Glamorgan Coast Geology – Witch’s Unconformity Mystery Cache

This cache has been archived.

Write and Mane: The recent rule change has removed the essential point behind this cache, which was never intended to be a 'cache-and-dash'.
It is hoped to resurrect the underlying objective as an Earthcache soon. The frustrating thing about this is that the series was planned when Earthcaches under the umbrella of Geocaching were okay but, just before the first cache in the series was released, the rules were changed to exclude them. The result was the less than satisfactory compromise that had to be used.
So two rule changes have forced us back to where we wanted to be in the first place ... but we could have done without all the avoidable hassle in between!

More
Hidden : 12/9/2006
Difficulty:
2 out of 5
Terrain:
2.5 out of 5

Size: Size: small (small)

Join now to view geocache location details. It's free!

Watch

How Geocaching Works
Looking for a different adventure?

Please note Use of geocaching.com services is subject to the terms and conditions in our disclaimer.

Geocache Description:

One of a short series exploring the geology of the Glamorgan Coast, this one looks at features on the foreshore at Dunraven Bay, including an unconformity. The cache is a small rectangular plastic container. Expect to walk about a mile, mainly on a foreshore which varies from sand to cobbles to rock platforms. Access is restricted to the low tide period.

DO NOT BE FOOLED INTO THINKING THIS IS A QUICK CACHE AND DASH – MAKE SURE YOU READ THE STING IN THE TAIL!

An unconformity is a time gap in the rock record. At Trwyn y Witch, the headland that forms the south-east side of Dunraven Bay, Sutton Stone rests upon Carboniferous Limestone. The latter dates from the first phase of the Carboniferous Period, about 350 million years ago, while the former was laid down at the beginning of the Jurassic Period some 200 million years ago. The 150 million year time gap means that rocks like Milestone Grit and the Coal Measures of the later Carboniferous Period, that can be found not many miles to the north, are missing. The whole of the Permian and Triassic Periods are also absent including, for example, the sandstones, shales and red marls found between Lavernock Point and Penarth.

In general, the gap in the rock sequence may be a result of the material never having been deposited in the first place, because the appropriate conditions never existed in this location, or the rocks were formed but then eroded away. In this case, it is probably a combination of the two. At the end of the Carboniferous Period, about 300 million years ago, continental collision caused considerable squeezing and folding and the Vale of Glamorgan was uplifted into a series of east-west trending folds. These were deeply eroded to form a chain of craggy hills. Sea levels rose to surround this upland area, with Triassic rocks being deposited around the margins, and then followed a major inundation during the Jurassic, when the Blue Lias was formed. By this time, the higher hills of the Vale of Glamorgan were just islands in the sea. At Trwyn y Witch, the initial stages of this rise in sea level would have seen the small limestone hill being planed off on the shore. Following complete inundation, deposition occurred over it – firstly the fairly massive bed of creamy white Sutton Stone, then the Southerndown Beds followed by the Blue Lias. If you look directly at the cliffs from Waypoint F1 and use the appropriate image you should have no difficulty picking out the unconformity.

There are fossils and other structures worth looking for while you are on the beach. At Waypoint F2, the Carboniferous Limestone of the lower cliff is heavily folded: at Waypoint F3, the Blue Lias beds are severely distorted at a complex fault where they abut the Sutton Stone, which here is draped over the Carboniferous Limestone: at Waypoint F5, a fault forms a dogleg in the cliff and then runs roughly westwards through the rock platforms and under the beach. As you cross this last fault on the rock platform, it may look like any other joint crack. But if you look carefully, it can be seen that the beds in the two faces do not match. Also, in the joint itself, fragments of rock have been ground at the interface and ‘welded’ onto the sides. Fossils are plentiful in the rocks: look out for cross-sections of the bivalve Pina, with its characteristic diamond shape (Waypoint F4): there are also bigger Ammonites (up to 200-300mm across) to be found (Waypoint F6).

*** PLEASE HEED THE FOLLOWING WARNINGS ***

1. The features noted are below high tide level, so 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 Porthcawl.

2. Take care on the foreshore: the rocks may be slippery, especially when wet. The cliffs in this area are very unstable, so stay well back from their base.

3. The foreshore immediately below the car park consists of cobbles and boulders – using the slipway down to the sand is advised.

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.

There is a pay car park at the Waypoint Point given below although, if you pick the right time/season, you may enjoy a free stay. Alternatively, you can park freely in the villages of Southerndown, St Brides Major or Ogmore and enjoy a longer walk. Between the car park and the cache location is the Glamorgan Coast Heritage Centre where, amongst other things, there is a small but relevant geological display. Unfortunately, the opening hours are limited to three days a week, plus times when there is a ranger resident.

There is also a regular cache hidden in the vicinity, at the reference coordinates of the cache listing. The cache had to be placed above high water mark, making it potentially a quick cache and dash.

However, we want people to explore the geology, so you should include an image of a large Ammonite (at least 150mm across) when logging a find.Just to confirm that from January 2009, any logs that do not have the required image will be deleted.

Additional Hints (No hints available.)



Advertising with Us

Inventory

There are no Trackables in this cache.

View past Trackables

What are Trackable Items?

Bookmark Lists

View all bookmark lists...