To reach this earthcache, walk down the concrete slope towards the bayou. To the far right of the slope, there will be a 4 ft cliff. Go to the land at the base of the cliff where you can examine the cliff. At times, the base may be covered by water, so I wouldn't recommend you visit this earthcache after a large rain. Viewing the outcrop from the side or from across the bayou is also satisfactory, but I prefer that you examine the outcrop right near it. Getting back to the trail will be more difficult than getting to GZ.
At GZ, you will see a clay outcrop carved by the bayou. The outcrop exposes a layer of sandy soil (where most vegetation grows) and a larger layer of clay and sand below the soil.
How did this clay get here?
Houston’s foundation consists of sediments washed from the Rocky Mountains from the past 60 million years. These sediments were primarily sands and clays. At this location, the thick sequence of sands and clays is known as the Lissie Formation. The sequence extends much further into the Earth than what is viewable at the outcrop.
Clay Geology
When you hold natural clay, you are holding what was originally feldspar in granite. Over millions of years, water containing carbonic acids from the atmosphere breaks down the feldspar crystals inside granite. The result is residual or primary clay- clay that remains near its parent material (granite). Eventually, the residual clay may be transported by water, wind, or ice to a distant location. Clays that have been moved a great distance are called sedimentary or secondary clays. Compared to primary clay, secondary clays are much finer and easily workable. Clay is considered a fine-grained sedimentary rock. Clay's internal structure is very fine and difficult to examine.
Left: Granite (pink color caused by feldspar); Right: Break down product of feldspar, clay
Pure clay is usually almost white, but other materials such as iron oxide often recolor parts of the clay. Clay is reddish, brownish, or tan in the presence of iron oxide. Clay also traps silt sediments carried by water turning the clay dark gray.
Logging Requirement:
Send me the answers to the following questions via my geocaching profile within a reasonable amount of time of logging your find. Use your observations and the educational notes to answer the questions to your best ability. "Found it" logs without the required email will be deleted. Happy caching!
- What color difference do you notice between the clay in the middle layer and the clay closer to the far top and bottom of the outcrop? Why do you think the layers are different in color than the middle layer?
- Estimate or measure the thickness of the layer containing iron oxide. How thick is the layer? What color is it?
- Are these clays primary or secondary? You can confirm your answer by wetting a piece of dry clay. If it's still fairly brittle it is primary. If it becomes soft and moldable it is secondary.
Pictures are most welcome! Just remember to not spoil the logging requirements.
Credit:
- The Practical Geologist by Dougal Dixon
- Intro to Ceramics by Robert Kibler, Richard McColl, and Mark Poore
- Clay by Wikipedia
- Lissie Formation by USGS
FTF goes to TexasWriter!
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