The Rarest Erratics

[written December 6, 2006]

The rarest glacial erratics are to the west, away from the high country, away from any obvious evidence of glaciation. They are old; but how old?

From about 5,500 feet in elevation and up, one sees abundant signs of the most recent, "Tioga" glaciation, which ended a scant 12,000 years ago. But the Tioga is only one of many. It is thought that, over the past 800,000 years, glacial climates have dominated, with warm "interglacial" climatic episodes (like today's climate) comprising as little as 10% of the total. And it has long been understood that the most recent glaciation of magnitude, the Tioga, was preceded by earlier, more intense glaciations.

By 1975 I was much on the alert for signs of these older glaciations. Such signs are not too easy to find, here on the west slope of the Sierra, where very abundant rainfall and snowfall tend to blur moraines into formless glacial till, and eventually, strip away that till altogether.

When glaciation takes place, there is a "firn line," usually imagined to be a contour line of fixed elevation. Above this elevation, snow accumulates into glacial ice; below this elevation, it may snow and snow aplenty, but it melts away faster than it accumulates, over the course of years.

So there is a zone of ice accumulation above the firn line, and a zone of ice ablation, below the firn line.

Of course, valley glaciers flow down below the firn line; so, for instance, we might reasonably place the Tioga firn line at 5,500 feet, but also reasonably expect to find that the valley glaciers, which formed "distributaries" for the ice constantly building above the firn line, to extend to significantly lower elevations. For instance, Tioga ice may have reached down the South Yuba to near the town of Washington, and down the North Fork American to near Humbug canyon. These locations are down near 2,500' in elevation.

One subtlety about the firn line is that it will run higher on slopes with southern exposures, and lower on slopes with northern exposures. Hence if it is 5,500' on southern exposures, it may be at only 5,000 feet on northern exposures. Now, our coniferous trees can be very sensitive indicators of climate, and microclimate.

Take the White Fir, Abies concolor. Its principal range or locus of occurrence is between 5'000 and 6'000 feet, where it often grows in nearly pure stands. Hence, it occurred to me away back when, the main population of White Fir could be used as a proxy for the lower, western extent of Tioga ice.

And this works out rather well. These White Firs often grow directly on Tioga-age glacial till.

But the White Fir is very sensitive to climate. On warm slopes with southern exposures one may not find any White Fir until one reaches 6,000', but on cold slopes, or especially, down in shady canyons or ravines, one can find the White Fir down to 4,000', and occasionally, still lower.

Blurring one's focus, then, one could take this approach: we lack direct evidence, let us say, of glaciation, at such-and-such a place; there is no till, no moraine, no exposed bedrock, and no glacial striae. We know, let us say, that the *main populations* of White Fir roughly coincide with the Tioga firn line, and we know that prior glaciations were more extensive. But those prior glaciations would have been subject to the usual microclimatic variations in their firn lines, higher on warmer slopes, lower on colder slopes and in shady canyons.

Hence one could take the tack of letting the White Fir once again stand as a proxy for these older firn lines, not in its principal locus of population, but in its western outliers. These more western, straggling populations of White Fir could help mark the extent of the earlier glaciations.

In "Mudflows, Incorporated" I described the sequence of andesitic lahars capping Moody Ridge. The top of the ridge, the top of these mudflows, is just above the 4,160-foot contour. A goodly number of White Firs grow up there, where the gentle slopes of the uplands trap cold air at night. On the adjacent south-facing slopes, however, dropping away into the North Fork, cold air flows away freely, and the White Fir disappears; whereas on the also-adjacent north-facing slopes, the White Fir remains an important component of the forest.

I made these observations in August of 1975, at which time I also found one glacial erratic, only a little below the summit of Moody Ridge, on the northern slopes, facing the freeway, I-80, and Canyon Creek. The erratic was a lone boulder of granite, six feet long, the one and only boulder of granite among ten thousand boulders of andesite.

The bedrock in this area is all serpentine and metamorphic rock of various stripes; whereas the nearest body of granite is ten miles away to the northeast. Hence to see granite is to see something quite out-of-place. There is very little to no granite involved in the ancient Eocene river channels in this area, either (for, the subtropical climate which prevailed then seems to have acted to rot whatever granite boulders there may have been in the sediments, into sand and clay).

Now, I am a cautious man. I could not discount the possibility that one of the andesitic mudflows, which predated the glaciations of the Pleistocene, had itself swept up this granite boulder, and carried it down here to Moody Ridge.

Were I to accept the boulder at face value, as a glacial erratic, I would have to accept that a valley glacier came down Canyon Creek itself, and that this valley glacier was 500 feet thick or more.

