Cobbler’s Hole EarthCache

Welcome to Cobbler’s Hole, a geological gem on this beautiful part of the Pembrokeshire Coastline. The down-and-up folding of the visible rock here, which is faulted on the landward side, demonstrates in miniature the large-scale folding underlying the whole of the south of Pembrokeshire. 

Most of the rocks exposed at the surface of the Earth are sedimentary - formed from particles of older rocks that have been broken apart by water or wind. The gravel, sand, and mud settle to the bottom in rivers, lakes, and oceans. These sedimentary particles may bury living and dead animals and plants on the lake or sea bottom. With the passage of time and the accumulation of more particles, and often with chemical changes, the sediments at the bottom of the pile become rock. Gravel becomes a rock called conglomerate, sand becomes sandstone, mud becomes mudstone or shale, and the animal skeletons and plant pieces can become fossils.

Sedimentary rocks are formed particle by particle and bed by bed, and the layers are piled one on top of the other. Thus, in any sequence of layered rocks, a given bed must be older than any bed on top of it. This Law of Superposition is fundamental to the interpretation of Earth history, because at any one location it indicates the relative ages of rock layers and the fossils in them.

Figure 1: Horizontal layers in rock, as they would have originally formed

Layered rocks form when particles settle from water or air. Steno's Law of Original Horizontality states that most sediments, when originally formed, were laid down horizontally. This is usually because of the presence of water - you will notice that the top of water is always horizontal. However, many layered rocks are no longer horizontal. Because of the Law of Original Horizontality, we know that sedimentary rocks that are not horizontal either were formed in special ways or, more often, were moved from their horizontal position by later events, such as tilting during episodes of mountain building.

Figure 2: Vertical layers in rock - at some point since they formed, the angle of the rocks has changed

This earthcache has been placed to demonstrate a particularly extreme case of rock folding. In front of you here at Cobbler’s Hole you will see a section cliff where you can see lines in the rock very distinctly – the layers between each rock bed – but you will notice the layers are far from being flat as they would have been originally. The folding here at Cobbler’s Hole was caused by movement of plates in the Earth’s crust pushing against one another. This folding causes anticlines and synclines. These are the up and down folds that usually occur together and are caused by compressional stress. On top of the Old Red Sandstone are layers of younger rocks – limestone, millstone grit and coal measures from the Carboniferous period (354 to 290 million years ago).

Figure 3: The process of folding - anticlines, synclines, compression and layer age

Anticlines are folds in which each half of the fold dips away from the crest.  Synclines are folds in which each half of the fold dips toward the trough of the fold.  You can remember the difference by noting that anticlines form an “A” shape, and synclines form the bottom of an “S.” 

There is a very distinct break in the folds in front of you, where one part of the cliff has clearly moved out of place from the rest. The folding has caused a fracture in the strata so that the beds on one side of the fault are displaced relative to the other. There are three common types of fault you may see, and these are shown in a basic form below.

Figure 4: Strike-slip, where the offset is predominately horizontal, parallel to the fault trace.

Figure 5: Dip-slip, offset is predominately vertical and/or perpendicular to the fault trace.

Figure 6: Oblique-slip, combining both the above; significant strike and dip slip.

Please read through all the information on this page and look at the diagrams. Then turn your attention to the cliff face you see in front of you and send me the answers to the following questions. Don't be put off by the number of questions, they are all straightforward and you only need to put a few words answer for each - reading the text and looking at the photographs and diagrams will make it very simple. Ideally please send me answers via the email service rather than the messenger if possible. Please send the answers before you log your find, or soon after. If no answers are received, your log will have to be deleted to comply with the guidelines. I appreciate everyone has different levels of geological understanding so I only ask you to make your best attempt at the answers.

Section A – The Folded Rock

Q1) Looking from left to right, what comes first – a syncline or an anticline? How many of each do you see?

Q2) For any synclines, turn your attention to the most folded layers. What do you estimate the inside angle of the sharpest fold to be?

Q3) Now, any anticlines, again look for the most sharply folded layer. Again, what you do estimate the inside angle of this fold to be?

Section B – The Age of the Rock

Q4) Remind me where the oldest and newest rock layers are found.

Q5) Describe the layers. Do you see different colours and textures? Do you believe it is all the same kind of rock, and if not, what different rock can you identify and what characteristics of the rock make you think this is the case?

Q6) Estimate the thickness of the thinnest layer, and also the thickness the thickest layer. Why do the layers vary in size?

Section C – Rock Faulting

Finally, look at the large and distinct rock fault that is clearly visible (I have explained where to look earlier).

Q7) How high is it from the foot of the cliff to the top of the fault?

Q8) At what angle do you think the vertical break lies, and at what angle might it have been originally?

Q9) From the three general fault types listed above, which type is this fault?

A photograph of you with the coastline or St. Anne’s Lighthouse would be good additional proof of your visit. Please try and avoid including photographs that show the cliff face to discourage armchair logging.

Please enjoy your visit to the Cobbler's Hole earthcache.