What is a Natural Bridge?
A natural arch or natural bridge is a natural formation (or landform) where a rock arch forms, with a natural passageway underneath.
Natural arches commonly form where cliffs are subject to erosion from the sea, rivers or weathering (sub-aerial processes); the processes "find" weaknesses in rocks and work on them, making them bigger until they break through. Usually a softer rock stratum under the cliff-forming stratum gradually erodes out creating a rock shelter which eventually spans the width of the cap rock thus forming an arch.
The choice of "bridge" vs "arch" is somewhat arbitrary. The Natural Arch and Bridge Society identifies a bridge as a subtype of arch that is primarily water-formed. By contrast, the Dictionary of Geological Terms defines a natural bridge as a "natural arch that spans a valley of erosion.”
Geology of the Barton Creek cliffs
Rocks of the Barton Creek cliffs are mostly Edwards Limestone that were deposited 100 million years ago during the Cretaceous period when this part of Texas was submerged under a shallow ocean.
About 20 million years ago during the Miocene period, West Texas was uplifted as the Gulf Coast area continued slowly to subside. Resulting tensional stresses caused the rocks to fracture, and continued deformation of these fractures created faults when fracture-bounded rock units began to move thus helping with the creation of cliffs and canyons.
Logging Requirements:
To answer the questions, you need to go to the posted coordinates at the top of the cliff where the bridge is located. Use extreme caution as the bridge is at the edge of the cliff. This cache is not recommended for people afraid of heights. Please note that you don't need to step on the bridge to answer the questions.
The goal of this earthcache is to try to understand the formation of this bridge.
- Observe the bridge compared to the layer of rock that forms the top of the cliff. Are they at the same level? If not how many layers separate them?
- From the level of the bridge, observe the layers of rock immediately below on the face of the cliff. What shape do they have? Based on this observation, what type of erosion do you think created the bridge?
- Do you agree with calling this land form a "bridge" or would you prefer "arch"? Why?
Not required, but pictures are always appreciated.
If you are unsure about an answer, don't worry just put your best guess. The goal is for you to experience this unique geological formation.
References
Credits
A big thanks to Melquiades for introducing me to this unique location with the now-defunct BridgeToTerabithia geocache.