Baby Bridge EarthCache
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When we see or read about geological formations, we usually consider time periods well before recorded history. It is indeed rare to see a "new" geological formation. This earthcache will afford you this rare opportunity because a natural bridge was born on Cibolo creek just a few months ago.
To log this geocache, complete the following earth tasks:
1. Attempt to identify the type(s) of rock composing the bridge. The following link might help: (visit link)
2. Read the article below and then identify the process(es) that likely created this bridge.
3. Observe the erosion around the bridge. Does it appear the bridge was formed from watershed flowing from above, from flash floods rising from below, or a combination of both?
4. Compare the erosion around the bridge with the structural integrity of the bridge span. Estimate whether the lifespan of the bridge (time before it collapses) will be months, years, centuries, millennium, epochs, eras, periods, or eons? Note: Please answer this question in your log. It will be interesting to compare your thoughts with other earthcachers!
E-mail to me your answers to 1-3 then log the cache as found:
Natural Bridges
A natural arch or natural bridge is a natural geological formation where a rock arch forms, with an opening underneath. Most natural arches form as a narrow ridge, walled by cliffs, become narrower from erosion, with a softer rock stratum under the cliff-forming stratum gradually eroding out until the rock shelters thus formed meet underneath the ridge, thus forming the arch. Natural arches commonly form where cliffs are subject to erosion from the sea, rivers or weathering (subaerial processes); the processes "find" weaknesses in rocks and work on them, making them larger until they break through. Following are specific processes which create natural bridges
Weather erosion: First, deep cracks penetrate into a sandstone layer. Next, erosion wears away exposed rock layers and enlarges the surface cracks, isolating narrow sandstone walls, or fins. Alternating frosts and thawing cause crumbling and flaking of the porous sandstone and eventually cut through some of the fins. The resulting holes become enlarged to arch proportions by rockfalls and weathering. Arches eventually collapse, leaving only buttresses that in time will erode. Many of these arches are found within Arches National Park and Rainbow Bridge National Monument in Utah.
Water erosion: Study a topographic map of Coyote Natural Bridge or Natural Bridges National Monument in Utah and you will see how a meandering river or gulch carved a shorter route through the rock under the arch. The old riverbed is now higher than the present water level. Some natural bridges may look like arches, but they form in the path of streams that wear away and penetrate the rock. Pothole arches form by chemical weathering as water collects in natural depressions and eventually cuts through to the layer below.
Cave erosion: Natural bridges can form from natural limestone caves, where paired sinkholes collapse and a ridge of stone is left standing in between, with the cave passageway connecting from sinkhole to sinkhole.
Like all rock formations, natural bridges are subject to continued erosion, and will eventually collapse and disappear. One example of this was the double-arched Victorian coastal rock formation, London Bridge, which lost an arch after storms increased erosion
Coastline erosion: On coasts two different types of arches can form depending on the geology. On discordant coastlines rock types run at 90° to the coast. Wave refraction concentrates the wave energy on the headland, and an arch forms when caves break through the headland. Examples are London Bridge in Victoria, Australia and Natural Bridge on Aruba. On concordant coastlines rock types run parallel to the coastline, with weak rock such as shale protected by stronger rock such as limestone. The wave action breaks through the strong rock and then erodes the weak rock very quickly. Examples include Durdle Door and Stair Hole near Lulworth Cove on the Dorset Jurassic Coast in south England. (Information from Wikipedia)
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Treasures
You'll collect a digital Treasure from one of these collections when you find and log this geocache:
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