Saturday, 16 May 2015

Focus On: The Geology of Wenlock Edge

Wenlock Edge is one of Britain's important and famous geological sites, which has even given it's name to an epoch of geological history.

The Edge is a limestone cuesta, meaning it has a steep escarpment on one side and a gentle "dip slope" on the other. This landform is created because when limestone is formed it is usually in horizontal layers or "beds", but tectonic forces from plate movements cause the beds to be pushed up at an angle. The dip slope is the top surface of the bed, while the escarpment is a slice through the bed. You can see this in the diagram below:

The limestone that makes up Wenlock Edge was deposited around 425 million years ago, during the Silurian period. At this time the UK was located in the southern tropics, not far from where Tahiti is now, and the climate was warmer and the sea levels higher. The Wenlock limestone was formed by a coral reef, and has beds made up of reef detritus, nodules precipitated from the sea water and small "patch" reefs.

Knowle Quarry, on the Lime Kiln Walk.

The Wenlock Edge limestone also contains numerous fossils associated with the coral reef. These include the corals themselves, as well as crinoids, brachiopods, gastropods and trilobites. Crinoids, or "sea-lilies" are similar to starfish but were attached to the sea floor by stalks; it is very rare to find intact crinoids but whole beds of limestone can be made up of their segments, which look a bit like Polos. Two types of coral are found at Wenlock Edge: Rugose, which are a solitary coral, and Tabulate, which live in colonies.

Fossils of Wenlock Edge: A - intact crinoids, B - tabulate coral, C - rugose coral, D - gastropod, E - trilobite & F - brachiopods.

Please be aware that fossils must not be hammered out of the rocks, and if one is found in a rock face permission must be obtained from the landowner before extracting it and removing from site. Loose material may be collected but any large specimens should be left for the enjoyment of others. If unsure, please check with the rangers before removing the fossil.

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