Glamorgan Coast Geology 5 – Erosion EarthCache
Glamorgan Coast Geology 5 – Erosion
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One of a short series exploring the geology of the Glamorgan Coast, this one looks at features on the foreshore around Penarth Head and considers an important geological process, erosion. Expect to walk about a mile, mainly on a pebbly/rocky foreshore.
But isn’t erosion destroying the geology? Well, yes – but the eroded material is deposited as sediments to create the rocks of the future. In fact, most rocks are sedimentary and the material of which they are made has been re-cycled many times. Re-cycling may seem like a relative new concept to us, but nature has been doing it since the earth was first formed. Usually, geology thinks in terms of millions of years and it is hard to visualise the processes in real time. But erosional changes can be relatively rapid. As you walk along the foreshore under Penarth Head, listen! Within the time span of this short walk, you will hear small stones falling down the cliff … or maybe something more spectacular! The main driver of erosion here is not the sea, but simply the action of the weather, wetting, drying, freezing and blowing. The main function of the sea is to wash the debris away: if it didn’t, the material would accumulate until it reached its natural angle of repose, vegetate over and stabilise the situation. You can see where this has happened behind the former dock office.
We must apologise at this point, as we are about to lower the tone of the cache listing! At the base of Penarth Head are the remains of a stone bastion, constructed to protect manholes on a sewer outfall about 100 years ago – but don’t worry, the sewer is no longer in use. At the time of construction, the sewer down to the foreshore was built in tunnel, well behind the cliff face: today, you can see part of it exposed about 8-10 feet above the north side of the bastion remains. This gives a clue to the rate of erosion here: in a century, the cliff has receded more than 10 feet. What price a “des. res.” on Penarth Head now? If this rate of erosion continued for a geological unit of time, a million years, Penarth, Barry and a lot of villages beyond would have gone, but Cowbridge might still be around! A further indicator of the rate of erosion can be found at the groyne which is crossed at Waypoint 3R. At the cliff end of the groyne is a gap of about 6.00 feet (Image 3), representing the erosion that has occurred since the groyne was constructed. If you take the date on the step where you cross the groyne as the date of its construction, you can calculate the more recent rate of erosion. THIS IS PART OF THE CHALLENGE! YOU NEED TO FIND THE DATE, AND DEDUCE THE RATE OF EROSION.
The recommended approach is from the Barrage car park (Waypoint 1P) - currently 50p for an hour, which should be long enough to do the cache. Alternatively, you can park at the roadside above Penarth Marina for nothing (Waypoint 2P). Access to the foreshore is from the corner of the Barrage car park (reference coordinates of the cache listing), along the outside of the fencing of the car park for the old dock office and Custom House (now a restaurant). There are steps to facilitate crossing over the groynes on the foreshore. While traversing the foreshore, look out for pinkish gypsum in boulders up to about a ton in weight (Image 5), ripple marked rocks (Image 4), a variety of fossils (bivalves, oysters, ammonites, etc) in rocks from the Blue Lias and examples of Radyr Stone (Image 6). The lower cliff here is composed mainly of black shales and thin bands of limestone of late Triassic age: the well-bedded grey limestones and shales at the top of the cliff are from the Jurassic Blue Lias. The boundary between the two geological periods represents a time of about 200 million years ago. The best examples of ripple marks will be found in thin sandstones from the end of the Triassic Period. Radyr Stone is a dark red breccia (coarse conglomerate) also from the Triassic, but not present naturally in this location. However, it has been a popular building stone throughout the centuries, especially in large structures requiring strength and durability – look for it in bridges, dock walls, wall capping (for example, on the Roman walls of Cardiff Castle), steps, etc. Here you will find it in the remains of the bastion (Image 2). Gypsum is also known as alabaster and is the main constituent of plaster of Paris: but perhaps it is seen to best effect where it has been used for delicate carvings which show off its translucency.
*** PLEASE HEED THE FOLLOWING WARNINGS ***
1. Only attempt this cache when the tide is in the lower half of its cycle. You can check tide times at several web sites e.g.
(visit link)
… and get the prediction for Cardiff.
2. Take care on the foreshore: the rocks are generally loose and may be slippery.
3. Do NOT attempt to measure the gap between the groyne and the cliff, use the figure given above.
To claim this cache you must comply with all of the following three conditions:
1. Upload with your log an image of Radyr Stone, with you or your GPS in the picture. As a result of a rule change effective from 01/01/11, this requirement is now optional. However, we hope that cachers will continue to upload photographs voluntarily to enhance the quality of their logs for the benefit of all who view this listing.
2. Upload with your log an image of a gypsum boulder, with you or your GPS in the picture. As a result of a rule change effective from 01/01/11, this requirement is now optional. However, we hope that cachers will continue to upload photographs voluntarily to enhance the quality of their logs for the benefit of all who view this listing.
3. Email to us the date on the groyne step at Waypoint 3R and the rate of erosion that has occurred since this date.
Please do NOT include the answer to 3 above in your log.
If you are looking for more information on the geology of the Glamorgan Coast, “Walking the Rocks” (£7.95 from the National Museum of Wales) outlines six exploratory walks, within the overall context of the geological sequence and timescale.
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