General Information and Warnings
Here at Cwmorthin you will find a stunning glacial valley but geologically it can be confusing due to the vast amounts of slate mining and surface quarrying that have taken place here. The earthcaches on this trail focus on both the original glacial features of the valley, and also the interesting slate that has been exposed from underground by the workings. Always remember that the huge piles of slate you will frequently walk over and past were not placed here naturally, this is all the work of man. As you walk around, try and keep an image in your mind of how the valley would have looked before the slate was quarried.
Our earthcache trail keeps to the tracks around the old quarry and mine areas. The mines are extensive and should all be gated and locked. The mines were abandoned because they were too unsafe to work. It is not recommended to climb the spoil heaps or explore away from the paths as there could be hidden dangers, and it’s not recommended to let children or pets run off unsupervised. Also be aware that slate can have a very sharp edge and can cut very deeply, so please treat this area with the respect it requires.
Information about the stones of Cwmorthin
The majority of the buildings here are likely to been built from the local stone available in the immediate area. We are standing outside some old buildings here which are predominantly slate, but not entirely. Look at the buildings closely and also the neighbouring drystone wall and you will see there are actually random stones that are not slate. It is very likely they are the igneous dolerite stones that would have been lying on the ground here in the valley long before the slate mining began. However, there could be granite here as well, as it is believed some was imported into the valley for building work at some point. Granite is an intrusive igneous rock, with larger crystals and more quartz than dolerite.
Over time, changes in the earth cause the rocks to alter, however, these changes cycle over long periods of time and all rock will fall into one of three standard categories.
Sedimentary - Sedimentary rocks are formed on the surface of the Earth, either in water or on land. They are called secondary, because they often result from the accumulation of small pieces broken off from pre-existing rocks. Most sedimentary rocks become cemented together by minerals and chemicals present when they are formed, and others are held together by electrical attraction. Some, however, remain loose, crumbly and unconsolidated. There are sedimentary rocks elsewhere in the valley, but they weren't used in any buildings.
Igneous - Igneous rocks get their name from the Latin word ignis, meaning "fire." They form from volcanic magma when a volcano erupts and are also referred to as volcanic rocks. Under the surface of the Earth the magna is kept liquid by high temperature and high pressure. As the volcano erupts hot magna reaches the surface. Afterward the lava rapidly cools down and solidifies. The crystals formed by cooling magma are usually small. Magma doesn't always reach the surface. Sometimes it is trapped underground in pockets of other rocks. In this case the magma cools down more slowly forming larger crystals and coarse-grained rocks. How the rocks form will depend not only on the different cooling temeratures of the magma but also its chemical composition.
Metamorphic - Sedimentary and igneous rocks which were subjected to intense pressure and/or heat and as a result underwent a complete change. Metamorphic rocks form deep within the Earth's crust. The process of metamorphism does not melt the rocks, but transforms them into other rocks which are denser and more compact. New minerals are created either by the rearrangement of a mineral's components or by reactions with fluids that enter the rocks.
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The buildings and walls you can see here are made from two distinctly different types of stone - metamorphic slate, and igneous dolerite or granite. It is really quite unusual to see buildings and walls constructed in this way, perhaps old stones from older buildings in the valley were utilised or old valley stones found lying on the ground. You will see two main types of stone here.
Slate
Slate is a metamorphic rock, formed under great pressure from what was originally mudrock. Its origins in Wales date back to the Paleozoic age between 350 and 500 million years ago. There are three ages of slate found in Wales - Silurian, Ordovician and Cambrian. Below is some information about these different ages of slate of Wales, but it is also important to note the mineralogy of the slates also differs due to the original local rock found in each of the areas, from which the slate was formed.
Silurian is the most recent and is found in the Llangollen, Glyn Ceiriog and Corwen areas - so this is the slate you will see near the 2016 Mega Campsite (Corwen) and 2016 Mega (Llangollen). It is not particularly durable but industry in those parts still developed because of its closeness to major markets. Its lack of durability is noticeable in the waste heaps which have even turned back to shale in some cases.
Cambrian is the oldest type and found in the north around Bethesda, Llanberis and in the Nantlle Valley. The colour is variable with a tendency towards purple, slates of this type are still mainly split and dressed by hand although there has been some success with mechanisation. So if you climb Snowden and spot any slate, it is the oldest in North Wales.
The slate we will see here today is Ordovician slate, which is found around Blaenau Ffestiniog and Corris, it is generally mined underground. It is almost uniformly blue-grey and is easily split using machines. This Ordovician slate is the middle age slate, formed later than the Cambrian slate at Llanberis and earlier than the Silurian slate at Llangollen.
Dolerite
Dolerite is a dark-colored igneous rock. It is a subvolcanic rock, in composition it is equivalent to volcanic basalt or plutonic gabbro. Dolerite dikes and sills are typically shallow intrusive bodies and are often fine grained or so fine grained the grains can't be seen by the untrained eye. Dolerite is a common rock type. It occurs mostly in shallow intrusions (dikes and sills) of basaltic composition.
Dolerite is hard and expensive to drill. Being an igneous rock it is difficult to cut as there is usually no predictable cleavage plane. It's properties mean it is useful for roadfill and gravel . As a building stone it is hard to come by good freestone i.e. stone which has already cleaved or can be worked into straight-sided pieces. If you try to shape a piece with a sledgehammer, most likely it will splinter into uneven lumps, that is if your blows are even successful at breaking it at all. There is a lifetime guarantee to the rock if you get a good piece. Felsite and Rhyolite stones could also be present here, they are also local volcanic igneous rocks which would share similar problems for building use, but this is likely to be why the stones are only used occasionally here.
Logging Requirements (Questions to Answer)
Please e-mail me the answers to the below questions via my profile. I do read all answers and try to reply to them all; I may not reply immediately so please do not wait for a reply before posting your find. Ideally, please send your answers at the same time you submit your log, or within a few days of your visit. I do check answers have been sent for every log, if you do not send answers within a week your log may be deleted. You are not expected to have any previous geological knowledge, your best attempt at the answers is all that is required.
1. Firstly, find a piece of 'metamorphic' slate. Please describe the texture of the slate, is it rough or smooth, does it have sharp edges. What colour is it? What shape has it easily been cut into?
2. Next, find an 'igneous' dolerite stone. Please describe the texture of the dolerite, is it rough or smooth, does it have sharp edges. What colour is it? Has it been cut by man to this shape do you think? Please highlight the differences from the slate.
3. As a building material, what do you think the strengths and weaknesses of each rock type may be?
We always like to see photographs of you and/or your GPS device with the geological features - plus this also provides additional proof of your visit, so please include one with your log if you wish.
This cache has been produced especially for the