I hoped to find other erratics, and bodies of till, but I did not.

It happens that the upland surface of Moody Ridge resumes to the northeast, on nearby Casa Loma Ridge, less than a mile away. Between the two is a gap or pass, which is directly above the Eocene-age "Nary Red" channel. There is, then, a thicker-than-usual section of the "Young Volcanics" of the Superjacent Series here, filling the old Nary Red valley.

It would make sense that a valley glacier, flowing down Canyon Creek, might have helped create this gap, or pass, in the ridge dividing Canyon Creek from the North Fork canyon.

But I am a cautious man. I had noted, in 1975, that in the Gold Run area, these same Young Volcanics had been stripped away, exposing the Eocene river channel of the Tertiary Yuba, below. What erosive mechanism could account for the disappearance of two or three hundred feet of rhyolite ash beds and andesitic lahars? Could my putative pre-Tioga Canyon Creek glacier have been at work in Gold Run?

I never liked *that* idea, but, being a cautious man, I have still not ruled it out. My own instinct was then, and remains now, that the weakness of the volcanic layers, combined with the beyond-normal quantity of groundwater within the broad confines of the Tertiary river valley, had somehow acted to remove the volcanics.

I imagined the Tertiary channel to contain beyond-normal groundwater partly because the underlying bedrock of the Subjacent Series would have itself tended to feed groundwater towards the center of the ancient valley.

But if I apply this rather diffuse model to the Nary Red channel, between Moody Ridge and Casa Loma Ridge, then why invoke glaciers at all? The mysterious pass could be the "same old story" of already-weak volcanic strata further weakened by an excess of groundwater.

And one lone granite boulder could not be enough to demonstrate a robust Canyon Creek valley glacier extending west to, say, between the Alta and Dutch Flat exits on I-80. No, one strange boulder from a strange land is not nearly enough.

There were various other clues, though, which suggested that such a glacier had existed: on the northeast end of Moody Ridge, cliffs of the "cement stratum" of andesitic mudflow are exposed. This a rarity, for, despite the seeming toughness of the Cement Stratum, it is very rarely directly exposed, below and west of the Tioga ice. Higher and to the east this same stratum, or its close analogues, are wildly well-exposed. But here, near the 4,000-foot contour, soil-forming processes outpace erosion, and keep the cement stratum well-hidden. Only the very steepest slopes, subject to mass wasting in the form of minor landslides and slumps, exhibit real outcrops, and these outcrops are typically scattered and small, and do not convince one that one is even seeing the Cement Stratum (the outcrops can easily look like mere boulders, not the massive and undissected lahar itself).

This tendency of the Young Volcanics, even in their most resistant strata, to hide themselves beneath a mantle of soil, is frustrating. Roadcuts are often the only way one can assure oneself just what is what, down here, below and west of the Tioga ice. In particular, the old strata of rhyolite ash, beneath the andesitic lahars, are almost never directly exposed. The rhyolite ash is not only covered by deep soils, but boulders of andesite have rolled down from the lahars above, so all one sees are these andesitic boulders, not the rhyolite itself.

Hence it seemed potentially significant that actual cliffs of the Cement Stratum are exposed on the northeast prow of Moody Ridge, facing directly into the gap or pass above the Eocene channel. I also noted, in 1975, that minor outcrops of the underlying rhyolite ash exist, also facing northeast across this same Eocene channel.

In 1976, while exploring Green Valley, I found huge boulders of this same stratum of rhyolite ash (it is the closest of all the local strata of rhyolite ash, to being a bona fide welded tuff), well above the river, concentrated just where a ravine leaves the steep slopes of serpentine, above, for the moderate slopes of glacial outwash, below. This ravine heads up in this same Eocene-age Nary Red Channel.

But what mechanism could have brought these huge boulders over a mile from their source, into Green Valley?

To me it seemed extremely likely that the rare cliffs of the Cement Stratum, the rare outcrop of the Welded Tuff, and the very very peculiar "region of giant tuff boulders" in Green Valley, all pointed to glacial ice flowing down Canyon Creek, and breaking out of the creek's own proper valley to flow south into the North Fork canyon. It is not impossible that the ice made it all the way down to the river, but if the Canyon Creek glacier were of *that* magnitude, why, the North Fork glacier itself could have reached down to Green Valley.

But I am a cautious man. If there was a Canyon Creek glacier big enough to rip through the Nary Red pass, in effect, creating the pass, and big enough to bulldoze huge blocks of rhyolite ash, fifteen feet through, down into Green Valley, then where are the moraines, and if no moraines are left, from the long blurring of erosion, where is the till? I could find none.

