Caldecott Tilted Beds
In California, United States
How Geocaching Works
First, use caution since cars enter the freeway at high speed. Don’t get hit.
This is one of those locations where that series of jokes about geologists apply.
You know you’re a geologist if:
• You consider road cuts a tourist attraction
• You consider Caltrans the most important department in state government
• You’ve ever run across 8 lanes of traffic to see if the road cut on the other side is the same (don’t do this).
Thousands whiz by in their car every day without wondering what the formations that straddle both sides of the Caldecott Tunnel entrance, but here YOU are. Look to the other side and you’ll see two obvious geological units in the road cut (see photo 1); the gray, gravelly Orinda Formation and the blocky Grizzly Peak Basalt (aka Moraga Formation). The Orinda Formation is the one closer to the tunnel entrance (to the left if you’re on the south side of HWY 24) and the Grizzly Peak Basalt to the right (from the same side). There are two formations visible in the road cut. These formations were deposited nearly flat lying, and have been uplifted by the tectonics that formed the East Bay Hills. There are two geologic units you see are the Orinda Formation
The Orinda Formation, more than about 9.8 million years old, was deposited by streams coming off hills/mountains in the west. Look closely at the deposits on the other side, in between the steeply uplifted layers you can see the outline of stream channels. Once can guess that one that on the alluvial fan, the stream moved suddenly (avulsion) and the old channel was filled by fine sediments. Look closely at the Orinda Formation near you, and you’ll see this unit consists mostly of stream gravel and some sand (see the close up picture). The size of the gravels tell you (if rocks could talk) that the hills that shed this material were close, as the rocks haven’t been ground to sand and silt.
Lying on top of the Orinda Formation is the Grizzly Peak Basalt. The blocky rocks you see here consist of basaltic and andesitic lava flows. Based on radio-dating the Grizzly Peak Basalt is approximately 9.8 to 8.6 million years old. The based of this basalt was deposited directly on the Orinda Formation, as attested by the baked surface of the underlying Orinda Formation. The reddish bands between the thick layers of basalt are period where no basalt was deposited, and thin layers of soil developed.
Why are these units closer to vertical than horizontal they way they were deposited? All the faults around have uplifted these geological units. Think of two layers of differently colored clay, one on top of the other. Then lift the two to edges to make a “U” shape, that is called a syncline. In front of you is the “left” arm of the “U”, the bottom of the “U” is under Siesta Valley at the Wilder exit to the east, and the “right” arm of the “U” is the hill and road cut east of the Wilder Exit before the Orinda Exit. If you look at the road cut before the Orinda Exit you’ll see the Grizzly Peak Basalt.
In accordance with the Earthcaching guidelines, to log you cache answer the following questions:
1) Examine 15 pieces of gravel in the Orinda Formation. How many distinct rock types can you identify? What does this tell you about the composition of the hills that shed this gravel?
2) Measure the longest axis of the 5 largest pieces of gravel you find lying about. Please do not further erode the slopes by digging pieces out. What is the range in size of the pieces of gravel you found? What does the size of the gravel tell you about how close the hills that shed this gravel where?
(No hints available.)
Close up of Orinda Fm gravels
View of contact. Orinda Fm to left - Moraga Fm rt
Last Updated: on 3/24/2013 5:43:12 PM Pacific Daylight Time (12:43 AM GMT)
Coordinates are in the WGS84 datum