This is one of a series of EarthCaches placed at sites within the Westmorland Dales area which the Cumbria GeoConservation group considers as being good examples of various aspects of the local geology. Our geological research has been made possible by the Westmorland Dales Landscape Partnership Scheme, jointly led by the Friends of the Lake District and the Yorkshire Dales National Park Authority and has been grant funded by the National Lottery Heritage Fund.
Brachiopods are shelled sea creatures. Although they have two shells like modern shellfish (e.g. cockles, scallops, etc.), they are not closely related. About 5 000 different species of fossil brachiopods have been identified, and about 100 species still exist today. While it is not possible to identify the exact brachiopod species here, they belong to a group called Productids. To imagine what they looked like in life, curve each of your hands to represent the two shells, and sit one hand on top of the other, both palms upwards. The body of the brachiopod would be in the small space between the palm of the lower hand, and the back of the upper one. In life, they sat on the sea bed, with the lower shell in the limy mud, that would later become hardened into limestone. The two shells were hinged together at one end and the other end could be opened for feeding or closed for protection. Inside the brachiopod was a structure called a ‘lophophore’, typically U-shaped, from which the name ‘brachiopod’ meaning ‘arm-foot’ is derived. The lophophore was used for filtering food particles from the surrounding seawater and also for gas exchange for respiration. A perfect cross-section of the two shells of a brachiopod would look a bit like this:
When brachiopods died, if the water was quiet, the two shell could stay together and they could remain in 'life position' i.e. embedded in the lime mud in which they were living. In slightly rougher water, many of the shells could be separated but remain in roughly the same place. In really rough water, or in a storm, most of the shells would be separated and could be picked up by water currents and dumped all together somewhere distant from where they were originally living. Geologists call this a ‘death assemblage’. When the shells are later fossilised, the original calcium carbonate (limestone) of the shell can be replaced by silica which is much harder.
Note: Please be very careful walking in this area, between about May and July, when the orchids are in flower.
Questions:
To claim this EarthCache, please send your answers in a message, either by e-mail or through the Message Centre. Do not put them in your log. You do not have to wait for a reply before logging; if there is a problem with your answers we will let you know. Pictures of your visit are very welcome in your log, but please avoid giving away any of the answers.
1. Briefly describe the three main beds of limestone here, including their approximate thickness, and identify where the brachiopods shells are to be found.
2. Describe the brachiopod shells. You can see the detail best where the shells are exposed on a corner of the rock. Is the outside of each shell smooth or ridged? What colour are they? Explain whether you think they still made of the original calcium carbonate or whether it has been replaced by silica.
3. State whether the brachiopods here are in ‘life position’ or whether they represent a ‘death assemblage’. Explain your answer.
4. Looking at the sketch of the two shells above, identify two features that would be present in the living animal that are not shown here.