Thursday, October 29, 2015

A Series of Fortunate Events: Climbing a Dome That's Not Exactly a Dome...It's Rock Mutton

There are domes and there are "domes". Yosemite National Park has a lot of domes of one sort or another, and there has always been a bit of confusion about their origin. From various points in Yosemite Valley, one can spy at least four them, famous Half Dome, less famous Sentinel Dome, North Dome, and Basket Dome. Although Yosemite is famous as a work of glacial action, the domes actually are not directly related to glacial erosion. As described in the previous post on Sentinel, these domes rose above the glacial ice. They formed instead because of exfoliation, the tendency of slabs of granitic rock to break off the corners and edges of monolithic chunks of exposed rock. In time they take their rounded shape.
Image from
In the high alpine parts of Yosemite, the situation is different. Tuolumne Meadows is not a "U"-shaped glacial valley. It has an open aspect, with high mountain ridges separated by wide meadowlands dotted with granite monoliths that rise above the dark forests. The granitic knobs look less like rounded domes, and more like Monstro the Whale breaching at sea, with a gentle slope on one side, and nearly vertical cliffs on the other. Two of the most obvious are Lembert and Pothole Domes. Both rocks are popular hiking destinations, and both provide stunning views of the high country of Yosemite. Pothole Dome was our destination on this trip.

The open country of Tuolumne Meadows resulted from a different kind of glaciation. In Yosemite Valley, the glaciers only occupied the valley itself. At Tuolumne, the glaciers covered the entire landscape apart from the highest peaks. The depth of the ice exceeded 2,000 feet (~700 meters). Such glaciers are called icecaps or icefields, and they produce a different set of erosional features. Pothole and Lembert Domes are examples of Rock Mutton.
Oh, excuse my French! The domes are examples of a roche moutonnée. This has been translated variably as greasy mutton wig rocks, fleecy rock, or rock sheep, and none of these is a particularly apt description. The first was related to the sheep grease that was used to hold the hair in place in the old wigs worn by French and English dignitaries. The fleece referred to the "resemblance" of these rocks to the locks of hair in the aforementioned wigs, and the last was said to be a description of the rocks appearing as grazing sheep from distant vistas. I don't quite get it, but c'est la vie.
Whatever you might want to call it, Pothole Dome provides an intimate understanding of what rock is like when subjected to intense glaciation. The gentle slope, which faced the oncoming ice flows, has been smoothed and abraded by rocks, sand, silt embedded in the ice. The passage of the gritty ice has left grooves, striations, and polished surfaces that make climbing the dome a great deal easier than one might expect. Most of the huffing and puffing comes from the high altitude (8,000 feet/~2,700 meters) rather than steepness.
The steeper western side of Pothole Dome was "plucked". Ice formed in cracks and fissures of the granitic rock and was pulled loose by the glacier as it flowed away from the rock. This made for a decidedly asymmetrical dome structure if one wants to call it a dome at all.

An alternate term for these rocks is a stoss and lee structure. "Stoss" comes from a German term for "push" or "thrust", referring to the gentle slope facing the flow of ice (the glacier "pushes" up the slope). The "lee" refers to the trailing steeper plucked side. I tend to prefer this description, although it still isn't all that descriptive. How about a "scour and pluck" structure?
The other unmistakable evidence of the passage of glaciers is the sheer numbers of boulders that lie scattered across the surface of the dome. There aren't many good explanations for how these boulders could have gotten here without invoking the passage of ice (the boulders came from a different kind of granite exposed off to the east). These out of place rocks are called glacial erratics.
There are marvelous examples of glacial polish and striations all over the surface of Pothole Dome. The ice was last here around 12,000 years ago, so the rough surfaces surrounding the polished sections provide an idea of how slowly the granite is weathered in this cold alpine climate. At lower elevations, the rock would weather much faster and the polish would rapidly disappear.
From the top of the dome, one can see some of the terrain that stuck out above the icecap, including the Cathedral Range (below). The peaks are classed as horns when glaciers flowed away from the mountain, plucking away at the flanks, or nunateks when the glaciers flowed around the peaks, scouring away at the higher slopes.
Pothole Dome's name reflects the presence of potholes on the south flank of the dome. These round holes formed when ice melted into the glacier through holes and tubes called moulins. The water flowing under the ice mixed with boulders and gravel in large swirling masses to grind out the holes.

The final glacial feature we noticed while climbing down were the chattermarks, concentric fractures caused when boulders being dragged at the base of the glacier skipped and chipped the surface. Usually the cusps of the crescents point away from the ice flow direction, so I found these a bit mysterious. The crescents open toward the east, presumably the direction the ice came from.
The day was passing and we had a few stops left on the itinerary. They'll be covered next time!

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