Thousand Island Lakes and Tuolumne Country
The Geology of the Thousand Island Lakes and Tuolumne Country
Photo by Jonathan Fox

As recently as 2000 years ago nearly all of Yosemite National Park, including the promontory of Cloud’s Rest where we sat, had been covered by a 60-mile-long, 2000 foot thick glacier that extended from Mt. Lyell, Yosemite Park’s tallest peak to our southeast, all the way down the Grand Canyon of the Tuolumne River to our northwest. Only the tips of the highest peaks in the area protruded from the top of the ice.

The lovely meadows of the Lyell River canyon that we traversed 3 days ago had been formed from silting in of a huge lake that formed when the glacier ice finally receded. The veneer was indeed young. The story of the characteristic, multi-textured granite domes and slabs we’d walked through during this segment of our journey was still being debated by geologists. There are only a few places in the world like it. Ancient rock, some 250-350 million years old, had been transported to the Sierra Nevada area by a wheeling, offshore plate that brought successive chunks of sedimentary seafloor and island arcs to slam up against the edge of the North American plate near the Utah-Nevada border, making what is now land from California northward.

As this plate subducted farther and farther under the North American plate, magma bubbled up in successive, massive blobs through cracks and dikes in the older rock above it 80-90 million years ago, heating and mixing in different combinations with the older rock it extruded through, and then slowly cooling into massive and varied granite formations.

The so-called Tuolumne Intrusive Suite alone includes over 400 square miles of exposed granite, and much more is buried below. When cataclysmic uplift raised the Sierra Nevada crest 4 million years ago, the ancient rock and the newer granite were pushed way up, and erosion plus the effects of multiple glaciations scoured the deep valleys, contoured the domes and vertiginous cliffs and smoothed the slabs all around us, finally backing away to leave the meadows, rivers, slabs and domes, erratic boulders and rock-strewn lakes that our journey had taken us through.

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