Gneiss is a very widespread rock type, especially in the lower parts of the continental crust, but it is also a common rock on the surface in some places (Scandinavia, Canada, and other shield areas where crystalline rocks are not covered by a layer of sedimentary rocks).
The term “gneiss” was brought to scientific usage from the German language. It was originally a mining term, meaning a country rock in the Ore Mountains (Erzgebirge) which contained metalliferous veins. The term (gneist) was first recorded in print by Agricola (Georg Bauer) in his famous posthumously published book De Re Metallica which remained the most important mineralogy and mining textbook for the next two centuries. The book was published in 1556.
Most of the mineral grains of gneissose rocks are visible to the naked eye. Banding in this rock is a result of mineral segregation into separate, typically light-colored and dark-colored layers. Light-colored layer is usually composed of feldspars and quartz. These layers are noticeable and have a discrete zone of the minerals. Most important dark minerals are hornblende and biotite. Individual bands are usually 1-10 mm in thickness. Layers larger than that imply that partial melting or the introduction of new material have probably taken place. Such rocks are called migmatites. It is often difficult to distinguish it from migmatite because there is a gradational transition from one to another. Hence, terms like “migmatized gneiss” are commonly used. It is not well understood how the segregation takes place, but it must be the result of extreme pressure and shear stress deep in the crust.
Flow banding occurs anytime the rock has been subjected at one time to deformation slowly enough or in the proper thermal environment to enable it to "soften" and flow (move) in the indicated direction. Flow typically results in the alignment of mineral grains, which gives a rock a "streaky" appearance.
The fountain is a weathered pink granite-gneiss that forms the rim of the fountain, the steps to the fountain, and the adjacent benches. There are obvious signs of flow band and segregation zones that show nicely. This is the focus of this earthcache. You will also see signs of chemical weathering on the granite-gneiss.
To log this earthcache, please send the answers to my email or geocaching account:
1. Look at the benches to the south of the fountain. You will see pink orthoclase (a type of mineral) grains. How long is the longest one you see?
2. Describe the flow banding that you see on the fountain rim.
3. Find an example of segregation zones on the rim of the fountain. Describe what you see.
4. Post a picture of yourself (face not required) or a personal item at the fountain.