This is an Earthcache, which does not have a physical container, but provides a learning exercise and a chance to show that knowledge in this setting. Please send me the answers to the questions below to validate your log.
There is a large rock here that does not belong. It also teaches us several lessons in geology. Some are rare lessons, and another is one that is in full view here.
The big rock in the garden is Texas pink granite (we will get to that in a minute) and was one of many rocks that shook off the flat cars of the HT&C Railroad when they were being transported from Granite Mountain in Burnet County to Galveston. These rocks were used in the building of the Galveston Seawall which was started in 1902 and finished in 1904. The train passed through Burton & Brenham dropping rocks along the way! It should not be here, as it is not mined here, nor was it used in local construction. We are lucky that it happened to fall off, to teach us even more about geology.
Granite is an igneous rock formed from magma that consists mainly of quartz, mica, and feldspar with colors ranging from black to gray. It forms from the slow crystallization of magma below the earth's surface. The grains in granite are large enough to be seen with the unaided eye (more later). It is widely used in construction because of its characteristic massive, tough, and hard properties. Some popular uses are in sculptures, memorials, buildings, and curling stones necessary for the sport of Curling. Granite is one of the rocks most prized by climbers, for its steepness, soundness, crack systems, and friction. A well-known venue for granite climbing includes Yosemite Park in the Western United States.
The Granite here came from the city of Marble Falls, Texas where operations began in the 19th century. The granite is also known as Sunset Red Granite which has a distinctive color. It is an enormous granite pluton rock formation located in the Llano Uplift, which is a circular geologic dome of Precambian Rock, primarily granite in Central Texas. The area is rimmed by limestone ridges. It is termed an uplift due to the raised status of the rocks in comparison to adjacent, completely buried rocks. The ancient faulting that produced the uplift may have happened around the time of the Ouachita Orogeny. It's like an island of granite in a sea of limestone. Most granite is primarily white, grey, and black. The color here is what makes it uncommon, the rarest granite color. Also, since Texas is covered by so much limestone, it is very unusual to find much of this color of granite. The rarer feldspar gives it the distinctive color.
Texture relates to how large the individual mineral grains are in the final, solid rock. In most cases, the resulting grain size depends on how quickly the magma cooled. In general, the slower the cooling, the larger the crystals in the final rock. Because of this, we assume that coarse grained igneous rocks are "intrusive," in that they cooled at depth in the crust where they were insulated by layers of rock and sediment. Fine grained rocks are called "extrusive" and are generally produced through volcanic eruptions.
Grain size can vary greatly, from extremely coarse grained rocks with crystals the size of your fist, down to glassy material which cooled so quickly that there are no mineral grains at all. Coarse grain varieties (with mineral grains large enough to see without a magnifying glass) are called phaneritic. Fine grained rocks, where the individual grains are too small to see, are called aphanitic. Basalt is an example. The most common glassy rock is obsidian. Obviously, there are innumerable intermediate stages to confuse the issue.
Fine Grain Coarse Grain
Logging Requirements:
1. Explain the depth of red color in the rock. Is it dark red, light pink, or in between? What is the reason for that?
2. What is the size of the grains primarily in the stone? Are they closer to the coarse large grains >5 mm, medium, between 1 and 5 mm, or fine <1mm?
3. Why is it so unusual to find this rock in Texas, or especially in this area?
Please message or e-mail me your answers. Logs without the supporting proof will be deleted. Please do not post answers in the logs. Go ahead and log after sending. I will contact you if there is an issue.
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Placed with full support and permission of the Brenham Chamber of Commerce.