Monday, April 11, 2005


Some of you expressed interest in the geology of this area and the North Fork. Hence I offer this.

A young man named Greg Stock has been investigating the rate of incision of canyons in the Southern Sierra. He has taken quite a remarkable approach: in the deep canyons of the Kings and Kaweah, there is some limestone/marble, and many caves. Some of the caves are now high on cliffs, but once upon a time, when the canyon was shallower, they were at river level, and the river deposited sediments in them. These seds are easy to recognize because, upstream, there is granite: so if you find rounded granite boulders in a marble cave, you know the river left them there (partly, also, because these caves are too far west in the canyons to have been touched by glaciers, ever).

Hence, date the seds, and measure how high the cave is above the present river level, and you have an idea of how long it took for the river to cut the canyon that deep. And that is what Greg Stock has done.

I have been trying to entice Greg Stock up here to see the North Fork's outwash terraces. We have corresponded for a couple years. Here is his most recent to me, combined with my response. But first, let me say that the various glaciations in the Sierra have been named, or rather, those which left the most obvious traces have been named. The most-recent ended ~12,000 years ago; it is called the Tioga. Then we have Tahoe II (~65,000 years) and Tahoe I (~130,000 years) and Sherwin (~750,000 years, call it ~800,000 years). There were others but these are the main ones named so far. They have all left moraines, most easily seen and studied on the dry east side of the Sierra.

Also, when we talk about river channels aggrading, we mean that, rather than cutting deeper ("degrading"), they are filling up with sediments. A valley with an aggrading river actually becomes shallower over time, the ridges lower, the valley floor rises. That's the kind of river we had here in the Eocene, such as is exposed at Gold Run and Dutch Flat. Lazy, meandering over an ever-thickening floodplain of sediments.

Finally, the granite which we see so much of in the Sierra was emplaced at depth over a long period of time, which ended roughly 80 million years ago (80 Ma). That's the magmatism Stock refers to. One of the mysteries of Sierran geology is, how did so much rock get eroded away, to expose these deeply-seated plutons of granite? They were exposed before Eocene times (55 Ma).

Hey Greg you wrote,

Sorry, I wasn't clear about Eocene agrradation: the incision was
Oligocene in age, with dominantly aggradation throughout the Eocene
(though do not simply assume constant aggradation for millions of years
- river systems tend to be more complicated than that). Check
Wakabayashi and Sawyer (2001) for details.

I have looked at portions of Wakabayashi/Sawyer (whatever was available on the internet). Very good stuff! I actually contacted Wakabayashi years ago to beg him to come look at the outwash terraces in the North Fork American. He refused. Thus he joins the ranks of girlie-men geologists too weak to explore the North Fork (heh heh). Strong enough to brew beer and fish for trout, not strong enough for the North Fork American, not strong enough to visit outwash terraces which show every potential for correlation to other glacial deposits, such as moraines and tills!

I mean, is Wakabayashi even serious about Sierran landscape evolution? If I tell him that I have a relict channel which is 400 feet above present river level, and associated to that channel is an outwash terrace all of 600 feet above present river level, doesn't it follow immediately that we have an opportunity to discover exactly how long it took the North Fork to incise that most-recent 400 feet?

My guess: Sherwin-age. ~800,000 years.

I've only been trying to get "real" geologists to take a look at these terraces since 1976. I suppose I should be patient. Heh heh. Heh.

There are volcanic clasts in the auriferous gravels around the
Stanislaus and Mokelumne rivers, also lots of metamorphics, but only
the rare granitic clast as you say. I probably should have said "some"
and not "most" clasts are volcanics. Sounds different in your neck of
the woods. Are there no volcanics in those gravels? This is actually
an interesting question because there were undoubtedly a few km of
volcanic rocks eroded off the range after the end of magmatism ~80 Ma
to expose the granitics, so did this all happen in the
Cretaceous/Paleocene? If no volcanics by the Eocene it would suggest
very high erosion early on.

Hmmm. Stanislaus and Mokelumne should be much the same as, say, Yuba and American.

But you don't seem to intend the word "volcanics" to apply to what I think of as the "young volcanics," Valley Springs/Mehrten/etc., all post-Eocene. There are many post-Eocene channels (and some of the Eocene channels are capped) with volcanic clasts; the so-called "intervolcanic" channels, on the interfluves around here. One such is right where I live: the "Nary Red" channel. These intervolcanic channels often robbed seds from the older Eocene channels and so we get a mix of young volcanics and older quartz, chert, etc. etc.

But so far as rocks eroded after the end of magmatism--that is a fascinating subject. Miles of rock stripped away. It does make sense that some of these would be unmetamorphosed surficial expressions of the magmatism. But I am not aware that such rocks have even the slightest presence in the Eocene channels, unless we were to count the quartz; for quartz veins must have threaded through all these missing rocks, and quartz is especially well-suited to endure the rock-rotting climate of the Eocene.

