Pebble Spotting On Abercorn Beach
Scotland has some exceptional beaches, often featuring pebbles of all shapes and sizes, which is why the European Stone Stacking Championships and the World Stone Skimming Championships are both held here. It's the varied Scottish geology that makes for a wide variety of pebbles on our shores.
On the face of it a simple question, but there are some formal geologist definitions for what a pebble is, and they define it as a clast (stone fragment) between 4–64 mm in diameter. However most pebble guides will describe a pebble as simply a small stone that fits in your hand, and feels good to hold. Pebbles can be further defined using geological colour charts such as the Munsell Chart, but also by the size of the particles they are made up of and whether those particles are visible to the naked eye or by magnifying equipment.
Pebbles are formed by different forms of Erosion. Most pebbles are local, having broken off from nearby rock exposures, and then been transported by rivers, sea currents, ice sheets or sometimes by people. As they have been moved along, often the corners have been knocked off and the rock has become gradually more smooth and rounded, a process referred to as Attrition. The waves on the beach itself can cause attrition, as pebbles are moved around by the tide and hit other objects. This is a comparitively fast process compared to the other factor that makes pebbles smooth, Abrasion, which is when the surfaces of two pebbles rub against each other, resulting in the wearing down of one or both of the surfaces. Abrasion takes a very long time to result in the pebbles you see here.
The waves help cause attrition and abrasion here on the beach, but those two processes were helping to form these pebbles long before they reached the beach. Many of the pebbles on our Scottish beaches, started life in The Ice Age, when Scotland was covered in a thick ice cap. When it eventually melted, ice flowed from higher areas down to lower lying ones. Flowing ice picks up rocks, moving them and shaping them, which is why Scotland is covered in erratic boulders – rocks that don’t match the local geology. As the ice flowed downhill towards the sea, it brought these new rocks to to the coast, but also changed their texture, shape and appearance on the way, breaking off pieces and rubbing the fragments into smoother textures along the way. The other factor that caused attrition and abrasion, and helped populate our beaches with such a variety of pebbles, is our many rivers and streams.
Well, rock of course. However, because of how the pebbles got here, that has determined what the beach pebbles are made up of. Some rocks are much tougher than others and when they are being ground down by a glacier or being knocked around by waves, crashing into each other, the tough ones survive. So the pebbles on the beach do not necessarily reflect the proportion of different rocks found in the local geology. For example, if the local area has lots of mudstone, you still won’t find mudstone pebbles dominanting the beach, because most would be broken up during the journey there. Below is a simplified geological map of Scotland, created by The Scottish Geological Trust. The local geology around the Firth of Forth would suggest that mostly igneous and sedimentary pebbles would be found on this beach.
Simplified Geological Map of Scotland
However due to some of the reasons above, a Scottish beach may have examples of any of the three types of rock – igneous, sedimentary and metamorphic – but in widely differing proportions, depending where the beach is.
Igneous rocks are created from cooling magma or lava, and as the liquid cools, crystals form, and these crystals lock together to make a very strong fabric. So igneous rocks tend to be tough and resistant to erosion, and as noted above, on the beaches - only the toughest survive - then you won't be surprised to learn that igneous rocks are often found as pebbles on Scotland’s beaches such as this one. There is a wide variety in the igneous pebbles here. Crystal size is a good way of identifying the type of igneous rock. All igneous rocks have cracks in them, a lot of these cracks (called joints) form as the rock cools and contracts. The joints in an igneous rock are its weakness and often control the shape of pebbles. Since igneous rocks don’t normally have layering, igneous pebbles are not usually flattened.
Pebbles made from sedimentary rocks are comprised of many layers of sediment piled on top of each other, then gradually buried and cemented. The layering of sedimentary rocks generally results in flat pebbles. The weak bonds between grains of sediment can sometimes mean that sedimentary rocks are softer than other types, and more easily worn away. There are exceptions however, such as limestone which has a structure that makes it a tough sedimentary pebble. Other than most sedimentary rocks being softer, the other way to know if a rock is sedimentary is if it has a fossil. A sedimentary pebble has grains that are cemented together, often with visible layers, or unique features like mud cracks or ripple marks.
As the map shows. Metamorphic rocks are common in the Highlands of Scotland but not so much here along the Firth of Forth. They are formed when existing rocks got buried and heated, causing chemical reorganisation to form new minerals. For this reason, Metamorphic rocks tend to be very tough and hard. So they may survive to become beach pebbles, but not in large volumes here near the Forth, unless originally carried near here by a glacier.
Metamorphic rocks tend to have more luster than igneous or sedimentary rock. If you notice any major grooves or 'ribbons' in a pebble, you are likely dealing with metamorphic rock. These ribbons might be very slight, but they will look like clearly defined veins throughout the rock. These aren’t like the layers of sedimentary rock, which have a texture to them, and appear as though the rock is made of stacked pieces.
You can see examples of each of these 3 types of pebble below
With your knowledge of pebbles and the processes involved in their creation, here are the questions you need to answer to claim this Earthcache as a find. There's also a task for you do at the end.
1) Pick up a handful of pebbles, make them as varied in appearance as possible, and take them to a nice uncluttered area, perhaps the grass nearby. What are the most common shapes? Why do you think that is and what caused it?
2) Do any of your pebbles have layers of different colours running through them? What type of rock is that?
3) Do all the pebbles you found reflect the geology of the Firth of Forth area? How do you think pebbles that are not reflective of the local geology, got here? A quick look round would perhaps point to a possible causes for that.
Please send your answers to me via the Geocaching App, or use the email function. I may not respond to every set of answers but if I don't you can assume it's because the answers are correct. As ever with Earthcaches, please send your answers before logging the cache. Those failing to do so may have their logs deleted.
As an optional task, look W/NW to a bearing of about 290 degrees (somewhere between 9 and 10 o'clock if north is 12 o'clock). You will see a prominent structure. Recreate it using some pebbles and stones on the beach, and put your photo in your log. Extra points if you get a shot of the prominent structure in the background of your photo. To reiterate, this is optional and not essential to claiming the cache as found.
Now put any pebbles you picked up, back, and if the tide is in, you might want to do it in a particular way - the cache title might give you an idea how.
This Earthcache is created using content from The Scottish Geology Trust Beach Pebble Guide, and with the help and encouragement of Clive Mitchell of The British Geological Survey, author of The Pebble Spotters Guide. Sincere thanks to both for helping to make geology more accessible to someone new to the subject.