Monday, August 6, 2007

Moraines in the East Fork

[written August 6, 2007]

Early Saturday afternoon Gay Wiseman and I drove up to Emigrant Gap and then south on Forest Road 19 towards the North Fork of the North Fork of the American River (NFNFAR).

There is a Tahoe National Forest campground where The Nineteen crosses the NFNFAR, a popular campground, and yet one often sees cars parked along The Nineteen, near the campground.

They park along the road in order to walk downstream to a beautiful waterfall and swimming hole. A close examination of the USGS 7.5 minute quadrangle reveals several of the 40-foot contour lines crossing the river near the falls; but for quite a ways further downstream, the gradient lessens, ergo, no waterfalls, and a cursory exploration twenty years ago or so had confirmed what the map suggested.

However, around a mile downstream, several more 40-foot contours cross the river in fairly close succession; hence, there ought to be more waterfalls, perhaps bigger and better waterfalls, with deeper and more dramatic swimming holes.

One always wants more drama while swimming--there can never really be enough--so Gay and I set out to find these supposed waterfalls, and their exciting pools.

While staring at the map, with its many contour lines, I noticed two strange spur ridges, one on either side of the river, one farther upstream, closer to the campground, the other farther downstream, close to Sailor Point. The area lies within Township 16N, Range 12E, in sections 7 and 18.

What drew my attention to these two ridges was their geometry. Imagine if you will a generalized canyon following a straight course. Let the typical cross-section be a simple "V" in shape. Now imagine the contour lines in such a canyon; they roughly parallel the river; they have a "global" direction which is nearly parallel to the length of the canyon.

Now further imagine that the walls of the V-shaped canyon are scored by ravines, and ribbed by intervening spur ridges, which are more or less at right angles to the river, to the length of the V-shaped canyon. Hence although the "global" direction of the contour lines parallels the river, locally, the contour lines bend in around the ravines, and bend out around the little spur ridges. We have defined the ridges and ravines as perpendicular to the river, and the "global" direction of the contour lines as parallel to the river. If we place a ruler so that it intersects all the little local outward bends in the contour lines, where a spur ridge is crossed, or so that it intersects all the little local inward bends, where a ravine is crossed, our ruler will itself be at right angles to both river and canyon.

However, these two little ridges which caught my eye do not exhibit this geometry. If one places a ruler so that it crosses all the little outward bends in the contour lines, it is far from being at right angles to the river. In fact, in both cases, the ruler would lie at approximately a 45-degree angle to the line of the river, to the trend of the canyon at large.

They are both moraines, portions of terminal moraines from a recessional stage or "stade" in the most-recent, "Tioga" glaciation. I say, recessional, because it is fairly clear to me that Tioga ice extended well down the canyon, miles down the canyon, during its maximum extent. The Tioga is somewhat poorly-defined in time: it is well known to have ended about 12,000 years ago, which is a geologic and climatologic blink of an eye, but its beginning point is harder to specify. We would not be drastically amiss, I think, to set the beginning of the Tioga to "about" 20,000 years ago.

To illustrate why the beginning of the Tioga becomes problematic, research has been conducted in recent years in which the sediments of Owens Lake, on the east side of the Sierra, were cored to a depth of a couple hundred feet, and the cores carefully analyzed. During periods of intense glaciation, one type of sediment reached the lake and was deposited. During interludes between glaciations, a different type of sediment reached the lake and was deposited. The sediment cores are longitudinally striped with these alternating types. All that remains is to attach a date to each stratum. This was done.

The Owens Lake sediment cores revealed no fewer than sixteen separate glaciations within the past 52,000 years. Sixteen!

And yet, it is a commonplace among Sierran geologists to refer to the Tioga glaciation of 12-20,000 years ago as having been preceded by the Tahoe II glaciation of 65,000 years ago. Clearly the glacial history is much more complicated, in detail. It is exciting to think that the story will continue to unfold as research progresses.

There is an important reason why geologists remain "stuck" in the Tahoe-Tioga model of glacial sequence: one the east side of the Sierra, so much drier than here, many terminal moraines are well-preserved, and in canyon after canyon after canyon one can see two principal terminal moraines: an older, more blurred moraine, farther down the canyon, and a younger, sharp-crested moraine, farther up the canyon. Sometimes only a little distance separates them. The younger, sharp-crested moraine is Tioga; the older, blurred moraine, farther down the canyon, is Tahoe.