No, this is a fascinating subject: what became of the miles of rock stripped away before the Eocene? Various lines of evidence suggest that it was miles of rock. So it is natural to imagine high erosion rates. And I believe that the magnitude of the erosion--i.e. miles--can only mean that this "ancestral" Sierra was being uplifted throughout this period, Cretaceous-Paloecene. Then the uplift stopped and the channels aggraded.

But I am not aware that any rocks in the Eocene channels are thought to derive from these vanished miles. Even the quartz might all have entered the channels in the Eocene itself.

Yes, Tahoe is always outside of Tioga, but the question is why.
Several people (my advisor, Bob Anderson for one) would argue that it
might have to do with basin hypsometry changes during each glaciation.
For instance, each successive glaciation bites glacial canyons deeper
into the landscape, lowering their mean altitude. So a Tioga glacier
probably spends more time at lower (warmer) altitudes within a canyon
than a Sherwin glacier did in that same canyon. Follow? Tioga
glaciers did not extend out as far because they melted closer to the
mountain front. That is one way to explain the ever-decreasing extent
of glaciers. Also, on the east side at least it is worth considering
how much the Mono Basin and Owens Valley have dropped (lowered their
altitude) over the past 1 Ma. Just some alternative ideas to the
notion of the Sherwin as the largest glaciation - though that is still
the simplest explanation.

Hmmm. Anderson may have something there, but I myself would wish to somehow discriminate between two models: (1) Anderson's model: canyons were lower in absolute elevation, hence warmer, hence glaciers melted before reaching Tahoe or earlier moraines; and (2) The other model: canyons were deeper in Tioga, hence the same absolute volume of ice would occupy a lower position. Or, simplest, maybe best, (3), Tioga was a lesser maximum than Tahoe, and much less than Sherwin.

But I just can't go along with Anderson: the depths of the canyons 65,000 years ago, Tahoe II, should not have been all that much less than Tioga canyon depths. If we accepted a 6 inches/thousand years rate, then, OK, roughly 26.5 feet lower in Tioga (65,000 minus 12,000 equals 53,000). Now, the "average lapse rate" of temperature with increasing elevation is 3.5 degrees F per 1000 feet (in dry air, higher, in wet air, lower lapse rates). So a difference in canyon floor elevation of 26.5 feet would represent a vanishingly small increase in average temperature.

Applying the same arithmetic to Sherwin, using the round number of 800,000 years, we'd have 400 feet of incision. Hence, say, an average temperature of 3 degrees F warmer now than during Sherwin times, at the bottom of the canyon. Well, OK. That really would be significant. So I kind of like Anderson's model, (1), for the Sherwin.

I have debated similar issues with Allan James, who has been doing cosmogenic dating of moraines in this area (unfortunately, another girlie-man gelogist--heh heh). The question I raised with Allan was, how do temperate-latitude valley glaciers, acting as distributaries to a high-elevation ice-field or ice-cap, respond to increases in depth of the source ice-field? Do the termini of the valley glaciers "strongly respond" or do the termini "weakly respond"? That is, if the ice-field gains "100 feet" in depth, does the valley glacier become "one mile" longer, or "one foot" longer? I guess Anderson would say that valley glaciers would weakly respond, because he would make temperature the dominant variable controlling their down-valley extent.

But Greg, don't I recall that we see the same pattern in the big continental glaciers, say, in the Midwest? That the Tioga equivalent was not as extensive as earlier Wisconsin-age glaciations?

Allan James wants to make the Tioga a kind of wimpy girlie-man glacial maximum around here. Whereas I have always imagined it to be at least similar in extent to most earlier glaciations. For instance, Allan pointed to a certain glacially-scoured lakelet above 7000' elevation near here and declared it was pre-Tioga, which is in direct opposition to my own idea of Tioga glaciation, based upon over 30 years of hiking in this area, combined with thousands of miles of hiking in the High Sierra to the south. And to me it is about ridiculous to say that any (glacially-formed) lake whatsover in the Sierra is Tahoe-age. Lakes are usually ephemera, they silt in and become meadows. To me if it's a Sierran lake in the high country, it's Tioga all the way, not that Tahoe glaciers might not have scoured out an earlier version.

Why did James think this lake was pre-Tioga, that is, Tahoe II? Because a prominent boulder-line left by Tioga ice is a little ways down the mountain from the lake. But I say, fine, the Tioga had a stade and left this line of boulders. That doesn't mean Tioga ice was never higher than the boulder-line.

I can't entirely discount James's girlie-man Tioga model. So, now I'm gun-shy. I see a body of till somewhere which in the old days I would have thought, Tioga-age every step of the way, and now, because of Allan James, I have to wonder: Tahoe-age? Because the way I see it we should *not* really be able to tell "at a glance" that such-and-such a boulder, in till, has been weathering for 12,000 years or for 65,000 years.

There are some patches of old till around here, all rotten and red with iron oxide. These at least can be seen to be older than Tioga, at a glance.

It is a really interesting subject and you, Greg Stock, are taking the bull by the horns and doing the work which will help us finally settle some of these issues.

I saw in your article that you visited ***300*** caves. That is totally amazing!!

Three hundred caves. You know, that is so--great. It is remarkable. The story of that exploration would make an interesting book.

Thanks Greg,

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