These boulders are prized by many rock climbers in Alabama as a unique and special feature of this area that affords many different types of rock climbing practice. People often come to Moss Rock Preserve to practice their rock climbing techniques and rope belay skills! The park is open from sunrise to sunset, so please plan your trip here accordingly and give yourself plenty of daylight to complete the tasks.
Tasks
There are four tasks you will need to complete this earthcache. Please send me the answers to the following via the "Send Answers" button before you log your find:
1. In your own words, briefly describe the way a boulder is formed.
2. What do we know about this area millions of years ago, based upon this boulder's strata?
3. Looking at the boulder, how clearly defined are the two sets of sand waves? Can you tell where each new sand wave system begins? Describe your analysis of this example.
4. Take a picture of yourself or your chosen totem item in front of the boulder structure here that demonstrates the cross-bedding. Post that picture with your log!
EarthCache Lesson:
Recipe for Baking a Boulder
Ingredients:
- Solid rocks, pulverized (quartz, feldspar, and other lithic grains)
- Organic materials (shell fragments, plant fragments) (OPTIONAL)
- Minerals (silicate, calcite, clay or iron oxide)
- Water (mineral water preferred)
Tools:
- Basin, like a sea bed or river delta
- Compactor, for pressurizing
- Timer (must be able to handle thousands to millions of year increments)
1. Let's begin by chopping up the quartz, feldspar and other solid rocks into the tiniest possible fragments called sediment. Chopping can be done by water or wind . . . and so will probably take you a very long time to get very far, but the end result will be quite finely chopped, so patience is key and just trust the process.
2. After you have accumulated several tons of sandy sediment, preferably harvested from the tops of mountains and transported by wind and water, transfer them to your depositional basin. The depositional basin can be any nearby basin, like a sea bed or a river delta, where water and gravity will help you gather a large amount of sediment without having to do much except be patient.
3. Once you have your sediment, mix those together with some mineral-rich water, enhanced with minerals like silicate, calcite, or iron oxide. These minerals will help bind your boulder together and make it quite strong. Try not to stir too much after this point so everything can settle. Over stirring creates tough muffins, but not so tough boulders.
4. Next, you will need to use your compactor to press the boulder under a great deal of pressure. Adding extra sediment on top is an excellent way to increase the pressure, as well as to create those beautiful layers you can see in a fully baked sandstone boulder. Add as many layers as you like, laying them down in waves that resemble the rolling patterns of the water waves that laid down the deposits. Set a timer for approximately 300 million years. During this time, your sandstone will undergo diagenesis and lithification, during which the rock will undergo chemical and mechanical processes that will make it less porous and solidify the rock.
5. Once the timer goes off, your stone is pretty much ready to go, but you will need to extract it from the Earth's crust. The quickest way to do this, and how geologists believe this particular boulder field came about, is through the forces exerted upon it by plate tectonics, which thrust this sandstone upward, in the process creating fractures that have over time weathered into smaller boulders.
6. Enjoy!
Cross-Bedding
In all seriousness, this is the basic process by which these sandstone boulders at the Moss Rock Boulderfields were created. At GZ, you see a very unique example amongst the boulders here in which you can see the erosional surface between two sand waves, also known as cross-bedding.
Cross-bedding happens when the sediments are laid down by currents. We can know by the presence of cross bedding here that there was a current, either a river or a coastal current, that was manipulating the sediments after they were laid down. The diagram above can demonstrate how the current creates these patterns in the rock.
Here is an example from another location in which you can see the directional change of the rock sediments in the stone:
Take a look at the smaller boulder in the path in front of you. Examine the patterns of the rock strata in this sandstone and answer the questions on the tasks at the top of the page.
References Cited:
Dr. Richard Carroll, Geologist, University of Alabama and the Alabama Geological Survey, provided on site analysis and discussion during a field excursion to the site.
Monroe, J.S.; Wicander, R.; Hazlett, R.W. (2006). Physical Geology: Exploring the Earth (6th ed.). Belmont: Thomson. pp. 203–204. ISBN 9780495011484.
Geology Is The Way. Cross-Bedding, by Clive Wardell. Michigan Tech. https://geologyistheway.com/sedimentary/cross-bedding/