4  - The complete sequence of rocks
 
The left (west) side of the Highway 101 roadcut is composed of the interbedded sandstones and shales. They are underlain by by hard, silica rich rocks that we will observe as we continue our trip (just above the Sausalito road sign). And at bottom of the sequence, the core of the Spencer Ave. road cut is in former volcanic rocks erupted under the ocean. The rocks are tilted from upper right to lower left. This sequence has been lifted from the deep ocean floor essentially intact. The resistant layer without vegetation that caps the hill beyond the first roadcut is the silica rich layer in the sequence.

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