That I could find no till, and I still can't find any to this day, does not mean it is not here. It could be quite well-disguised. For instance: almost all of Canyon Creek to the east and upstream, up-ice, down which my putative glacier would have flowed, is incised into the Young Volcanics. Now, it is reasonable to expect that this ice was flowing from points still farther east, where there is granite exposed. So, if there is till from this glacier, there could be, and maybe should be, granite boulders in that till.

On the other hand, suppose very very few granite boulders were along for the ride, in the Canyon Creek glacier; the glacier would in any case have been quarrying the andesitic lahars, and whatever tills are left, will be made of andesitic material, and, most problematically, these andesitic tills will be resting, quite often, directly upon andesitic lahars, which in turn are subject to deep soil formation, so that one sees a smattering of andesitic boulders embedded in soil, and from experience one deduces that under that soil is the lahar itself.

But one could deduce wrongly; that same smattering of andesitic boulders embedded in soil could quite easily be a glacial till, perhaps only a few feet thick, mantling the andesitic lahar underneath.

Yesterday I went in search of that granite boulder I saw back in 1975. I also wished to use GPS to identify the elevation of the top of the Cement Stratum.

I did not find the boulder, but the steep slope to which it clung was logged in 1977 and 1978, and bulldozed skid trails criss-cross the steep slopes. Countless tons of topsoil have been displaced due to these 1977-78 skid trails. Apparently we Americans are so rich in soil, we can just throw it away.

I walked to the top of the northeast-facing cliffs of the Cement Stratum, and GPS showed the elevation to be about 4010'. Above the Cement Stratum is my Stratum of Big Boulders, and I followed up the spine of the ridge into this higher layer. This Stratum of Big Boulders seems to be, really, just the basal part of the Stratum of Rotten Mudflow, which forms the uppermost lahar in the andesitic sequence, in this immediate area.

I had walked this bouldery spine many times since 1975, but yesterday I was very pleased to find something new: a small granite boulder, perhaps two to three feet in diameter, visibly weathered and old-looking, compared to the Tioga-age granite erratics one often sees farther east and higher in elevation, as for instance near Emigrant Gap.

So. Granite Boulder #2. Of course, there is still nothing to show that it was not brought down with a lahar, that it was a glacier what done it, but I have never yet seen a granite boulder embedded in a lahar, here at Moody Ridge. They do exist elsewhere, for instance, a little west of Blue Canyon, along the railroad, are some granite boulders embedded in a lahar; and although rare, I have seen a few here and there, in the higher elevations.

I was excited to find Boulder #2. I dropped down the north side, off the bouldery spine of the ridge, and followed skid trails down to Moody Ridge Road. I examined hundreds of boulders torn up by the logging, all andesite. And then I found Granite Boulder #3.

It looked very much as I recalled Granite Boulder #1, which I last saw in 1976 or 1977, being about six or eight feet long, and four or five feet thick. However, it was not in the same place, and was pretty clearly in its natural spot, undisturbed by skid trails. It was so well covered in moss I could not be sure it was granite, but, kicking away the moss, I was able to break off a dirty flake a foot across, and then break that flake, and I saw that, yes, it is granite.

Having observed three granite boulders in the near vicinity of the Nary Red pass, I am not so cautious, now, and I declare that:

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Once, if not many times, a glacier flowed down Canyon Creek, and once or several times this glacier was at least 500 feet thick, or, more pertinently, its top was at least as high as 4100' in elevation, near Moody Ridge. This glacier (or glaciers) broke through the ridge dividing Canyon Creek from the North Fork, creating the Nary Red Pass, and bulldozing huge blocks of rhyolite tuff into Green Valley. This glacier also broke out of Canyon Creek to the north, deepening the pass or gap near Lake Alta, and leaving the rare outcrops of rhyolite tuff exposed, near that lake. From the degree of weathering of the granite boulders, and the degree of weathering of the rhyolite boulders in Green Valley, and the degree of weathering of the rhyolite outcrops near Lake Alta, and the degree of weathering of the rhyolite outcrops flanking the Nary Red Pass on Moody Ridge, all of which degrees suggesting age but not "great age," I attribute the most recent Canyon Creek glacier to the Tahoe II glaciation, of approximately 65,000 years ago.
*******
If not Tahoe II, then to Tahoe I, ~130,000 years ago, do I turn. It is quite possible that several distinct glaciers broke through these passes, for instance, the Sherwin Glaciation of about 800,000 years ago, or the McGee Creek of ~1.3 million years ago; but I cannot accept that either of these older glaciations could have left these fairly sound and only slightly weathered granite boulders.

