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Geologic Trip, Yosemite
Valley Half Dome From
Tenaya Bridge you can get a good view of the steep northwest face of Half
Dome. Half Dome is formed from a large block of unjointed Half Dome
Granodiorite that is bounded on the northwest and southeast sides by major
joints. This block of granite was extremely resistant to erosion, and stood
above the general landscape most of mid- to late-Tertiary time. Through this period, the sharp corners of the block were rounded by
exfoliation to form a steep-sided elongated dome. During the
Pleistocene glacial episodes, the largest of the glaciers that flowed down
Tenaya Canyon reached to within 700 feet of the summit of the dome, but none
of the glaciers ever covered Half Dome. However,
the jointed granite
above the glacier was quarried by the freeze and thaw action of ice along the
joints. By the end
of the Pleistocene, the joint zone on the northwest flank of the dome had
become the familiar steep cliff that we see in this photo. The crest and backside of the
dome were unaffected by the glaciation, and retained the curved surfaces of
the exfoliated granite. The trail
to the summit of Half Dome goes up the steep slope of exfoliated granite on
the northeast end of the dome. Exterior Websites National Park Service: USGS: Yosemite Hikes: _____ |