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Entombed sea lilies and William Smith EarthCache

Hidden : 07/02/2008
Difficulty:
2 out of 5
Terrain:
1.5 out of 5

Size: Size:   other (other)

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Geocache Description:

This cache shows Bradford on Avons fascinating geological history. This pit was specially excavated in 2007 and is famous for its fossils.

The heyday of the canals was in the first half of the 19th century. From the 1850s onwards, trade was taken over by the railway. Whilst carrying out engineering surveys for the nearby Somerset Coal Canal, William Smith (1769 - 1839) the 'Father of English Geology' was able to work out and publish the order of strata he encountered in the excavations. He published lithographs of fossils from the Bradford Clay in his pioneering collection, which was probably made in this or a nearby quarry.

At Bradford, the Kennet and Avon canal follows the southern side of the Avon valley. The cache site is on the dip slope of the Cotswolds Hills in the mid jurassic forest marble formation, some 165 million years old.

This rock, some 24 metres thick consists of alternating limestones and clays. A thick layer of clay (Bradford Clay) occurs at this cache site above the limestones. The lower part of this clay is rich in fossil debris, derived from creatures that lived on or burrowed into the surface of the underlying limestone that formed the sea floor. Currents swept banks of broken shells around, breaking them down into shell-sand : lenses of more complete shells are found occasionally. Several different sea-floor environments are preserved in the rocks here complete with fossil fauna, buried by an influx of clay particles brought in on the current. This was probably the result of some sudden event, perhaps a river in flood, or the collapse of a pile of sediment nearby, maybe triggered by an earthquake.

The most famous fossil here is the sea-lily Apiocrinites Elegans which grew fixeed to the solid sea bed (Ancliff Oolite). The shell-sand grains that make up much of this rock were by then cemented together with calcium carbonate dissolved in the sea water to form a hard, irregular limestone pavement. This provided a variety of habitats for burrowers, borers, and encrusting sea-creatures, as well as free swimmers. The site was first described in the 16th century, when it was realised that the fossils at the base of the clay bed had, in life, been living on the underlying limestone and that the clay bed had buried his fauna. Fine specimens can be seen in the Museum in the town centre by the bridge.

Fossils you are also likely to find here are Brachiopods (like tiny clawshells), Rhactorhyachia, Digonella & Epithyris.

To claim the cache as a find please estimate and E Mail us the length and width of the pit.

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