a very weak southerly wind (the "wind aloft") was causing lee-slope uplift in Giant Gap, and much more weakly, in Green Valley. Hence a very respectable fog-mass had formed in Giant Gap, while only a thread of fog at the 3900-foot level showed that lee-slope uplift was happening in Green Valley too.

Both were flat-bottomed, showing strong stratification and weak mixing of the atmosphere.

Only the south canyon rim was affected; the north canyon rim was on the windward slope, and was untouched by fog.

Here is a subtlety: although above the canyon rim a southerly wind was causing the lee-slope uplift and thus the fog, down inside the canyon the flow was from the southwest; one could see the fog moving slowly to the left, to the east, up the canyon. But as it entered Giant Gap Ravine, the lee-slope uplift diminished, and the fog trailed away into a narrow point, and evaporated altogether.

One often sees this in the canyon: a fog bank forming at one end, and disappearing at the other.

Not far from the Giant Gap fog mass is a spot about 500 feet below the canyon rim, on the principal spur ridge descending to the river from Giant Gap Ridge, where the spur ridge levels out briefly. It is almost a pass. It so happens that a branch from the western side of the Melones Fault crosses the ridge at this quasi-pass, and a grove of Kelloggs Black Oak grow in the fault shattered rocks of the pass, where soils have developed to a greater depth and a richer yield.

When conditions are right, an arch of fog drape across the spur ridge, right over the pass, right over the Black Oaks of the Fault Zone, and one can watch it come into being on one side, and evaporate into clear air again on the far side.

It can flow in either direction, this fault-zone fog arch. It spans a couple-few hundred yards. Sometimes it is the only patch of fog in the canyon!

It is caused by the entrainment of up- or down-canyon flow of air; all things being equal, if a spur ridge lies athwart the wind, and has a straight, uniform profile, air should move across it more or less smoothly and at the same speed. There should be the usual lee-side uplift.

But let that ridge be interrupted by a quasi-pass, and suddenly the air is able to move more swiftly, across the pass, than higher or lower on the spur ridge.

This can lead to warmer and therefore moister air rising into the pass-entrained winds from the windward side of the spur ridge. And so--let the temperature and humidity be suitable--a fog arch forms.

As I climbed up the trail, moving more rapidly than normal since I realized that the general gloom was deepening and sunset was near, I watched while the global temperature declined. The result was that the flat bottom of the Giant Gap fog bank lowered; it had been at 3900 feet, now it was 3800, 3700 feet, and suddenly the thin thread over the south canyon rim in Green Valley was a large and continuous mass in its own right; it hooked up with the Giant Gap mass, and with other masses rapidly forming from this same lee-slope function, farther up the canyon; the air was astoundingly stratified, and within minutes I was looking at a flat-bottomed fog bank extending east into Humbug Canyon and beyond, and still lowering!

Looking above me, I saw that the dew point had finally lowered enough for fog to begin to form on the north canyon rim, in isolated small patches.

When I finally reached the top of the trail, I was in the fog, and the last light of the day was dwindling into dark.

Such was another fine day in the North Fork.

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