The Elan Valley Estate was created in 1892 by an Act of Parliament to facilitate the creation of a series of reservoirs to provide drinking water to Birmingham. It encompasses approximately 80 square miles of very sparsely populated, most upland moorland, providing countless opportunities for walking, birdwatching and simply enjoying the stunning views. This EarthCache brings you to one of the busier locations on the Estate, close to the Elan Valley Visitor Centre. Various facilities are available here, including parking, a cafe, bike hire, picnic areas and free WiFi (mobile phone signal is generally very poor or absent across the Estate).
The rocks underlying the Elan Valley Estate and much of the surrounding upland areas were formed from sediments laid down under water during the late Ordovician and early Silurian periods, more than 400 million years ago. (Coincidentally, both of these geological periods are named after two of the British tribes living in Wales during the Iron Age and the Roman occupation.) Over time, the sediments grew so thick that they became unstable and collapsed in the manner of underwater avalanches, leading to large delta fans of material on the deeper sea bed and called 'turbidites' after the turbidity currents that created them. Individual turbidite beds are characterised by grading or sorting of the material within them, typically with the heavier sediments (large rocks or stones) at the bottom and the lighter ones (sands, silts and clays) nearer the top. The consolidation of these fragments of material within a matrix that holds them together is termed a conglomerate. Well-sorted conglomerates contain sediments of the same or similar size while poorly-sorted examples display a mixture of different sized sediments close to each other. The process by which turbidites are formed is illustrated below (acknowledgements to the Elan Valley Trust).
Over millions of years, the water between the material was squeezed out, crystals formed and the whole mass was transformed into an especially hard type of rock called a conglomerate. The rocky outcrops either side of the Caban Coch dam are formed of this material and its very hard and tough quality was the reason why the dam was constructed here. The local rock was used for a variety of purposes during the construction of the dams and reservoirs and examples can be seen as capstones on the various pillars and walls beside the roads that pass through the Estate.
The coordinates for this EarthCache take you to the southern side of the Caban Coch reservoir dam. Access is via a pleasant woodland path with one steep section, meandering up from the stream at the base of the dam, along from the Visitor Centre. At the top of the path, you will reach a square area, bounded on three sides by walls constructed when the dam was built. Examine the pillar on the right-hand, south-easterly corner of this square and consider the following questions. Your answers should be sent to us via either email or the Geocaching message service.
1) The capstone on the pillar provides a fine example of the characteristic 'turbidite' type of conglomerate from this area. Describe the colour, texture and size of the largest stone you can find embedded in the matrix.
2) Now examine the smaller pieces of stone in the matrix and compare the colour and dimensions of these with your answer to Q1.
3) Using the description given in the text above, is this a well- or poorly-sorted conglomerate?
4) Based upon your observations, do the sediments you have found appear to have come from the same type of rock or a variety of different types? How consistent is this conclusion with the process by which turbidites are formed?
5) (Optional) By all means include a photograph of the beautiful scenery or surroundings of this EarthCache but do not reveal the answers to any of the questions.
The cliff face behind you, to the south of the dam, shows a large exposure of the turbidites. You should admire this from a distance and not approach the base of the cliff as rock falls can occur at any time and without warning.
We gratefully acknowledge the permission of the Elan Valley Trust and Dwr Cymru for the placement of this EarthCache.