One Hundred Hikes in Yosemite

Evolution of the Yosemite Landscape — The Holocene Epoch

The last 10,000 years have been classified as the Holocene epoch, which is misleading, for it implies that the glacier-dominated Pleistocene epoch is behind us. Notwithstanding global warming through man's zealous production of greenhouse gases, nothing could be farther from the truth. There will have to be a lot of future movements of the earth's giant plates before the ocean's circulatory patterns are sufficiently changed to permit substantial warming of the northern latitudes. We are only in a short-term interglacial period similar to a preceding, major one that lasted from about 130,000 to 120,000 years ago.

Since 13,000 years ago, the unofficial start of the Holocene for the Sierra Nevada (when its glaciers had all but disappeared), the Yosemite landscape has changed very little. The massive face of El Capitan has weathered back only 1/4 inch, if even that, though parts of it have spalled rockfalls. Rockfalls generally occur in winter and early spring, as water works its way behind large flakes, freezes and expands, and thereby pries the flakes loose. While all flakes are initially formed through unloading, they can be pried loose by processes other than freezing water. On March 26, 1872, John Muir, Galen Clark and others witnessed massive rockfalls due to the distant Owens Valley earthquake. A sufficient tonnage of rocks fell from Liberty Cap to temporarily dam the Merced River just above the brink of Nevada Fall.

Lightning can be another cause of rockfalls. A large-scale rockfall is unlikely to result, but in June 1976 a lightning bolt broke off a 220-foot-high 1000-ton slab from Upper Yosemite Fall's massive wall. Finally, from the 1930s, especially during the '60s and '70s, climbers drove countless pitons into thousands of cracks, briefly accelerating the natural rate of exfoliation.

Major rockfalls occur every few years in Yosemite Valley. The largest one identified is the prehistoric fall that dammed Tenaya Creek to create Mirror Lake, estimated to be about 15,000,000 cubic yards. The 1987 Middle Brother rockfall, perhaps the largest historic one, pales in comparison at about 800,000 cubic yards. Fresh rockfall sites on cliffs are easy to identify, since they expose fresh rock. Over decades, medium-gray lichens grow over the exposure, blending it in with the rest of the wall. Without lichens, Yosemite Valley would have walls mostly creamy or light gray.

While rockfalls built up talus slopes in Yosemite Valley and other parts of the Park, rivers and creeks worked ineffectually at removing them. Rivers and creeks were also ineffective at cutting through bedrock, and they typically have carved only a foot or so into it in the last 13,000 years. Examine the brink of Vernal or Nevada fall and see just how little erosion the Merced River has done.

By the start of the Holocene epoch, all of the Sierra's Tioga glaciers had disappeared. However, a minor period of glaciation began about 1250 A.D. and became pronounced from about 1550 to 1850 A.D. This was the historic Little Ice Age, and John Muir was the first to discover a real glacier in the Sierra Nevada, not just a year-round snowfield. This was the Merced Peak glacier, which Muir had discovered in 1871. By late 1977, a severe drought year, all traces of ice and snow had disappeared. With continued climate warming, this may be the fate of the few remaining Sierran glaciers.

Before closing this chapter, we should look at the origin of Yosemite's meadows. It had long been assumed that High Sierra meadows once were lakes that had developed after Tioga glaciers had disappeared. According to this view, a lake was gradually filled in with sediments until at last a meadow existed. Frangois Matthes had proposed it for his Lake Yosemite-the most extreme example. But sedimentation, it turns out, is a very slow process in the Sierra Nevada, and most lakes formed back then are still lakes today. The average Sierran lake typically has between 10 and 20 feet of sediments-about one foot of deposition per thousand years.

Actually, as Spencer Wood has shown, many of the High Sierra meadows, such as Tuolumne Meadows, were forested up to at least the start of the Little Ice Age, when colder conditions developed and the level of ground water rose, drowning the trees. We can therefore be thankful for the Little Ice Age, for without it we wouldn't have the many meadows from which we can view the snow-clad High Sierra landscape. The view-packed Yosemite Valley meadows are also in existence largely due to the presence of a high water table. The meadows once were much more extensive, covering about 745 of the Valley's 1,141 acres, but they were greatly reduced by humans in part due to the lowering of the water table in order to reduce both flooding and mosquitoes, two problems that plagued early tourism. Humans locally can be powerful agents of geologic change.


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