Double Rock consists of two granite towers at the top of a ridge overlooking the Grand Canyon of the Tuolomne River. This is the widest point of the canyon, only about 4 miles across, which makes it much narrower than the Grand Canyon in Arizona.
To reach the cache, start at the Ten Lakes trailhead, climb about 2100 feet in 5 miles to Ten Lakes Pass, and head west along the ridge approximately 2 miles to Double Rock. I have seen reports of people who started at Lukens Lake or White Wolf and hiked on the trail towards Ten Lakes until reaching the county line, and then heading north from that spot. Either way, this would be easier to do as part of an overnight backpack than as a single-day trip.
From the NPS web page on the geology of Yosemite:
"Massive granite dominates the Yosemite area and much of the Sierra Nevada as well. Mount Hoffmann and most of the terrain visible from it are composed of granite, formed deep within the Earth by solidification of formerly molten rock material and subsequently exposed by erosion of the overlying rocks. Because of its massiveness and durability, granite is shaped into bold forms: the cliffs of Yosemite and Hetch Hetchy Valleys, many of the higher peaks in the park, and the striking sheeted domes that can form only in massive, unlayered rock. Although granite dominates nearly the entire length of the Sierra, the granite is not monolithic. Instead, it is a composite of hundreds of smaller bodies of granitic rock that, as magma (molten material), intruded one another over a timespan of more than 100 million years. This multiplicity of intrusions is one of the reasons why there are so many varieties of granitic rock in Yosemite and the rest of the Sierra. The differences are not always apparent to the casual observer, but they are reflected in sometimes subtle differences in appearance and in differences in response to weathering and erosion acting on the rocks.
"The vast majority of Yosemite is comprised of plutonic igneous rocks. Plutonic rock forms deep underground when molten rock cools and solidifies very slowly, allowing large crystals to form. In contrast, volcanic igneous rocks form at the surface when molten rock cools and solidifies quickly, resulting in small crystals. Granite, granodiorite, tonalite, quartz monzonite, and quartz monzodiorite are all forms of plutonic rock that are found in Yosemite, and are loosely referred to as granitic rocks. Quartz diorite, diorite and gabbro are plutonic rocks found in Yosemite, but are not technically considered to be granitic rocks. Plutonic rocks are primarily comprised of 5 minerals: quartz, potassium feldspar, plagioclase feldspar, biotite, and hornblende. Plutonic rocks, including granitic rocks, differ primarily in the relative proportions of quartz and feldspar, although texture is also an important consideration. The plutonic rocks were generally formed during the Cretaceous period."
From Wikipedia:
"Starting about 2 to 3 million years ago a series of glaciations further modified the area by accelerating mass wasting through ice-wedging, glacial plucking, scouring/abrasion and the release of pressure after the retreat of each glaciation. Severe glaciations formed very large glaciers that tended to strip and transport top soil and talus piles far down glacial valleys, while less-severe glaciations deposited a great deal of glacial till further up in the valleys." "Glacial systems reached depths of up to 4000 feet (1200 m) and left their marks in the Yosemite area. The longest glacier in the Yosemite area ran down the Grand Canyon of the Tuolumne River for 60 miles (95 km), passing well beyond Hetch Hetchy Valley."
Although the Grand Canyon of the Tuolomne was primarily formed by glaciers, it does not have the typical "U"-shape of glacial valleys. The reasons for this fact are not entirely understood. In Yosemite the granite is so hard that the primary glacial erosion mechanism is a mechanism called "plucking," in which water freezing in the cracks of rock causes pieces to flake off. These pieces are then carried away by the glacial ice. It is thought that because the granite in the area of the Grand Canyon of the Tuolomne has very few cracks and joints, the glaciers were not able to pluck large flakes off, as occurred in the Yosemite Valley. As a result, the canyon is "V"-shaped.
To log this cache, send me the answers to the following questions:
- On the first line, please put "GC5E351 Double Rock at Yosemite"
- Give the number of people in your party and their geocaching name(s)
- What elevation does your GPS indicate for this point? What is the elevation difference between you and the bottom of the Grand Canyon of the Tuolomne?
- Describe the material at the base of Double Rock. Estimate the grain size. Do you think this material was eroded by glaciers? Why or why not?
- The rocks at Double Rock include both veins and inclusions. What are the differences between the material in the veins and the material forming inclusions?
- Describe the distinctive rock feature between the two towers of Double Rock. How do you think this feature formed?
- (optional) include a picture of you and your party at Double Rock.