Millions of Years
  1. Metamorphic Rock
  2. Migmatite
  3. Dolomite
  4. Joints
  5. Folds
  6. Fossils
Metamorphic Country Rock

 metamorphic rock in contact with granite
Dark metamorphic rock contact white foreground granites in this view north from Mt. Conness.

Looking north from the summit of Mt. Conness in Tuolumne Meadows, Yosemite California one sees dark rocks in contact with the white rocks underfoot. A closer look at the rocks underfoot shows that they have huge crystals, over a centimeter across: crystals of milky quartz and white cubes of feldspar. In general terms the rock is granite, to a geologist it is Cathedral Peak granidiorite, to a mountain climber it is heaven. The granite is solid, on the surface it is rough enough to allow boots to stick yet smooth enough to not cut flesh.

The dark rock was here first, before the granite. As the older rock it is called country rock. The dark rock started out as lava and volcanic ash. It is dark because it is rich in iron oxides. The white granite has little iron content. When the cubic kilometers of granite melted their way up through the layers of volcanic rock they cooked the rock and squirted it full of hot, mineral rich, water. The heat, water and minerals metamorphosed the country rock to metavolcanic rock.

How Cathedral Peak Granidiorite became Yellow.

Yosemite valley cuts through many different types of granite. On a geologic map each granite gets its own pink color and pattern to identify where it is at the surface of the bedrock. They had run out of patterns when they reached the small patch of granite at the north edge of the Yosemite geologic map and so they gave the small piece of cathedral peak granidiorite a yellow color usually reserved for sediments. Then to the horror of the geologists almost the entire geologic map of Tuolumne meadows turned out to be one type of granite, cathedral peak granidiorite. According to tradition they kept the color the same from the earlier Yosemite map. So that is how the largest outcropping of the same granite in the United States was given a color on a geologic map the usually indicates a sediment.


migmatite, Wind Rivers Wyoming
Migmatite, a highly metamorphic, mixmaster rock.

Notice the mixture of light and dark rock in this photo. The dark rock is the country rock, the light rock intruded it. Unlike the great separation between dark and light rocks in the Yosemite photo above, here the rock is intimately mixed together. The dark country rock is highly metamorphosed. Just a little higher temperature and it would have melted to form igneous rock.


A wall of Dolomite in the Wind River Mountains.

Notice the solid gray rock in this photo. It has been intruded by white rock along straight lines. There are also white lumps sprinkled throughout. Dolomite is a deep ocean sedimentary rock. It is made from calcium carbonate just like limestone except that half of the calcium has been replaced with magnesium. After this rock was deposited it was fractured and hot, mineral laden, water flowing through the cracks deposited the white minerals.


Hunk Cracks near Olmstead point Yosemite CA
The granite of Yosemite is fractured on many scales.

In this photo you can see the flat faces of fracture planes that result from fractures in granite known as joints. The granite batholiths of Yosemite Park have been stressed and fractured into sets of parallel planes. Joints are tens to hundreds of meters apart. Glaciers have removed the rock that used to be pressing against these joints.

The sunlit faces of the rocks in this photo make one set of joints, the shadowed faces make another. The flat surface at the top of this wall is another joint surface as is the invisible flat rock surface I am standing on to take this photograph.

The glacier flowed from right to left across this face plucking away blocks of granite from between the joints, rounding the edges of, and grooving and polishing the granite left behind. The resulting cracks up the less than vertical wall look like fantastic rock climbs.


squaretop wyoming view of folds
From Squaretop Wyoming you can see folded rock at the edge of a sedimentary mountain.

Look across the valley of the green River in Wyoming, just upstream from the Green River Lake and you will see a mountain with wonderful horizontal lines etched into its side. Without a doubt this layers mountain is made of sedimentary rocks. Look closely at the right edge of this mountain and you will see a very strange thing, the sedimentary layers bend around a corner and head straight up into the sky.


tree  bark fossil imprint Nevada fossil imprint tree bark Nevada
Fossil imprint of tree bark.

My wife Ellen and I were hiking up a canyon in White Pine County Nevada when she spotted these imprint fossils alongside a streambed. Notice that these rocks look like they were run over by a tire that left treadmarks. They are actually the imprint of the bark of a large tree either a palm or a cycad. I walked right by them and didn't see a thing. ou have to be alert to see the science around you.



To Time Index

Scientific Explorations with Paul Doherty

© 2000

4 October 2000