[written November 15, 2006]
What a glorious season! Indian Summer grading into the storms of winter. The Black Oaks, Bigleaf Maples, Pacific Dogwoods, Blue Elderberries, Thimbleberries, Bracken ferns, and all manner of deciduous plants are blushing into all manner of golds and reds and purples and browns. Wow.
This has been a time of no expeditions down into the canyon, but many short hikes near my home, on Moody Ridge, near Dutch Flat. I have decided to make a precise geological map of Green Valley, a Gold-Rush-era mining camp of huge proportions, down along the North Fork, below Moody Ridge to the south. I actually began work on this map years ago, but now I will collate results, and also use GPS to exactly locate the boundaries of the various Quaternary units and bedrock structures in and around Green Valley.
I decided, too, that I would map Moody Ridge, for a big part of the story I want to tell has to do with the actual incision of the North Fork canyon; when it began, how quickly it proceeded, and so on.
Now, it happens that all the main ridges in this area, about half-way between the great plains of the Sacramento Valley, and the frozen summits of the Sierra, heh heh--in this area, I repeat, at around four thousand feet in elevation--it happens that all the main ridges are "accordant," that is, their summits are all at very much the same elevations, and these summits all slope very very gently to the southwest.
Moody Ridge is one of these accordant ridges. They are without exception relics of the Pliocene-era volcanic mudflow plateau which covered all this region, for many dozens of miles south and north. The plateau extended almost to the Tuolumne River on the south, and on up through the Feather River country to the north. It was pervasive, huge.
But then a gigantic slab of the earth's crust was tilted up like a trap door, a slab four hundred miles long and about one hundred miles wide; and we call this slab the Sierra Nevada.
It is worth noting that the crest of the Sierra itself is self-accordant, as it were, in that the many summits gradually increase in elevation, southward; we go from a paltry 8000' in the northernmost Sierra, to 9000' near Donner Pass, 10,000 feet at Highway 50, 13,000' at Tioga Pass near Yosemite, and 14,000' at the Palisades and Mt. Whitney itself, away down by the great canyon of the Kern, that strange Sierran canyon, unlike all others, which parallels the crest, instead of running away at a right angle.
At any rate, this self-accordant, smoothly-rising-to-the-south Sierra crest suggests that more uplift has occurred in the south than in the north. It is thought that there has been 10,000 feet of fairly recent uplift near Mt. Whitney, but only 4,000 feet, or thereabouts, near Donner Pass. There is some debate about Sierran uplift, its timing, and its magnitude, but I cannot go into the details now.
Here in the Dutch Flat area, the general pattern obtains, and all the major canyons trend from northeast to southwest, as do the long axes of all the ridges dividing such canyons; and Moody Ridge is among these many accordant ridges. If one had a giant ruler and laid it across the North Fork canyon, on a right angle, from Moody Ridge, to, say, the accordant Giant Gap Ridge, across the canyon to the southeast, one would find that that ruler was about dead level.
One can actually fit a plane to these accordant ridges, and thus restore implicitly the surface of the Pliocene andesitic mudflow plateau. I have done experiments with such plane-fittings using software which allows me to create virtual landscapes, based upon USGS Digital Elevation Model data, for this area--very very accurate models in which elevations are known on a square grid of 30-meter intervals. The software requires me to express the planes I use for fitting in terms of their surface normals and distance form the origin. It is a little cumbersome.
The Pliocene plateau sat atop the Sierran slab, and when the slab tilted up, the plateau tilted with it. There appears to have been about 4000' or 5000' of uplift at the Sierra crest near Donner Pass. It occurred over a few millions of years, and is still occurring. We shall have earthquakes which will thrown down many or all our old brick and stone buildings, in towns like Dutch Flat, Colfax, Grass Valley, and Nevada City.
So, the whole slab is tilted up like a trap door, sloping gently down to the southwest, and a system of "consequent" streams developed on the plateau; geomorphologists might say that the streams (the Feather, Yuba, Bear, American, etc.) are "consequent upon" the gently sloping surface. In other words, they incised their courses directly downhill, to the southwest.
Then came the Pleistocene, less than three million years ago, and glaciation seems to have spurred rapid incision of the canyons, so that they cut through the andesitic mudflow, cut through the rhyolite ash beneath it, cut through whatever river gravels etc. which may have been underneath that rhyolite ash, and then hit the bedrock, and just kept right on cutting, down and down and down.
