The Machado Postpile is found within the El Dorado National Forest near the alpine lake of Silver Lake. It is approximately 3.3 miles from the Kit Carson Rd parking area near the Minkalo boys camp to the top of the postpile (using cairn or ducks). This field of diabase extrudes from the surrounding granite imposing a wonderful contrast to an otherwise uniformly colored landscape.The Machado Postpile has almost a north south orientation, but points slightly to the NW when viewed from a Quad or Topo map. Make sure to explore the smaller field of dolerite (diabase) just to the east of the larger field.
E-Mail me your observations and answers. DO NOT POST YOUR ANSWERS. Enjoy and protect this wonderful environment.
1. Give the GC code and Earthcache name.
2. Facing the Machado Postpile from the West, the columns lean from vertical toward the East at an angle of approximately 30-40 degrees. When facing the post pile from the East (opposite side), what approximate angle from vertical to the columns lean?
3. Columns often times have distinct shapes when viewed from the top. For example they may be six sided (hexagonal). What shapes as you observe the tops did you find? Estimate which shape you think is predominate.
4. When sitting on the top, the columns are not very distinct in some areas. They look like shortened mushrooms. Do you think this was due to weathering or glacier slide? From observation, why might you think so? (optional observation)
5. The much smaller field of diabase columns to the east have some very unique features; It has only one wall that you can observe. When looking at this wall from the east facing west, describe what you see.
6. Did you find any feldspar in the rock composition? And if so, did it have striations in it? What color was feldspar?
7. (Optional) Post a picture of yourself with your favorite postpile location behind you.
That's all! Go ahead and log your cache. Only if you have incomplete logging requirements will I respond to you.
What is Diabase?
Diabase is an igneous rock similar to basalt. It is dark green to black and often has white crystals scattered through it. When exposed to the weather its surface often turns brown. It has a medium grain size (you can see them without a magnifier, but they are smaller than rice). It is a tough, hard rock.
Diabase forms from a magma that is rich in iron and magnesium, and poor in silica (quartz). It generally forms dikes; that is, it fills fissures or cracks in some other rock near the surface of the earth. Diabase is from the same kind of magma as basalt, but because it cools more slowly, it develops slightly larger crystals.
What is Feldspar?
The feldspars are a family of silicate minerals which occur in igneous rocks. There are many different members to the feldspar group. Silicon and oxygen form the foundation for the group; calcium, sodium, and potassium are also present. One of these elements is usually dominant, but most of the feldspars contain all 3 in varying amounts. It is the proportions of these 3 elements which help determine which specific feldspar is formed. The feldspars are divided into 2 broad categories: plagioclase, which contains calcium and sodium and orthoclase, which contains potassium. See graph below. The feldspars constitute about 50 percent of the earth's crust.
Reference material:
http://malindi.com/Albums/Machado/Machado.htm
http://www.rockhounds.com/rockshop/rockkey/#diabase
http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry34.html