Stop #3

 

This figure shows the location of stop #3, to the south of the volcanic stock.

Above Figure #5: This figure shows the location of stop #3, to the south of the volcanic stock.

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Volcanic Flows

Stop #3 (33 degrees 10’ 3” N and 117 degrees 17’ 7” W)

Stop #3 presents a series of volcanic flows going over the top of the tan and coarser-grained Green Valley Tonalite. The tonalite seen in the road cut is deeply weathered, crumbly, and much lighter in color than fresh tonalite.  The weathering is mostly due to the weathering of the plagioclase feldspar to light-colored clay minerals.

 

This is the first of two pictures showing the gray fine grained unit as a flow over the top of the tan coarse grained unit.This is at a distance.

 

This is the first of two pictures showing the gray fine grained unit as a flow over the top of the tan coarse grained unit.This is closer.

 Above Pictures: These 2 pictures show the gray fine-grained dacitic unit as a flow over the top of the tan coarse-grained tonalite.

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The Adit

This is at the end of Stop #3.  Notice the small opening in the lower left corner. This is called an “adit”. It is an exploratory excavation for valuable minerals. It is common for geologists to look for mineralization and ore deposits at the contact of two rock units. The next 2 pictures show the gray, fine-grained dacitic lava flow going over the top of the tan, coarser-grained tonalite here.  The “adit” marks the contact between the two units. The rocks to the right of the “adit” opening are not in place so basically ignore them.

These 2 pictures show the adit, this one is at a distance.

 

These pictures show the adit, this one is close-up.

 Above pictures: These pictures show the adit, the first is at a distance and the second is close-up. Note the volcanic flow on top in both pictures.

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Local Weathering

This area gets very muddy when it rains.  This is because of the way in which the rock units’ weather. The main mineral that causes this is plagioclase feldspar. Plagioclase feldspar weathers relatively quickly into light colored clays, which become very slippery when wet.