So that, for instance, right here at the northeast end of Moody Ridge, the North Fork canyon began by cutting through 250 feet of mudflow (4200' to 3950'), then 50 feet of rhyolite ash (3950' to 3900'), then 2,100 feet of the Mesozoic serpentine of the Melones Fault Zone (3900' to 1800').
And all this happened within, let us imagine, the last four million years.
So with simple arithmetic we can express the *average* rate of incision, in, say, inches per thousand years, or small fractions of an inch, per year (nowadays geologists like to express incision in terms of millimeters/year; I like inches per thousand years, myself). We say, the canyon has incised itself 2,400 feet in 4 million years, and find the average rate to have been, then, 7.2 inches per thousand years, or .72 inches in a hundred years.
This is quite a rapid rate of incision. My own instinct is that it is slower than that, right now, and that it accelerates during glacial maxima, of which maxima there have been many, over the past few million years.
Now, the sources of the andesitic mudflows which had built up the plateau, over millions of years time, were volcanos near, or on, the present Sierra crest; so it is natural, and correct, to surmise that these mudflows are thicker near the crest, and that they thin as one gets farther and farther away from the crest.
A geologic map of this whole part of the Sierra confirms that surmise; if one focuses upon the mudflows, which are typically given some kind of orange or tan color on the map, one sees as it were the many accordant ridges, with the dendritic branches of the canyons dividing one patch of mudflow, one ridge, from the next; and the patches are larger and much concentrated in the middle and upper elevations, but become scarcer and smaller down lower in the foothills.
Nevertheless, to this day one can find andesitic mudflows which reached the Sacramento Valley itself. One is exposed along Sierra College Boulevard, near Rocklin, south of I-80.
All the above is prologue, then, to a simple question: given that the northeast end of Moody Ridge is capped by 250 feet of andesitic mudflows, how many different mudflows can be identified within this section?
At higher elevations, where recent glaciation has exposed huge expanses of mudflow, one sometimes obtains quite a good look at multiple different layers of andesitic mudflow. These would probably obtain the status of distinct named "formations," if a precise geologic map were made.
Since I aim to make a precise map here, it behooved me to go out hiking and walking and thrashing through the brush and try to establish exactly how many different mudflows exist, right here.
I was surprised to find only two, or possibly four, different mudflows within this 250-foot section.
From the summit of the ridge, at about 4200', down to about 4100', there is one mudflow, characterized by very rotten boulders of light brown and yellowish and tan andesite, embedded in a matrix of very light-colored, grey and tan andesitic (?) ash.
This stratum, of about one hundred feet in thickness, is well-exposed in a certain roadcut. The rotten boulders were cut in a perfect plane; it is quite amazing and even pretty, in its way, to see all these thousands of spheroidal andesite boulders, in cross-section. I hesitate to even call this stuff andesite, for it is so deeply weathered and rotten now, despite being the highest and youngest mudflow locally, that it simply cannot be the same rock as the very sound and solid boulders of the lower, older strata.
As one drops lower in the upper, rotten mudflow, there begins to be an admixture of real, sound, andesite boulders. And as one nears the base of the stratum, there is a discrete layer containing many large andesite boulders, perfectly sound, and in freshly-broken chunks this rock is seen to be dark grey to black, with small white flecks of feldspar, such as one should see in a proper andesite. These boulders range up to four feet in diameter. There are occasional very small fluvial deposits, or so they seem, within this Stratum of Big Boulders. The Stratum of Big Boulders is only about ten feet thick. It seems to grade directly into the Stratum of Rotten Mudflow above it, yet I am tempted to separate the two into distinct formations.
Immediately below the Stratum of Big Boulders, and extending from about 4100' down to 4000', is the Stratum of Cement. This is a classic andesitic mudflow, identical in appearance to many exposures in the higher elevations, such as on the very summit of Castle Peak, near Donner Pass. Boulders and cobbles of sound andesite, up to a couple feet in diameter but commonly less than one foot in diameter, and sometimes well-rounded, but often quite angular and only slightly rounded off, and all these boulders and cobbles are embedded in a light grey matrix of andesitic ash.
