There is a small parking area at a trailhead on the south side of the hill. A short, but steep trail takes you up to the top of the rock.
About 2 million years ago, a volcanic vent erupted basaltic magma through gravels that were either at the bottom of a shallow lake or completely saturated. This kind of eruption through water is called phreatomagmatic . Typically it is quite explosive. The resulting volcanic deposit consists of basaltic tuff breccia with rounded and angular fragments.
More recently during the Tahoe (80,000 to 130,000 years ago) and possibly the Tioga (about 18,000 years ago), glaciers flowed over this hill leaving glacial erratics. Two characteristics largely define a glacial erratic. First is that it must be transported a significant distance by a glacier or iceberg. And second, the erratic is typically a different rock than the surrounding rock that it is deposited on. Size does not define an erratic. While shape does not define an erratic, they are often very round because the rough edges are eroded off while the erratic is being transported within the ice. Many of the erratics found on the top of Eagle Rock are likely Rockbound Granodiorite. This rock formation is found south and southwest of the head of this canyon.
Erosion at higher elevations in a glacier extract pieces of rock that become encased in the ice. As the glacier moves downhill, the encased rock is transported along. During transport the sharp edges of the rock are typically smoothed out forming a rounded rock. At the end of the glacier or when the glacier retreats, the ice melts dropping the rock onto the ground. In this way, pieces of rock formation can be found just sitting on bedrock that is nothing like the transported piece of rock.
Logging requirements:
Send me a note with :
- The text "GC42QG9 Eagle Rock Tahoe National Forest" on the first line.
- The number of people in your group (put in the log as well).
- What is the general color of the bedrock of the mountain and the shape of the larger rocks in it?
- What is the general color and shape of the large rocks sitting on top of the mountain?
- Comparing your answers from 3 and 4, which are the erratics?