OLD L. A. ZOO PICNIC AREA TO
Saturday, March 13, 2004
FRYMAN CANYON OVERLOOK
Saturday, March 13, 2004
|Geological map above is from Thomas W. Dibblee Geological Foundation Maps DF-30 and DF-31. These maps 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 Old L. A. Zoo Picnic Area in Griffith Park at 8:30 a.m. The hike was about 10 miles in length and the last half was on paved city streets.
The Day 1 hiking group assembled in the Old L.A. Zoo Picnic Area before starting the hike. Seated in the front row, left to right, are Sam Shadman, Gene Fritsche, and Sue Fritsche. In the back row, left to right, are Doug Yule, Jamilynne Shadman, John Alderson, Ian Yule, and Elizabeth Nagy.
Here's Sam, Jamilynne, Ian, Doug, and John (hidden in the back) marching down a part of the trail.
As the morning progressed, Jamilynne and Ian decided it was time to make a short rest stop. I don't think the adults minded stopping either.
We couldn't pass up the opportunity to pose in front of the world-famous Hollywood sign.
Elizabeth had her car parked at the halfway point and we made it there for lunch. Jamilynne and Ian had been real troopers all morning, but they'd had enough by this time, so they and their parents left Gene, Sue and John to carry on to the end.
Here's Sue and Gene at the end of Day 1 at Fryman Canyon Overlook on Mulholland Drive. John was there too, taking the picture. The rather hazy San Fernando Valley is in the background. The San Gabriel Mountains are back there too, but you'd never know it.
The oldest rocks along this leg of the hike are two different intrusive, plutonic igneous rocks: a light-colored granodiorite on the left side of this photo and a dark-colored quartz diorite on the right side of the photo. You can see in this photo that the contact between the two rock types is abrupt and that in the upper right corner is a granodiorite dike in the quartz diorite. These observations show that there were two separate intrusions of different composition and that the quartz diorite is the oldest (it had to be there first in order to be intruded by the granodiorite dike). The granodiorite has been dated at about 102,000,000 years old.
Closeup of the quartz diorite.
Closeup of the granodiorite.
Overlying the quartz diorite (on the left side of this photo) are sandstone beds of the Middle Topanga Formation (right side of the photo). Because the quartz diorite cooled into a rock deep below the surface of the Earth, a long period of uplift and erosion of the quartz diorite is required before the sandstone can be deposited on top of it.
The Middle Topanga Formation consists of interbedded sedimentary layers and lava flows. In this photo a dark basaltic lava flow on the left is covered by light-colored sandstone layers on the right.
Fossils present in the Middle Topanga Formation sandstone layers indicate that the sandstone was deposited in the ocean. These clam fossils, called Anadara, were found along Mulholland Drive, just west of Laurel Canyon Blvd.
Another line of evidence that the Middle Topanga Formation was deposited in the ocean is this basalt lava flow containing broken fragments of lava pillows. Lava pillows only form when lava is extruded from a lava flow under water. This basaltic lava has been dated at about 17,000,000 years old.
This closeup of a Middle Topanga Formation conglomerate layer shows a light-colored granodiorite boulder at the point of the hammer, dark-gray quartz diorite cobble in the lower right and a brown basalt cobble at the center-top part of the photo. These eroded rock fragments in the conglomerate prove that the granodiorite and quartz diorite were exposed and were being eroded when the Middle Topanga Formation was being deposited. The basalt cobble proves that even the Middle Topanga Formation lava flows were being eroded and reincorporated into the Middle Topanga sedimentary layers.
After another period of erosion, sandstone and conglomerate deposits of the Upper Topanga Formation were deposited on the Middle Topanga Formation. This photo shows a closeup of cobbles and pebbles that make up the Upper Topanga Formation conglomerate beds. The salt-and-pepper gray cobbles are quartz diorite, and to the right of the hammer is a small, greenish basalt pebble, and next to it a cobble of brown sandstone. These cobbles and pebbles indicate that all the older rock units were still exposed in the vicinity and were contributing eroded material to the Upper Topanga Formation sedimentary deposits.
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