In this Stratum of Cement (for this mudflow looks much like a rough concrete) one can find the remains of branches and limbs of trees, and fragments of leaves and conifer needles. Usually the wood rotted away to nearly nothing and left a hollow, a perfect mold, in the mudflow; but sometimes one can find the punky remains of wood millions of years old, within these hollows.
I speak of the Cement Stratum here on the northeast end of Moody Ridge; elsewhere, and more typically, the same formation, or its close relatives, will not display any fossil organic material whatsoever.
On the other hand, near Donner Summit I myself found a perfectly-preserved impression of a Madrone leaf, in just such a mudflow stratum, a stratum which contained a thin layer of finer sediments; and in these fine sediments are many leaf fossils, mainly, though, fir needles, by their appearance. The Madrone leaf was quite a lucky find. Geologist David Lawler saw that the fossil found a proper home, with the paleontology people at U.C. Berkeley.
Back to the northeast corner of Moody Ridge: below the ~100-foot thick Cement Stratum is a very poorly-exposed stratum which for all the world looks like something intermediate between the out-and-out andesitic mudflow above, and the out-and-out rhyolite ash strata, below: it is a light-grey stratum of volcanic ash, perhaps andesitic ash, perhaps grading towards rhyolite in chemical composition, and embedded in this ash stratum are just a few cobbles and small boulders of sound andesite. The stratum appears to be about twenty or thirty feet thick, maybe a little more.
So, naming this the Intermediate Stratum, I have tentatively identified four distinct mudflows or "formations" in the global andesitic mudflow "cap" at the northeast end of Moody Ridge. Listing them in descending order, youngest first and oldest last, we have, then,
1. Stratum of Rotten Mudflow.
2. Stratum of Big Boulders.
3. Stratum of Cement.
4. Stratum of Intermediate Composition.
rhyolite ash, fluvial deposits, several to many unconformities
I write "unconformity" wherever there has clearly been a break in the deposition, that is, many thousands of years, perhaps even a million years or several millions of years, could fit into any one of these unconformities. During those time periods the uppermost stratum was an erosion surface.
Between the Rotten Stratum and the Big Boulders Stratum I am not sure if an unconfomity exists. It could well be that the Stratum of Big Boulders is derived from, say, a few million years of erosion of the underlying Stratum of Cement. This erosion freed big boulders from their matrix as the fines washed away, and thus slowly concentrated these very slowly-eroding boulders at the surface. Then, as I imagine it, the Rotten Mudflow roared over the erosion surface of the Cement Stratum, and very likely swept up and entrained into its own messy mass many of these boulders. Being dense and heavy, they tended to remain at the bottom of the flow. Hence they remained concentrated, and hence the Stratum of Big Boulders.
Beneath the andesitic mudflows are various strata of rhyolite ash, in this area interbedded with volcano-fluvial deposits, since the Eocene-age Nary Red Channel is quite close by on the northeast. This ancient prevolcanic river channel, akin to those in Dutch Flat and Gold Run, was apparently never entirely blocked up by the rhyolite ash eruptions of the Oligocene and early Miocene, so the river continued to flow, but the erosion surfaces upstream were fairly well covered in volcanics, so most of the cobbles and sediments in the upper strata of the channel are of rhyolite.
However, the andesitic mudflows subsequently filled up the Nary Red Channel valley to overflowing, and that was that.
Well, I haven't been writing much lately, but I have been at least thinking about the North Fork, and its geology, and its trails, and so on. I have studied the geology of the Sierra for about forty years now. So far as my understanding of purely local geology goes, I find myself collating results as it were, re-examining my own observations and my readings and drawing new conclusions, of which I hope to wrote more in the future. For instance, I am about certain that in older glaciations, such as the Tahoe I and Tahoe II, of 130,000 and 65,000 years ago, ice came down Canyon Creek and broke through the divide just northeast of Moody Ridge, directly over the axis of the ancient Nary Red Channel (no coincidence, the dividing ridge was weaker there), and hung down the canyon wall at least a little ways towards Green Valley, all the while quarrying out the gigantic chunks of rhyolite ash which one finds, today, away down on the floor of Green Valley itself.
This patch of rhyolite mega-boulders is drastically out of place and will definitely appear on my precise map of Green Valley. It is one of the more interesting formations down there.