FRYMAN CANYON OVERLOOK TO
Monday, March 15, 2004
ENCINO HILLS DRIVE
Monday, March 15, 2004
|Geological map above is from Thomas W. Dibblee Geological Foundation Map DF-31. This map may be purchased online from the Dibblee Geological Foundation branch of the Santa Barbara Museum of Natural History at: http://www.sbnature.org/dibblee/newweb/orderinfo.html.|
Participants met at the Fryman Canyon Overlook at 8:45 a.m. The hike was 8.8 miles in length and was entirely along Mulholland Drive.
When we arrived at Fryman Canyon Overlook to start the Day 2 hike, we were surprised to find John Brooks, a KFWB radio reporter waiting for us. John interviewed Gene and later in the day, after we had returned home, we heard the report on the radio. It was an unexpected, but pleasant, start for the day.
We had no extra hikers to accompany us on Day 2 and the entire hike was through the residential communities along Mulholland Drive. After a rather uneventful morning, we stopped for lunch in a shady spot at the Stone Canyon Overlook.
Soon after lunch we crossed the 405 Freeway on the Mulholland Drive bridge. Traffic was unusually light at this time of day.
In the early afternoon we made it to the end of the day's hike at the intersection of Mulholland Drive and Encino Hills Drive. Although it was hazy, it was still clearer than it had been at the end of Day 1, and you could actually see the San Gabriel Mountains across the San Fernando Valley.
The oldest rock present along this leg of the trip is quartz diorite, the same igneous intrusive rock that was seen in Griffith Park on Day 1 of the hike. This photo is a closeup of what the quartz diorite looks like.
Deposited on the quartz diorite along this part of the hike are sedimentary rocks of Cretaceous and Paleocene age, but they are not very well exposed. Deposited on top of the Paleocene rocks is a sandstone unit called the Lower Topanga Formation, which is exposed in a roadcut along Mulholland Drive just north of the Summit Overlook. In this photo of part of the exposure, a small fault cuts across and displaces the alternating beds of brown and white sandstone.
Fossils in the Lower Topanga Formation indicate that the sandstone was deposited in a shallow, marine environment during the early Miocene epoch, about 20,000,000 years ago. These little marine snails belong to the genus Mesalia.
Sue spotted a large fossil bone fragment in the Lower Topanga Formation. It looks like a fragment of a whale rib bone, which is another indication that the Lower Topanga Formation was deposited in shallow ocean water.
Here Sue is pointing to some dark brown dikes that have intruded the Lower Topanga Formation. These dikes consist of an intrusive igneous rock of basaltic composition that is called diabase. The fractures through the Lower Topanga Formation, that are now occupied by diabase, served as conduits along which melted rock flowed to the surface to appear as the lava flows that make up part of the overlying Middle Topanga Formation. The dikes are about 17,000,000 years old.
The rounded appearance of this funny little "face" is the result of spheroidal weathering of the structureless diabase. The "eyes" in the face were made when drill cores were removed from the diabase by geologists who wanted to determine the direction of magnetic north that was frozen into the rocks at the time they cooled from the melted condition to solid rock.
As was seen in Griffith Park on Day 1, the Middle Topanga Formation consists of alternating layers of lava flows and sedimentary rocks that were deposited about 17,000,000 years ago. Here is an exposure of some thin sandstone beds in the Middle Topanga Formation.
Following a short period of uplift and erosion, the Upper Topanga Formation was deposited on top of the Middle Topanga Formation. This exposure reveals sandstone beds of the Upper Topanga Formation. Compare the sandstone beds in this photo with the cobble conglomerate seen in the Upper Topanga Formation in Griffith Park on Day 1. The difference in size between the cobbles being deposited in Griffith Park and the sand grains being deposited here, illustrates that the Griffith Park area was closer to the shoreline and closer to the source of the sediment that was being transported to the site of deposition.
After a short period of extensive deformation and erosion, the Monterey Shale was deposited on top of the Upper Topanga Formation. This photo, taken on the east side of the 405 Freeway and south of the Mulholland Drive bridge, shows the pebble conglomerate that was deposited at the base of the Monterey Shale. Note that the pebbles are mostly made of black slate. The black slate comes from the Santa Monica Formation, the oldest rock unit in the Santa Monica Mountains, which will not be seen until Day 3 of the hike, but which occurs here immediately under the Monterey Shale. This occurrence proves that all the Cretaceous, Paleocene, and Miocene rock units described above were removed by erosion prior to deposition of the Monterey Shale.
This photo shows a typical exposure of the thinly bedded, dark gray Monterey Shale exposed in a roadcut on Mulholand Drive just east of the 405 Freeway. The Monterey Shale contains abundant fossil microfossils and fish scales and some whole fish fossils that show that it was deposited in a deep ocean basin.
Within the Monterey Shale are some sandstone beds that result from turbidity-current deposition on the fringes of submarine fans that reach into the deep ocean basin. These sandstone beds become unstable when undercut by road or house construction that produces slopes that are too steep to support them. When heavy rains saturate the rock, it falls down onto the roadway as shown here.
Shale beds, like those in the Monterey Formation, are not very competent, and when they were pushed upward out of the ocean and deformed during the rather recent formation of the Santa Monica Mountains, they were folded into the crumpled layers seen in this photo.
When the Monterey Shale was deformed into the folds shown in the previous photo, the sediment was still saturated with water and had not yet been hardened into rock. Fractures that formed in the shale during deformation were injected with a slurry of watery sand that rose from underlying sand beds. This process produced the brown sandstone dikes that pass diagonally through the shale layers shown in this photo.
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