We were visiting the Old Station in Tintern, and right in the middle of six wooden statues there is an interesting stone. There are lots of Earthcaches where you have to work out if it's a Conglomerate or a Breccia - here I'm going to tell you it's a conglomerate, and you will tell me why that is.
There is a fee to park at the Old Station Car park, though this Earthcache can be approached from any number of public footpaths if you have parked elsewhere.
1) Look at the individual stones within the large - are they sharp or are they rounded? Why does this tell you it's a conglomerate?
2) Using your answers to question 1, does this mean they have travelled a long or short distance - how does this influence the shape of these smaller stones?
3) Look at the individual stones within the large stone - do they look the same, what colours can you see?
4) Using your answers to question 3, is this a monomict or polymict stone?
5) Take a photo of yourself at GZ , or an identifying item, with your favourites of the statues.
Please submit your answers via message though the Geocaching website or by sending me an email - there is no need to wait for a response before logging your find.
Conglomerates
Conglomerates are like natural concrete made from rounded stones. Imagine smooth river rocks that have tumbled downstream for ages, getting all their sharp edges worn off. These rounded rocks then get stuck together by natural glue (like sand or mud that hardens), forming a new, solid rock. So, if you see a rock with lots of smooth, rounded pebbles cemented together, it's probably a conglomerate – a sign that those pebbles traveled a long way in water. Think of the smooth, flat pebbles you find on a beach have been rounded and shaped by the constant motion of the waves.
Breccia
Breccia is also like natural concrete, but instead of smooth stones, it's made from jagged, broken pieces of rock. Think of a rockfall where rocks smash and break into sharp, pointy bits. These broken pieces then get cemented together before they've had a chance to get rounded. So, if you find a rock with lots of sharp, angular rock fragments stuck together, that's a breccia. It tells you the rocks probably broke apart and then got stuck together very close to where they originated.
The big difference between the two is simple: conglomerates have smooth, rounded pieces, while breccia has sharp, angular pieces. It's all about how far those rock fragments traveled before they became a solid rock.
Monomict vs Polymict
When we talk about conglomerates and breccias, we can also describe what the rock pieces inside them are made of. This leads us to monomict and polymict types.
A monomict conglomerate or breccia has rock pieces that are all from just one type of rock or mineral. Imagine a rock made only of broken bits of granite, or only smooth pebbles of quartz. This tells us the source area for these rocks was very uniform, meaning the sediment came from one specific type of rock.
On the other hand, a polymict conglomerate or breccia contains rock pieces that come from many different types of rocks or minerals. Picture a rock filled with a mix of sandstone chunks, bits of volcanic rock, and some limestone pebbles. This kind of rock tells us the sediments came from a variety of places, where different rock types were being broken down and mixed together.