Why, one might ask, was this Canyon Creek glacier so eager to break out of the confines of Canyon Creek? I answer, because the valley of Canyon Creek narrows to a gorge between the Alta and Dutch Flat exits on I-80. This would have inhibited passage of the ice, which would have backed up upon itself, deepening until it was able to break through the two divides.

Actually, both passes, the one near Moody Ridge and the one near Lake Alta, are linked to the same Eocene-age (and also intervolcanic), Nary Red Channel. The southern pass connects Canyon Creek to the North Fork American, the northern, "Lake Alta" pass connects Canyon Creek to the Bear River.

After discovering Granite Boulder #3 I walked north to Casa Loma Ridge, and climbed it, searching for more granite erratics, but finding none. I used my GPS to locate the top of the Cement Stratum at about 4020', or about ten feet higher than at the northeast corner of Moody Ridge.

Although I have walked around Lake Alta and explored nearby ridges and swales several times over the years, I have never seen a granite boulder there. If my hypothesis about a Canyon Creek glacier breaking through both passes is correct, there ought to be at least one or two such granite boulders kicking around over that way.

I guess it's time for a fresh look.

OK, that's all for now folks, except, I will provide a limited bibliography, of works I have consulted over the years, below. For some of the best and most recent discussions of landscape evolution here in the Sierra, see the papers by Greg Stock and John Wakabayashi. My friend L.A. James has also made very significant contributions, although I disagree with his interpretation of the Tioga glaciation, among other things.




Bibliography:

Bateman, P.C. and Wahrhaftig, C., 1966. Geology of the Sierra Nevada. In: Geology of Northern California, Calif. Div. Mines and Geol., Bull. 190, pp.107-172.
Birkeland, P.W., 1964. Pleistocene glaciation of the northern Sierra Nevada, north of Lake Tahoe, California. J. Geol., 72: 810-825.
Blackwelder, E., 1931. Pleistocene glaciation in the Sierra Nevada and the Basin Ranges. Geol. Soc. Am. Bull., 42: 865-922.
Brewer, W.H., 1966. Up and down California in 1860-1864. Reprinted in: F.P. Farquhar (Editor), 3rd Ed. Univ. Calif. Press, Berkeley.
James, L.A., Harbor, J., Fabel, D., Dahms, D. and Elmore, D., 2002. Late Pleistocene Glaciations in the Northwestern Sierra Nevada, California. Quat Res., 57(3): .
Lindgren, W., 1897. Description of the gold belt: description of the Truckee Quadrange, California. U.S. Geol. Surv. Geol. Atlas, Folio 39; 1:125,000; 8 pp.
Lindgren, W., 1900. Description of the Colfax Quadrangle, California. U.S. Geol. Surv., Geologic Atlas, Folio 66; 1:125,000; 10 pp.
Lindgren, W., 1911. Tertiary Gravels of the Sierra Nevada of California. U.S. Geol. Surv. Prof. Pap. 73, 226 pp.
Matthes, F., 1930. Geologic History of Yosemite Valley. U.S. Geol. Surv. Prof. Pap. 160, Wash, D.C., 137 pp.
Muir, J., 1873a. Discovery of glaciers in Sierra Nevada. Am. Jour. Sci., 3rd series, 5: 69-71.
Muir, J., 1873b. Explorations in the great Tuolumne Cañon, Overland Monthly. Reprinted in: A. Gilliam (Editor), Voices for the Earth: A Treasury of the Sierra Club Bulletin. Sierra Club Books, San Francisco, CA.
Russell, I.C., 1889. Quaternary History of Mono Valley, California. U.S. Geol. Surv. 8th Ann. Rpt., Pt.1, Wash., D.C., pp. 261-394.
Stock GM, Anderson RS, Finkel RC. 2004. Pace of landscape evolution in the Sierra Nevada, California, revealed by cosmogenic dating of cave sediments. Geology 32: 193-196.
Wakabayashi J., Sawyer TL. 2001. Stream incision, tectonics, uplift, and evof the Sierra Nevada, California. Journal of Geology 109: 539-562.
Whitney, J.D., 1865. Geology of the Sierra Nevada. Geologic Survey of California, Geology, Vol.1, Calif. Legislature, CA, 498 pp.
Yeend, W.E., 1974. Gold-bearing Gravel of the Ancestral Yuba River, Sierra Nevada, California. U.S. Geol. Surv. Prof. Paper 772, Wash., D.C., 44 pp.