Direct observation leads to the Tioga-Tahoe model.

Of course, as a glacier melts away it retreats up its canyon, and leaves a series of terminal moraines. Actually, if its retreat is steady and rapid, it may leave a formless mass of glacial till slathered over everything. On the other hand ...

It may well happen that during this retreat up its canyon, a retreat which may require well over a thousand years, the glacier stops retreating for a century or two. During such a "stade" the glacier will deposit a much more strongly marked moraine along its terminus.

My two ridges-of-odd-geometry, in the canyon of the NFNFAR, represent two such stades, the younger one half-a-mile up the canyon from the other. The upper, younger ridge-of-odd-geometry is in the SE 1/4 of Section 7, T16N R12E, near the word "Fork" as seen on the 7.5 minute quadrangle, and is on the southeast side of the NFNFAR. The other ridge-of-odd-geometry is to the west, in the NW 1/4 of Section 18, T16N R12E, near the surveyed elevation of 5137'. This moraine is northwest of the river, and The Nineteen cuts right through its upper end.

Both are really rather minor ridges. On the topographic map, they are expressed as a series of kinks in the contour lines.

It is not nearly as easy, here on the well-watered west slope of the Sierra Nevada, to see moraines. They do exist, but they are often inconspicuous. I have a rather short list of such moraines, in my mind: there is a fine figure of a moraine north of Lake Spaulding, visible from I-80 at a disiance of several miles. There is a long, brushy moraine complex on Black Mountain, also visible from I-80. Red Mountain has a blurred-into-till portion of a lateral moraine of the Fordyce Glacier, high above Fordyce Creek. And there is quite a distinct moraine at the lower end of Bear Valley. And there are others.

So, having noted the two ridges-of-odd-geometry, I was excited to see whether I was right, or wrong, when I actually got out there on The Nineteen and passed Sailor Point, entering the NFNFAR canyon.

Of course I was right.

And the waterfalls? Oh, well, Gay and I had quite a nice time, following the good old Bradley & Gardner ditch, and then dropping to the river and boulder-hopping along, farther and farther and farther down the canyon. We saw many a one-foot waterfall. We saw many a three-foot waterfall. But we found no big waterfalls. Pools, yes; but no "dramatic" pools. Oh, they were very nice I'm sure, and Gay went swimming. But they lacked drama.

It is a very beautiful reach of river and canyon, with pretty stream-polished exposures of the Shoo Fly Complex metasediments, and the giant-leafed Indian Rhubarb all along the water, and many many many giant granite eggs left there by the ice, 12,000 years ago. During our explorations we discovered several old narrow-gauge logging railroad grades which had been pressed into service by Tahoe National Forest as skid trails, twenty-five or thirty years ago. In fact, I was fuming quietly to myself as Gay and I finally climbed back up to the good, the old, the huge Bradley & Gardner, the Placer County Canal. A bulldozer skid trail angled steeply up the slope, and we followed it for a time. Under what possible pretext did Tahoe National Forest allow the canyon wall to be thus scarred for the next thousand years or more?

I can forgive the glacier, in fact, I rather admire its scars.

I can forgive the loggers of the 1890s, who made a few carefully-thought-out railroad grades and rolled logs right down onto flatcars. The loggers of the 1890s lived in an era of rapacity, in which rapacity was so universal that "everyone" did it. But to come along in the 1980s, and make scars on the canyon walls which will easily last a thousand years? If it was Sierra Pacific Industries, whose only view is the bottom line in a ledger, well, that would at least be intelligible.

But since the scarring was on public land, under the management of Tahoe National Forest, I can't shake the feeling the the public trust was violated. It's not a new feeling. When I stop and wonder how it could be that Tahoe National Forest either itself orchestrated the destruction, or stood by and did nothing to avert the destruction, of so many historic trails, in this same area: the Burnett Canyon Trail, the Monumental Creek Trail, the Mears Meadow Trail, the Big Valley Trail, the Sugar Pine Point Trail, the China Trail, et cetera, well, when I recall all that, I am worse than displeased.

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