[written July 27, 2007]
Early one morning I met Ron Gould and Catherine O'Riley for an expedition to Terra Incognita.
Terra Incognita: oh the magic, ah the mystery, of Terra Incognita! Or, as some would have it, the North Fork of the American River, between Mumford Bar on the east, and Italian Bar, on the west.
To reach the Mumford Bar Trail, we must somehow cross the North Fork, hence we traced infinite curves past Iowa Hill to Sugar Pine Reservoir, thence to Foresthill Road, and on up the Divide, to the trail.
Much of the Divide burned in the 1960 Volcano Fire, but here, one pretty little patch of timber escaped alive, and in this island grove nestles the trailhead, and a small campground. Around ten in the morning, we set off down the Mumford Bar Trail, leaving the Divide at 5400', aiming for the river, very near 2600', a descent of nearly 2800'. This is accomplished over the course of four miles and forty switchbacks, for the Mumford holds a gentle grade, and takes its sweet time about ever actually reaching the river itself.
It is really quite a nice trail, the Mumford. It is almost entirely a forest trail, but at one point, high on the trail, a view opens northeast, slanting across the east-west trending North Fork Canyon, to Snow Mountain, Devils Peak, and Big Valley Bluff. More typically, one walks through a classic fire-evolved, fire-adapted, Ponderosa Pine-Kelloggs Black Oak woodland, today being overwhelmed by shade-tolerant Douglas Fir. Ah, the Douglas Fir is on the very point of winning the battle: thousands upon thousands are reaching a height of fifty feet or more, and are shading out the Kelloggs Black Oak. Ghostly white dead oak trunks thread the forest: already half the oaks, in many areas, have been shaded to death.
There are some huge trees, the genuine first-growth timber, along the trail. Often these centuries-old trees show big fire scars on their uphill sides, near the base of the trunk. These betoken the almost steady state of frequent wildfires which governed vegetational patterns here for thousands of years. These "cool" fires left larger trees alive, but killed small conifers and shrubs. Hence the forest was much more open, and relatively clear of brush and small trees.
It is considered likely that the native Californians, the Indians, burned off the land regularly. For thousands of years, wildfires were a commonplace.
Then, in an almost unintended experiment, we said, as it were, "Let's take these forests, swept by fires every twenty years, for lo these last ten thousand years--let us take these forests and suppress all fires, and, in the meantime, let's cut down the biggest trees, so that the increased sunlight reaching the ground helps a million conifers take seed, and sprouts a hundred million shrubs."
And now the fire, when it does occur, will burn all the more intensely, likely killing even the largest oldest trees, trees which survived the cool wildfires of centuries past, but now die in the infernos of the modern forest.
Tahoe National Forest has done some interesting work on the Foresthill Divide, using fire to help thin pine plantations. I think we will have to devise more ways to bring fire back to our mountain landscapes. If we could burn a few tens of thousands of acres per year, we could gradually restore fuel loading to the lower, more natural levels of aeons gone by.
Hmmm. The Mumford Bar Trail. It winds through the woods, passing a few exposures of glacially-polished bedrock, but much more typically, glacial till covers everything, and as one at last nears the river, the trail bears east and drops onto the top of a glacial outwash terrace. This is Mumford Bar. All the "bars" of the 49ers are either glacial outwash terraces outright, or the reworked sediments of such terraces, as in gravel bars low to the river, which are swept regularly by high flows.
The last, Tioga glaciation ended a scant 12,000 years ago. As the North Fork glacier retreated up the canyon, it dropped a huge volume of bouldery sediments into the river, which volume exceeded the carrying capacity of those raging, glacier-melt-fed waters; hence a kind of narrow floodplain developed within the canyon. Once the glacier upstream had melted entirely away, say, 12,000 years ago, the sediment load quickly diminished, and the river, itself diminished, began cutting a channel into this long, sinuous floodplain of glacial outwash sediments.
Fast forward twelve thousand years, and we have only vestiges of the narrow, canyon-bound floodplain left, in the form of outwash terraces. Often these terraces stand sixty or a hundred feet above the river. Rarely, higher, older, even more fragmentary terraces exist, as well. At Mumford Bar, the terrace is well-formed and flanks the river for nearly a mile. Often these terraces support rich forests with large trees, and are full of springs.
So, as one descends the Mumford Bar Trail, one eventually reaches the principal outwash terrace, with its rich forest of larger-than-usual trees, and its many springs, and a sometimes spectacular understory of dogwood and maple. An old cabin of logs hewn square stands in a meadowy opening on the terrace. The North Fork ripples gently through a bouldery bed sixty or a hundred feet below.
