Climbing in California, September 2006

This was a short rock climbing tour of California. First, we went and climbed the popular After Six (5.6) in Yosemite Valley. After that Vian and I went up to Tuolomne Meadows to mess around on Stately Pleasure Dome to practice slab climbing. The two days were spent climbing near Mammoth Lakes and Owens River Gorge.

As you look out across the Gorge from the west rim, exposed on the east wall you see a dark grey zone at the bottom with a pink layer above. These are two flows of volcanic ash and cinders that erupted out of Long Valley, a few miles to the northwest. The two layers are termed 'cooling units' because they were produced by separate eruptions, but geochemical and stratigraphic evidence suggests that they were deposited within a few days to weeks of one another.

All the climbs are on the lower grey layer, because the pink layer is soft and loose. The rock in the Gorge is a welded tuff, welded because heat is what bonded it together; and a tuff, because it's composed of fragments of volcanic rock. It's called the Bishop Tuff, and it's the same formation as the Deadman's bouldering areas. The lower cooling unit was still quite hot when the upper cooling unit buried it, so the additional compression of the overlying upper unit contributed to the dense welded nature of the lower unit. In the lower unit, you see pumice rocks that have been flattened out due to compression, and the letter slot pockets that are so sweet to sink your hand into are also elongated due compression as the rock cooled. In the upper unit, the pumice rocks are not squished like they are in the lower unit and the rock is much less dense, because it was not under as much pressure and cooled quicker. Also, the welding in the upper unit is largely due to hot gases passing through the tuff, rather than heat and pressure as in the lower unit.

I'm not sure why the two units are different color, but I'm guessing the primary mineralogy of the two units is similar enough that the it must be due to textural differences. The lower unit is dark, because it's dense and glassy so it's slightly obsidian colored; the upper unit is light colored because it's light and glassy so it's kind of pumice colored. I'm not sure what actually gives the pink color, but I'll sure look next time I'm there.

The columnar jointing in the Gorge is renowned amoung geologists. Rather than columns that go straight up, ala Devils Tower, the colums in the Gorge start at points along the contact of the two cooling units and radiate outward and upward, like giant candelabras. This is because the tuff was riddled with fumaroles (steam vents) which caused it to cool faster near the vents -- roughly, imagine what the isotherms would look like in a slab with point sinks on the bottom and the top held at constant temperature. Then imagine the pattern made by the normals to these isotherms -- that's the pattern we see. Look out across the surface east of the gorge and you see little mounds scattered across the landscape. These are caused by minerals deposits from the fumaroles locally hardening the rock, making it more resistant to erosion.

At this point, you might be thinking, "What a mondo bogus bozo post; this has nothing to do with climbing." Wrong chumpo! This has everything to do with the FA style that's developed in the Gorge. Because the lower unit is much more densely welded than the upper unit, the rock is much more solid, and the routes all end below the contact of the two units. The upper unit is choss -- one of the few accidents in the Gorge occured when some knuckleheads decided they'd explore a new approach through the upper unit and ended up surfing down the slope on some refrigerator sized blocks. In most places you can't even get to the contact to rappel down the lower unit, so routes have to be put up on lead drilling off hooks. Typically, one route gets put in on lead and then it's anchors are used to establish a rap bolted route or two.