Driving across Yosemite National Park made GG wonder once again about the origin of Yosemite Valley. Within the valley, the basic story is the variety of rock types that allowed the valley glacier to cut so deeply. But there is another part that is more subtle.
If you hike the valley floor, you would think the main stem river would be Tenaya Creek, which heads up the valley under Half Dome with only a small rise in elevation, and not the Merced River, which rises dramatically over the Giant Staircase of Vernal and Nevada Falls to Little Yosemite Valley. This is quite odd, and what it tells you is that a really large amount of ice came pouring in over Tenaya Creek.
This ice was not created by snowfall in the relatively small Tenaya Creek drainage. The vast majority had fallen in the Tuolumne Meadows area on the immense ice sheet that capped the upper Tuolumne River watershed. The natural drain for this ice was, of course, down the Tuolumne River.
This is where things get interesting. Had the Tuolumne Glacier knifed down through what is now the Grand Canyon of the Tuolumne, there would have been a large conduit for ice to descend down the river channel. But in fact the Grand Canyon of the Tuolumne, despite having the longest glacier in the Sierra churning down it, remains essentially a V-shaped fluvial river canyon.
What this means is that the ice would pile higher and deeper near Tuolumne Meadows than you might normally expect, which means that more ice could escape over drainage divides. One was to the east, where this ice poured through Tioga Pass and carved the truly impressive canyon to the east. Another was to the southwest, over the divide into tiny Tenaya Creek’s drainage. Here the ice found rock more willing to yield to glacial erosion, and the deep hole thus created in Yosemite Valley reinforced the diversion of ice into that valley.
So while the diversity of rock types–particularly the weak diorites of the Rockslides area in Yosemite–was probably essential to getting Yosemite carved to its present dimensions, equal tribute should be paid to the Yosemite Creek, El Capitan, and Sentinel intrusions to the north which did not yield to the glacial ice, forcing most substantial erosion in Yosemite itself.