Here the American River Trail leads away east, up the canyon, following a long series of such outwash terraces up to Sailor Canyon; although, in point of fact, this being one of the secrets I have divulged to you, my readers, the old trail continues right along past Sailor Canyon, following the good old glacial outwash terraces just as before, sixty or a hundred feet above the North Fork. One can even find and follow this same trail past Wildcat Canyon, where the Sea Of Talus is met: the old trail crosses above the rocky ocean.
Still another trail connects here, the Government Springs Trail, which drops to Mumford Bar from the crest of Sawtooth Ridge.
Ron and Catherine and I took a lunch break at the base of the trail, where we met many mosquitos.
Before I leave the subject of outwash terraces, however, I wish to set forth a certain model I have developed on glaciation in a canyon environment, in a canyon, let us say, much like that of the North Fork American; in fact, I will apply the model to this part of the North Fork canyon.
In the first place, glaciers are powerful agents of erosion, and while scouring away at the mountains, they carry bits and pieces along for the ride. They act as conveyor belts, gradually moving sediments down, including huge boulders, including sand and silt. A valley glacier, like the one which filled the North Fork canyon at Mumford Bar, will often, from above, show long lines of dirty, bouldery sediment on its surface. What is less easily seen is the very large amount of sediment hidden within the ice, or being dragged along below the ice.
So when we think back to the North Fork glacier, whatever boulders may have been up on top of the ice, we should imagine that at the base of the ice there are boulders, too, and many of them, but also, all kinds of fine sediments.
We should also imagine that rivers of meltwater flow both through the ice river, and beneath it, especially, of course, below, or down-ice, from the "firn line," above which ice accumulates, below which, it melts away. Moreover, we should imagine that although the principal river will be the deepest in the ice, along the floor of the valley or canyon the ice inhabits, other rivers may form and disappear, most often, along the margins of the ice; but these will find a way into the depths eventually, and join the main river of meltwater, at the very base.
Typically valley glaciers extend well below the firn line of the their tributary glaciers and ice fields. We might imagine the firn line, during the Tioga, at about the 5500' contour, higher on south-facing slopes, lower on north-facing slopes.
Near Mumford Bar the North Fork canyon is incised into the Shoo Fly Complex of early Paleozoic metasediments. The original horizontal beds of shale and sandstone have been metamorphosed and tipped up on edge, rotated almost ninety degrees, so that now the tops of the beds face east.
The sandstone has often metamorphosed into tough and massive quartzite, the shale, into relatively weak slate. It happens, then, that there is often an alternating sequence of stronger quartzite and weaker slate.
These tipped-up, east-facing beds of quartzite and slate are at right angles, often, to the line of the canyon. If the canyon were never glaciated, it would develop a strongly ribbed form, with side canyons incised into the softer slates, and spur ridges on the stronger quartzite.
But the North Fork has been glaciated, and the great mass of ice flowing down the North Fork tended to plane the quartzite ridges down. That is, we should imagine that, between glaciations, the "ribbed" pattern would begin to take hold, begin to develop, with ravines in the slate, ridges on the quartzite; and then along would come this big brute of a glacier, and it would plane down all those little nascent spur ridges.
OK. What this is all leading up to has to do with the sediments trapped between the ice and the canyon wall, and between the ice and the bedrock floor of the canyon.
Ice flows, and yet it is a solid. It easily breaks. And while ice at the surface is one sort of ice, ice three thousand feet down in the North Fork Glacier is another sort of ice. At any rate, the model I am struggling to propose is a simple one: as the ice, which does flow, which does bend, but which can only bend so much, before it breaks--as the ice flowed down the North Fork canyon, in the global, down-canyon, direction--west, let us say--it *of course* hit the crests of the spur ridges, tending to plane them down. But what happened in the little ravines between those spur ridges? According to my model, the sediment dragged along by the ice was such that it tended to fill those little ravines.
There is a subtlety here. Of course we expect a generalized mass of till to blanket many slopes, after such a monstrous glacier melts away. But what I propose is that linear masses of this till occupied the little ravines on the canyon wall while the ice was still there. So that, when one came right down to it, the North Fork glacier was flowing partly over bedrock, *and partly over its own till*. For the ice was too brittle to bend in and out of these abrupt little fluctuations in topography; the ice just carried majestically along. "Go West, Young Ice," etc. etc.
So the model is that the North Fork glacier skimmed along the crests of the spur ridges, planing them lower as it went, with the intervening ravines chock-full of that bouldery sediment we call glacial till.
