In Texas, United States
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This is a relatively easy hike but at least one creek crossing will be necessary. Be gentle to the land. Follow trails as much as possible.
Chalk Ridge Falls Park, just downstream of the Stillhouse Hollow Dam, is a fascinating place of rugged terrain and flowing water managed by the Army Corps of Engineers (visit link) This is a free area that is open from 8:00 am - Sunset. Dogs, glass containers, alcoholic beverages, bicycles, nor firearms ARE NOT ALLOWED in the park. There are over 5 miles of hiking trail and a number of worthwhile geocaches in the park, including another Earthcache: GC26DNK : CTU – WATER FROM A STONE (Edwards Aquifer).
Except for a little topsoil, the exposed rock of Chalk Ridge Falls Park is comprised of Edwards Limestone. In brief, the Edwards Formation consists of massive limestone beds with bands of chert nodules. The limestone is greatly attributed to “rudistid biostromes” (e.g. fossilized reefs, especially the corals). Various marine fossils are common.
Limestones are typically formed in marine environments and are one of the commonest rock types worldwide. The primary mineral that limestone is made from is calcite, with the chemical formula CaCO3. Oddly enough, most other minerals are made up of silicon, Si, and oxygen, O, often combined with aluminum, Al. One of the best known and commonest of these is quartz, SiO2. Quartz and calcite do not have any common chemical bonds, so they do not combine chemically and form new compounds. Calcite is a relatively soft mineral that breaks down easily, both chemically and mechanically. Quartz, on the other hand, is very hard and resistant to both mechanical and chemical weathering. This is why most ocean beaches are essentially quartz sand: all the other common minerals have been broken down and weathered away.
The above facts lead us to this Earthcache. If you look up a geologic description of the Edwards Formation, there is usually a line saying the ‘. . . Formation contains chert nodules.’
So what is chert? Chert is rock made of microcrystalline quartz. What does ‘microcrystalline’ mean? It means that the crystals are too small to be seen without the aid of a microscope. Nodule means a little lump or knot. Flint is another name for chert found in association with limestones.
When you got to the coordinates, you will find a cliff face of weathered limestone. Embedded in this cliff face are harder lumps which aren’t weathering. These are the ‘chert nodules’ mentioned in the descriptions of the Edwards Formation. Search around the foot of the cliff and you can find some broken pieces of these nodules. Looking at the exposed interior, the material appears glassy and is light to very dark grey in color. If you examine the broken areas even closer, you can see that the breaks are often circular and that the edges can be very sharp. Indeed, the Native Americans and other ‘primitive’ people worldwide used chert nodules as one of the main materials for their stone tools.
What is a bunch of chert nodules (made up of very tiny quartz crystals) doing in limestone, made up of the very different mineral calcite? As mentioned above, most minerals are made up of silicon and oxygen. All water, except man-made distilled water, contains various dissolved minerals in solution. Since silicon and oxygen are so common, ‘silica’ (which is essentially dissolved quartz) is a relatively common solution mineral, even in the lime rich waters that form the lime muds which eventually become limestones. However, there is nothing in limestone for the silica to chemically bond to. Birds of a feather, flock together – and so do minerals that share an ineffable ‘chemistry.’ Minorities in the limey mix, when two silica ions meet in the limestone, they bond together. Any further silica ions that come into contact with them also bond. Over time, these silicate compadres form a neighborhood, and a chert nodule is formed.
Pictures are not required but we would appreciate them!
THE TEST ================================================
So. To log this cache, email us the answers to the following questions.
1> As an estimate, how much of the exposed cliff face do you think is comprised of chert nodules?
2> DO NOT TRY THIS ON ANYTHING YOU VALUE! Take a piece of the chert and try to scratch various common objects like a penny, a steel knife blade, a piece of the unweathered limestone, glass, whatever strikes your fancy. Do this for at least three different objects and email me the results of what scratched what.
3> Based on the ‘scratch test’ in 2>, would you consider chert to be very soft, soft, medium, hard, or very hard? Why?
LOGS WITHOUT VERIFYING EMAIL WILL BE REGRETFULLY DELETED (though not without due notice).
We don’t demand it, but pictures with your logs are encouraged (of you and/or the scenery … whatever strikes you as NEAT!), as are logs with actual content, telling us about your adventures in the park and what you thought about this earthcache worthy spot.
(No hints available.)
Last Updated: on 8/11/2013 3:20:41 PM Pacific Daylight Time (10:20 PM GMT)
Coordinates are in the WGS84 datum