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.
The Rhosydd Geology Trail follows the track up the south side of the lake and back around to Plas Cwmorthin, a former quarry managers mansion. It extends a little further on a less used path that can be boggy in places, the rangers have asked that you return via the same route from the farm, please don't try and cross the boggy area between the farm and the old mines. This trail is generally further away from the mines and spoil heaps but you will pass old buildings that may be unsafe and there could be other hidden dangers - please take appropriate care. 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 Quartz Veins
Quartz is a defining constituent of granite and other felsic igneous rocks, and this is what you see here today. As individual crystals it is very common in sedimentary rocks such as sandstone and shale and is also present in variable amounts as an accessory mineral in most carbonate rocks. It is a common constituent of schist, gneiss, quartzite and other metamorphic rocks. Quartz has the lowest potential for weathering in the Goldich dissolution series and consequently it is very common as a residual mineral in stream sediments and residual soils. There are various ways quartz can crystallise, but the majority of quartz crystallises from molten magma. Well-formed crystals may reach several metres in length and weigh hundreds of kilograms, although unfortunately, you won’t see one that big today as we are only viewing quartz veins.
As well as crystals, Quartz can also occur in what appear to be veins, and this is how they are usually described. The term is slightly misleading, as the ‘veins’ you see are not thin tubes, they are usually thin sheets but because we only see the cross section they appear to be veins. The veins can form under various conditions, and depending on these conditions, may or may not bear quartz crystals in them.
The simplest type of a quartz vein is the filling of an already present crack in rocks. The crack might form for a number of different reasons, which include:
- Cracks formed during folding of the rock in a mountain-building process
- Cracks formed by shattering during tectonic events (movements in the earth’s crust)
- Cracks formed by a decrease in pressure during the uplift of a rock
- Cracks formed when a hot rock cools down and shrinks.
At some point, the rocks will have been greatly heated due to changes in the earth. In this area, volcanic activity is quite possibly involved. Hot brines will filter gradually through the already cracked rocks to greater depths. The process will filter the minerals the brines carry into the cracks where they will remain as the temperatures and pressure drop. This process may continue until the crack is completely filled or may stop before, leaving "pockets" in the vein that are sometimes outlined by crystals.
So why is there a variation in the colour of quartz veins? Hot brines that enter a crack in the rock from some distant hot source often cool and filter most of their load rather quickly. When that happens, the result is milky coloured quartz. Milk quartz or milky quartz is the most common variety of crystalline quartz. The white colour is caused by minute fluid inclusions of gas, liquid, or both, trapped during crystal formation and this makes it of little value for optical and quality gemstone applications.
Later, when the crystal growth slows down, the crystals may get less milky or even clear. In the majority of quartz veins, most of the quartz is formed as massive, milky quartz, and well-formed crystals, will be at most only a small portion of the vein filling, and may well not be found at all. In some places here you will see some red colouring within the quartz. Like with the slate, this may be caused by a presence of iron.
Logging Requirements (Questions to Answer)
Please e-mail me the answer to the 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. Take a look at the quartz veins. Explain their distribution on the rock - are they all linear or formed in a pattern, or does it just look random?
2. What is the width of the thickest vein and what is the width of the thinnest vein?
3. Turn your attention to the colour. Describe the predominant colour you see in the veins. What does this tell you about how quickly the brine cooled? Do you see any red colouring in the quartz? If so, why? (See information in the ‘colour’ paragraph for guidance)
4. Examine the veins closely. Can you see any crystals that have been successfully formed?
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
