Natural Bridge EarthCache
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The coordinates will take you to the natural bridge located in Maquoketa Caves State Park, Iowa. Or you can park at the coordinates for parking and follow the sign. Please use the trail do not attempt to cut across country.
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 or bridges form from a narrow ridge, walled by cliffs. The ridge becomes narrower from erosion, the softer rock stratum under the cliff forming stratum gradually erodes out leaving the arch or bridge. Natural arches commonly form where cliffs are subject to erosion from the sea, rivers and streams, and weathering. These erosive processes find weaknesses in the rocks and work on them until they eat there way through forming openings, which are further eroded to form the final arch or natural bridge.
On coasts two different types of arches can form depending on the geology. On discordant coastlines the rock ridges run at right angles to the coast. Wave refraction concentrates the wave action on the headland, and an arch forms when caves break through the headland. On concordant coastlines the rock ridges run parallel to the coastline. Weak rock (such as shale) is protected by stronger rock (such as limestone). Wave action breaks through the stronger rock and then erodes the weaker rock very quickly.
Weather eroded arches:
1. Deep cracks penetrate into a sandstone layer.
2. Erosion wears away exposed rock layers and enlarges the surface cracks, isolating narrow sandstone walls, or fins.
3. Alternating frosts and thawing cause crumbling and flaking of the porous sandstone eventually cutting through some of the fins.
4. The resulting holes become enlarged to arch proportions by rock falls and weathering.
Water eroded arches:
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 under the bridge.
The natural bridge you are looking at was formed by cave erosion. Although not by paired sinkholes. If you could have visited this area eons ago you would be standing inside the cave looking out through the entrance. Overtime a sinkhole developed behind the entrance. When this sink hole collapsed the original entrance to the cave was left as the natural bridge. You are standing on the collapsed ceiling of the cave. The collapse nearly filled in the original passageway. If you look behind you, you will see were the debris from the collapse nearly filled in the entrance to the passageway to the remainder of the cave.
Natural bridges occur through out the United States and all over the world.
Like all rock formations, natural bridges are subjected to continual erosion and will eventually collapse and disappear..
Information for this cache was obtained in part from Wikipedia, the free encyclopedia.
To log this cache please include a picture of the natural bridge in your found log. Send the answers to the two following questions in an email do not include the answers in your log.
Information to answer the questions can be obtained from the information boards on site.
From what type of rock is this bridge formed?
What type of erossion is primarily responsible for the formation of this bridge.
Congratulations to Waterfallfrog for being first to visit this Earthcache.
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