Geologic Trip, Yosemite Valley
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.