Stop #1

 

Figure #3: This shows the locations of the stop #1

Above Figure #3a: This shows the location of the stop #1, just southwest of the volcanic stock.

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Green Valley Tonalite (Cretaceous)

Stop #1 (33 degrees 10’ 13” N and 117 degrees 17’ 14” W)

Stop #1 provides excellent exposures of the Cretaceous Green Valley Tonalite. The overall color of this rock is intermediate and the crystals can be seen easily with the naked eye. Using a hand lens on a fresh surface you will notice it consists of three minerals. The primary mineral is grayish-brown and has two distinct cleavage planes. You may note some striations on the cleaved faces.  Most of us are used to seeing a lighter variety of this mineral in hand specimen. This is a dark variety of plagioclase feldspar. There are also smaller amounts of amphibole and quartz in this rock. This unit is mapped as a tonalite which is between a diorite and a granite in composition (Figure #2b).

 

 

 

This picture shows Stop #1, note the fresh outcrop in the bottom of the ditch. The rocks here are intermediate in color and medium-grained.

 Above picture: Stop #1 is in a ditch just before we reach the dam. There are excellent exposures of the Green Valley Tonalite in this area. Note the fresh outcrop in the bottom of the ditch.  The side of the ditch in the background is the same material but heavily weathered. The cracked tonalite blocks in this weathered area represent the early stages of spheroidal weathering.

 

 

 

 

This shows the rocks at stop #1 at a distance. Medium-gray and medium-grained tonalite/diorite.

 

This shows the rocks at stop #1 at a distance but closer than the previuos picture.

 

This shows the rocks at stop #1 in a close up where you can see the grains and texture of the rock.

Above Pictures: This series of 3 pictures show the rocks at stop #1 first at a distance and then closer and a close-up. Note the grey color at a distance. The greenish hues are not olivine but a byproduct of the photograph.

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Spheroidal Weathering

 

On the way to Stop #2 we pass some large intrusive igneous outcrops that exhibit spheroidal weathering. It looks like an “onion skin” peeling off of the rocks.

 Above Picture: On the way to Stop #2 we pass some large intrusive igneous outcrops that exhibit spheroidal weathering. This is a common type of weathering for this rock type and it gives an “onion skin” appearance to the exposures.