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. The processes "find" weaknesses in rocks and work on them, making them larger until they break through.
The choice between bridge and 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."
On coasts two different types of arches can form depending on the geology. Ondiscordant 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.. When these eventually collapse, they form stacks and stumps. 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. When Stair Hole eventually collapses, it will form a cove.
Natural bridges can also 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.
As a special form natural archs can also form when a lava stream flows into the sea and it comes to magma extrusion and phreatomagmatic explosions. In an liquid environment like the ocean liquid lava behaves erratically and forms weird and funny structures. Then the numb lava (possibly in form of trachyte) often forms bizzare structures like rock needles or natural archs, like here. Sometimes in the structures certain objects or beings can be seen with imagination. Of course these rock formations are also suffering from erosion furthermore.
Your task to log this Earthcache as found:
Send your answers to the following questions via message to my account (in English or German):
1.) What's the (estimated) hight and width of the natural arch?
2.) What kind of rock builds up this natural arch in your opinion? Give me a short description of the rock (surface, colour, texture...)!
3.) Give me a short description (in your own words) - how do you think this natural arch has been formed?
Optional you can post a picture of yourself or your gps in front of the natural arch, if you like!
Attention: There is no need to wait for a log permission! You can log the cache as found after you sent me the answers - if there's something wrong, I'll contact you!
Have fun and enjoy the location! Silberschakal
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