 | (Map site 1.) Roadcut exposure of a hornblende diorite-gabbro (Rubio Diorite) whose age is not known. Only three exposures of this rock occur along today's route. Large dark clumps of hornblende and plagioclase separated by more granitoid intrusives.
 | (Map site 2.) Typical exposure of the gneiss that occurs on the south side of the San Gabriel fault. The layering in these metamorphic rocks is called foliation and is caused by the parallel alignment of the mineral grains in the rock. The age of this gneiss is unknown.
 | (Map site 3.) Roadcut exposure of gneiss that has been intruded by a pegmatite dike. Within the pegmatite the pink mineral is orthoclase feldspar and the white areas contain quartz and other feldspar minerals. Pegmatites are the last rocks to cool from a molten intrusive mass and the crystals in a pegmatite are large (one inch or so) because they cool very slowly. After intrusion of the dike the whole rock mass was cut by a small fault that is clearly visible running from the upper left side of the photo diagonally down toward the feet of the observer on the road.
 | (Map site 4.) This alluvial fan conglomerate deposit accumulated several tens of thousands of years ago in a valley along the San Gabriel fault. As the San Gabriel Mountains have been uplifted more recently, most of these deposits have been eroded away as the valley along the fault deepened. The fact that these deposits have not been offset along the fault is an indication of the inactivity of the San Gabriel fault for many tens of thousands of years.
 | (Map site 5.) This roadcut along Little Tujunga Canyon Road is of the Mendenhall Gneiss which occurs only on the north side of the San Gabriel fault. Cutting more or less horizontally across the top of the roadcut is a dark gabbroic dike. These dikes are common in the oldest rocks in the San Gabriel Mountains, but do not occur in the younger rocks. They must, therefore, be of an intermediate age.
 | (Map site 6.) Closeup of the Mendenhall Gneiss. This is the oldest rock in the San Gabriel Mountains and the metamorphic age of the rock is about 1.7 billion years old.
 | (Map site 7.) The sharp contact in the middle of this photo is the San Gabriel fault. The 1.7-billion-year-old Mendenhall Gneiss is on the right side of the fault. Granite dated at about 90 million years old is on the left side. The difference in rock ages across the fault is more than 17 times.
 | (Map site 8.) The gray material exposed here is graphite. It is soft and can be easily scratched by a fingernail. A small piece of it will write on a piece of paper. Graphite is a metamorphic mineral and might have originated as the result of metamorphism of a coal layer in the Paleocene Santa Susana Formation that is caught here between two strands of the San Gabriel fault zone.
 | (Map site 9.) Roadcut exposure of granite, the youngest of the intrusive igneous rocks in the San Gabriel Mountains. It has been dated at about 90 million years old.
 | (Map site 10.) Closeup of the granite shown in the previous photo. The white and pink minerals are feldspar, the glassy gray ones are quartz, and the black minerals are most likely biotite and hornblende.
 | (Map site 11.) Looking back at the valley eroded along the San Gabriel fault zone. The day's hike was started on the backside of the mountain on the skyline and progressed along the road that diagonals down the side of the mountain.
 | (Map site 12.) The flat spot in the center of the photo is a Quaternary alluvial deposit that was created when a wide river valley occupied the center portions of the photo. Later uplift of the San Gabriel Mountains caused the local streams here to deepen their canyons by cutting through the old river bottom deposits. The flat spot remaining, called a river terrace, is all that is left of the previous wide valley bottom.
 | (Map site 13.) Roadcut exposure on Little Tujunga Canyon Road of the Paleocene Santa Susana Formation. It is caught here as a thin sliver between the DeMille and the Watt faults. It is not shown this way on the attached Dibblee geologic map, but the Dibblee map does show it a little farther to the east. Because the Santa Susana Formation is a marine deposit, its presence here in the middle of the San Gabriel Mountains shows that the mountains were under the ocean and did not become a mountain range until after the Paleocene epoch.
 | (Map site 14.) Closeup of a conglomerate layer in the Paleocene Santa Susana Formation.
 | (Map site 15.) Sandstone and mudstone layers in the Paleocene Santa Susana Formation.
 | (Map site 16.) The Watt fault follows the yellow line at the bottom of the photo. To the left of the fault is the Plio-Pleistocene Saugus Formation and to the right is quartz diorite. The fault moved and juxtaposed the two rock units after deposition of the Saugus Formation. Erosion removed the overlying rocks down to the level of the nearly horizontal yellow line. Alluvial river deposits covered this unconformity surface as described in photos 4 and 12. Most of these alluvial deposits have since been removed by erosion following uplift of the San Gabriel Mountains, leaving only those shown here along the road. Note that the Watt fault cannot have moved since deposition of the alluvial deposits.
 | (Map site 17.) Canyon outcrop of the Plio-Pleistocene Saugus Formation that was deposited on a river floodplain. The more recent river floodplain seen from above in photo 12 is in the background. At the time the river was depositing that floodplain, the canyon exposed here had not yet been eroded.
 | (Map site 18.) Roadcut exposure of the gneiss that occurs underneath the Saugus Formation in this area.
 | (Map site 19.) Roadcut closeup exposure of the Plio-Pleistocene Saugus Formation. The red-brown mudstone in the bottom of the photo was deposited on the floodplain of a river by overbank floods. After the floodwaters returned to the river channel, the mud dried out, and became vegetated and oxidized to produce its red-brown color. The area subsided and eventually a river migrated across the area and eroded a channel in the floodplain mudstone. Sand and gravel were deposited in this channel as shown in the top half of the photo.
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