Now, turning to the case of the North Fork itself: it raged along beneath three thousand feet of ice, through twisted caverns of Stygian darkness.
Suppose, now, that a previous glaciation was more intense, and suppose that more intense glaciation to comprise either or both of the two "Tahoe" glaciations, of 65,000 and 125,000 years ago. The previous glaciation deposited even more glacial outwash, as the glacier retreated up the canyon from Mumford Bar. This bouldery outwash is a very effective agent of erosion when added to a raging, perpetually-at-flood-stage river, i.e., the North Fork. So, when at last the glacier was gone, and the river etched down through the narrow floodplain of outwash, to the bedrock buried below, with the infinitude of boulders rolling along in the flood, it was a matter of a few thousand years to nick a gorge into the bedrock.
A gorge, say, one hundred feet deep. Picture a vast canyon, three thousand feet deep, and right at the bottom, the river flows through a narrow gorge, with cliffs on both sides. But the gorge is only a hundred feet, two hundred feet, deep.
Now come forward to the Tioga glaciation. We have proposed that the North Fork Glacier flowed both directly over bedrock, and directly over till. At the base of the glacier the North Fork rages along through its caverns. But is it only water which moves? No. There is a huge sediment load, of boulders, cobbles, sand, silt, even clay, being dragged down through those buried abysses in the ice.
And the ice, not bending too well, not being able to conform to every minor fluctuation in topography, flowing in the global direction, west, would, I propose, skim right over the tops of these little inner gorges, inner gorges incised during the *previous* glaciation. I envision that, the instant the North Fork canyon was re-occupied by (Tioga) ice, perhaps 25,000 years ago, these little inner gorges filled with glacial till, with the bouldery sediment load being rushed along by the ice-bound river of meltwater.
So that when one enters such an inner gorge along the North Fork, say, the one a couple hundred yards downstream from the cabin at Mumford Bar, one should *not* imagine that the North Fork Glacier actually occupied the gorge.
Imagine rather that the gorge was buried in glacial outwash, that the hidden, "glacial" North Fork raged along above the buried gorge, and above the river in turn, there were three thousand feet of ice.
It is quite clear, in some locations, as for instance, Green Valley, that inner gorges along the North Fork were created, then buried in glacial outwash, and finally, exhumed, during Post-Tioga erosion.
I believe we see exactly the same thing near Mumford Bar: inner gorges created 65,000 years ago, say, which were filled with till (or better, sub-ice glacial sediments) as soon as the Tioga glacier re-occupied the canyon, remained buried for the entire time the Tioga glacier was there, and then, when the glacier finally melted, 12,000 years ago, and the sediment load dropped to something like modern (low) levels, these little gorges were exhumed.
They do not seem to have been much deepened in post-Tioga time, in the Holocene.
The idea of a major valley glacier flowing, not over bedrock, but over till, is exemplified by Yosemite Valley, where earlier, more intense glaciations hewed the bedrock nearly a thousand feet deeper than the present floor of the Valley. This was revealed by drilling out sediment cores. For a time, there was a Lake Yosemite; but it silted up full with glacial outwash brought down the Merced River and Tenaya Canyon. When the Tioga ice re-occupied Yosemite, it did not bulldoze this thousand feet of outwash sediments out, but skimmed along over the top. Oh, it carved a bit of a broad trench for itself through the Valley, but it never came close to the bedrock floor. And the broad trench has since been filled in.
Returning to the North Fork, that a long narrow floodplain of glacial outwash sediments choked the canyon downstream from the North Fork Glacier, has long seemed obvious; but near Mumford Bar we are presented with evidence that a kind of outwash "floodplain" may have existed beneath the ice itself, as well.
It is by this same model that I explain the Monuments, at Monumental Creek: they were beneath the ice, buried in glacial outwash. They were created by river erosion, in the churning maelstrom, in the roaring abyss below the ice. Such tall spindly towers of rock would fall immediately if exposed to the inexorable flow of a glacier. No, they were protected from the ice by sub-ice glacial sediments.
Since the tallest Monument is about one hundred feet high, we should probably imagine the sub-ice sediments to have been at least one hundred feet deep, there.
Ron and Catherine and I had all three studied the topographic maps, and had noted that the North Fork canyon, below Mumford Bar, suddenly narrowed, the walls steepened, and we expected to find any number of inner gorges in our Terra Incognita. We expected to find cliffs falling into deep pools; we expected to have to swim through these deep pools, as one does in Giant Gap. We had somewhat more than three miles of river to follow down to Italian Bar; those three miles could become an infinity, and we might not reach Italian Bar before dark. Hence our early start.
