This earthcache will teach you about the impact organisms can have on the local geology. There is no physical container to find here; to log this earthcache, you will need to read through the earthcache lesson below, make observations based on the reading and what you see, and send us the answers to the questions below.
You're looking at layers of limestone that were exposed when this road was cut during development in 2014. The primary limestone layer in San Antonio is Kainer (Edwards) limestone, which varies in thickness but can be several hundred feet thick. It was formed during the early Cretaceous period, around 140 million years ago, when this area of Texas was covered by a warm, shallow sea. The stone is made of sediments and the remains of aquatic life. The road cut here displays how the layers of limestone differ from each other over the millions of years they were deposited.
What is biological weathering?
There are three types of weathering that can affect rocks. They can be physically affected by forces, they can undergo chemical changes, or they can be affected by living organisms. This last category is called biological or organic weathering. Living organisms can affect rocks in several different ways; this can in turn leave the rock more susceptible to additional physical or chemical weathering.
Plant roots
It's common for cracks in the rock to accumulate soil as leaf litter and other detritus accumulates and degrades into soil. Plant and tree roots penetrate the cracks seeking moisture and nutrients. As the roots grow, they can physically force the rock apart, widening the crack. Some plant roots also produce acids that can eat away at the rock surface, again providing nutrients to the plants.
Microbial activity
Bacteria, mosses, lichens, and algae have evolved to draw nutrients from rocks by breaking them down using organic acids. Lichen in particular can contribute to rock weathering; this is not a single organism, but actually a symbiotic colony of fungi, which secrete the chemicals that break down the rock, and algae, which subsist on the minerals producted by the weathering. The longer a rock has been exposed to the elements, the more likely it is that these organisms have attached themselves to the rock and worked to weather it. Their presence also creates a moist chemical climate that can accelerate weathering.
Animal activity
Animals can weather away rock as well. Some animals actually drill into rocks, boring holes in them to seek protection. Others may just burrow around rocks, digging up rocks or otherwise exposing rock surfaces to further physical or chemical weathering. Animals can also wear paths in stone by continually traveling over them, breaking rocks, and so on.
Logging this earthcache
To log this earthcache, email us or send us a message, and copy and paste these questions, along with your answers. Please do not post the answers in your log, even if encrypted. There's no need to wait for confirmation from us before you log, but we will email you back if you send a message or include your email address in the email. Group answers are fine; just let us know who was with you.
1. The name of this earthcache: Biological weathering at Cibolo Canyons
2. Describe the rock surface at the coordinates.
3. Name as many other potential causes of biological weathering you see at the coordinates. This would be the effects of living things that one can see with the naked eye, please. :)
4. From the description, we know sea life had the biggest impact in forming this rock. In your opinion, what organism has had the biggest impact on the rock at this location since its formation? Explain how you came to that conclusion.
Photos are encouraged but not mandatory. Please do not post any photos that reveal the details discussed in the logging questions.
Sources
Earth Eclipse - What is biological weathering?
University of Nebraska - Lincoln, Plant and Sil Sources eLibrary
Carbonate Geology and Hydrology of the Edwards Aquifer