EARTHCACHE REQUIREMENTS:
Each cacher must send in their own answers when logging this find. Review the materials/lesson below and answer by message/email the questions to best of your ability. Questions are listed after the lesson. Please follow all logging requirements and do not post any answers with your log. Failure to send answers risks deletion of your find.
ATTENTION: There are gates along the road on either side of Brushy Creek that will be closed in the event of high water/flooding in the area. DO NOT attempt this earthcache if there is closure for your safety and those with you.
CHISOLM TRAIL:
The Chisolm Trail meandered through Texas to Kansas, approximately 1000 miles, with numerous branches of trail feeding into it along the way. Just over 500 miles of the Chisholm Trail traveled through Texas. It was used primarily between 1867 to 1884 as the route herds with mainly longhorn steer. Traveling across the land with cattle, wagons and horses, these riders looked for the best and safest way to travel through the sometimes unforgiving landscape. This location shows several prime examples of how the land and formations would help serve as a guide for cowboys to keep the steer, wagons and other cowhands moving in the right direction. The Round Rock served as the landmark for a low water crossing across the Brushy Creek, with a safer route to travel. It still serves as a reminder of the historic past and the city was named after it.
Which brings us to Round Rock, Texas, location of the famous Round Rock and wagon wheel ruts. If you stand atop the pedestrian walkway that crosses over Brushy Creek, look down and you will see an amazing creation of nature....the Round Rock. If you stand along the path that follows Brushy Creek, you will notice how nature, weathering and the environment has affected this incredible stone. As the name implies, it has a an oval/round appearance, hence it's name. Interesting is the fact that this rock and the land surrounding it, as well as a vast area of this state is comprised of limestone.
LIMESTONE:
Limestone is a sedimentary rock, having formed from the accummulation of shell, coral, algal and organic debris. It can also form by a chemical sedimentary process, such as precipitation of calcium carbonate originating from lakes or ocean water. In terms of hardness, it ranks at a 3 on the Mohs Scale (1 to 10), is relatively soft and scratches easy. Limestone is also porous in nature, with voids or tiny openings within its structure. This porosity is a result of mineral content within the limestone and how it is formed.
Limestone has two primary origins. One is by biogenic precipiation from seawater, the primary agents being lime-secreting organisms and foraminifera. The second origin is by mechanical transport and being deposited in preexisting limestones, creating clastic deposits. This includes travertine, tufa, caliche, chalk, sparite and micrite which are all varities of limestone.
STRATIGRAPHY:
Stratigraphy is a branch of geology concerned with the study of rock layers and layering. The rock formations at Brushy Creek is a subgroup of the Fredericksburg Group being under the Comanche Series. Specifically, it belongs to the Edwards Formation of the Fredericksburg Group. The Edwards Formation is made up of massive limestone beds with rudistit biostromes and bands of chert nodules. Three units may be recognized in the Round Rock area, include 1. A lower unit made up of white to dark gray coarse grained , thick to thin bedded, chert-bearing rudist biostrome. Other fossils in this portion of the Edwards Formation includes gastropods and pelecypods. 2. White to tan middle unit made up of coarse to fine grained, thin to medium blended, chert bearing dolomite. 3. An upper unit made up of white to cream, fine to coarse grained, think to medium bedded limestone with rudistid and caprinid bioherms. Rocks from the Edwards Formation were deposited in a variety of carbonate environments, on n an extensive, shallow water, medium to high energy, marine platform reef, lagoonal, shoal, basinal and supratidal. It is charachterized by carbonate grainstone and rudist bioherms and biostromes, periodit restruction led to depositing of intertidal facies rock and evaporation. Dolomitization and the persence of chert nodules are nocally important in this section. (1)
There are multiple variations in the color of limestone, which will give you an idea of their compositoin and formation. Since the color of limestone rocks has various influencing factors, which includes chemical composition and presence of impurities. There are generally three color formations which includes:
COLOR: FORMATION PROCESS:
White pure limestone with high calcium content
Gray presence of organic matter or fossil remains
Beige sand and mud deposits
WEATHERING:
Weathering also affects limestone, as it can on other rock formations, depending on their level of hardness. Weathering involves physical, chemical and biological process, acting separately or more frequently together to cause fragmentation of the stone, eventually transforming into sediment. On the other hand, disintegration and alteration of limestone's surface in its natural state and position is through hysical, chemical and biological process induced and modified by wind, water and climate. As discussed above, the hardness of limestone is a level 3 on the Mohs scale, so it can be affected easily by multiple types of weathering. Human impact, intentional or unintentional, can also leave visible reminders that limestone has a lower level of hardness. Sedimentary rocks, such as sandstone and limestone are more susceptible to weathering due to their porous nature and presence of minerals that more easily dissolve in water.
BIOLOGICAL:
Biological weathering refers to weathering caused by organisms, such as plants, fungi, bacterial microorganisms or animal impact.
FREEZE/THAW:
Freeze/thaw weathering can affect limestone. Water expands in a frozen state upwards of 9-10%. Additional water can seep into the cracks during this time. It is a cycle of freezing, thawing and repeating. With each resulting cycle, the crack grows. Over time, it will allow for the rock to completely separate.
PHYSICAL OR MECHANICAL:
Physical or mechanical weathering, is a process that causes the rock to break up without chemical change. The primary process is abrasion allowing the particles to reduced in size. Water is one of the main caues of physical weathering since it is a constant and ongoing movement of water against, around and over rock.
CHEMICAL:
Chemical weathering changes the rock's composition, transforming them then water interacts with minerals to create various chemical reactions. It is also a gradual and ongoing process.
ORGANIC or BIOLOGICAL:
This type of weathering refers to weathering caused by organisms such as plants, fungi, bacterial microorganisms or animals. Lmestone is weakened as these microorganisms by obtaining their nutrition through a combination of nitrogen from the surrounding air as well as minerals from the stone. These minerals may include silica, phosphorous and calcium. Stone is made more vunerable to breaking down as a result.
HARDNESS OF LIMESTONE AT BRUSHY CREEK:
Remember how this land was traveled by cowboys, longhorn steer, wagons and horses, physical evidence of the shoftness of limestone at Round Rock, is visible by looking for the reminders from this long gone era. Wagon wheel ruts are clearly visible in the limestone near the infamous Round Rock. When water levels are low, you will be able to see them from your location on the pediestrian bridge or you may want to walk to the trail that you can actually walk out and see them clearly. Years and years of travel, the wagon wheels would have followed over the same indentions made by previous wheels, deepening them a bit more with subsequent trips.
Stratigraphy Materials Credit: https://repositories.lib.utexas.edu/items/04e92365-0520-4479-98d7a1f5e2e5cd39
QUESTIONS TO BE ANSWERED:
1. Using the above chart, which color and formation is the limestone of the Round Rock and creek bed?
2. What type of weathering process has impacted this area of Brushy Creek? Explain how you arrived at this conclusion.
3. What do you observe at the Round Rock and Brushy Creek that supports limestone being a 3 on the Mohs Scale for hardness?
4. From the path that parallels Brushy Creek, what do you think the long term affect of weathering will have on the Round Rock? Describe what you see and your theory.
5. Observing the Round Rock itself and surrounding rock formations, do you see physical evidence of shell, coral or other fragments? Describe wht you observe.
6. Nice, but not necessary, post a photo of the Round Rock, wagon wheel ruts or any other interesting feature you see while at this site. You do not need to be in the picture if you don't want. (optional)