We dropped down to the bouldery river bottom at Mumford and almost immediately met our first gorge. Spectacular exposures of water-polished metamorphic rock were all around us. I noted an old iron pin set into the bedrock above our first gorge pool, and now, in retrospect, I believe it to be evidence of when the North Fork was turned right out of its bed, into a wooden flume, a common strategy in the early days of gold mining, in the 1850s. We found a gap in the cliffs which allowed us to continue downstream; I believe this gap is where the flume once led. The actual point of diversion would have been a quarter-mile upstream, near the cabin, say.
It strikes some geologists oddly that glacial outwash sediments could be gold-bearing; but it is not widely realized that the "bars" of the 49ers are exactly such glacial outwash terraces. As Ron and Catherine proceeded slowly down the narrow canyon, often within the still-narrower inner gorges, I saw that almost all vestiges of outwash had been eroded away. However, wherever a patch of outwash clung to the canyon wall, there were signs it had been mined.
The sky was blue, the sun was hot, the water was clear and cool. We entered a gorge which showed every sign of forcing us to swim, but found our way on foot almost entirely through, before a long and deep pool barred further progress. We stopped and repacked our packs, and changed into river shoes of various types, before entering the pool. But, it transpired that we didn't really need to swim. We could merely wade, holding our packs up, and we needn't have bothered with all our special dry-bags and plastic bags.
Briefly, we were in an incredibly beautiful gorge, walled by vertical cliffs, and we could more or less just saunter along, hop from boulder to boulder, do some mild wading, ford the river from one side to another, and at the bottom line, it was easy going.
We found two old miners' camps strewn with garbage. Here and there along this gorgeous gorge, it could be that a miniscule flat above the river, with a spring, and a grove of alders, would make for a nice camp-site. But, once a little below Mumford Bar, though we crossed many a gravel bar, many a bed of sand or fine gravel, we saw not one single human footprint, not one sign that anyone had been along the river this summer.
Bizarre! I'm not complaining, I don't visit the North Fork for the crowds.
We chased ouzels slowly downstream, and wondered about the shy, the mysterious, Purple-Pooping Bird of Paradise, which is only to say that, in many places, the rocks were splashed by ... purple bird poop. There were ripening blackberries in the area.
About a mile, a mile and a half, below Mumford Bar, massive quartz veins laced the cliffs, and some rather large boulders had broken away, exactly along the vein itself. While clambering over these jumbled, quartz-encrusted boulders, we found incredible masses of quartz crystals, some inches in diameter, studding the surfaces.
We did not pause very long to examine these crystals. In one place, a wonderful Crystal Cave was formed by two such boulders leaning together; one could actually enter, and be surrounded by quartz crystals. Clearly, many of the larger, nicer crystals had been broken off in years, or maybe centuries, past. We left them untouched, which is only as we should all do. "Leave No Trace" means, among other things, don't drag arrowheads and quartz crystals and old bits of ornamental iron from the Gold Rush home with you. Leave them there for future generations to ponder and puzzle over.
Take photographs, but leave the crystals alone.
Before we knew it we had made two of our three miles down the river. The sun was still high. So we stopped and swam in this pool, stopped and swam in that pool, we found patches of shade and just lazed around. One of the principal spur ridges dropping from Sawtooth Ridge down to Italian Bar was silhouetted against the western sky. We were close. Finally we continued on our wandering, boulder-hopping, river-fording way, turned around the spur ridge, and saw the dark mouth of a tunnel driven into the slate. We had reached Italian Bar.
It only remained to switch back from river shoes to hiking boots, and then to climb up and up and up and up through the woods, 2700' or thereabouts, to Ron's truck. It seemed to take forever. I had my loppers and many a small Douglas Fir fell to my wrath. However, since there are no fewer than a thousand (or is it five thousand? ten thousand?) small Douglas Fir which should be cut from near the trail, and I only accounted for a hundred, it hardly looks as though I did anything at all. Still, the sweat literally poured into my eyes as I combined the trudge up a viciously steep trail, with heavy lopping. Eventually I had to give it up, and save my energy for the climb alone.
We reached the truck just at sunset, after a long day in the great canyon, after discovering the Crystal Cave, after scaring any number of trout and water ouzels, after swimming the clear cool pools, after jumping and diving from the rocks. We were pretty beat up, sunburned, bruised, scratched, soaked with sweat. It was all more than worth it.