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 Geological Tilting
Ideally you will have completed our geological trail in numeric order, and therefore you will have visited #6 first, which introduces sedimentary rocks. If you look up from the given co-ordinates you will see some more rocks that are clearly sedimentary as you can see the layers in the rock. The layers are much thicker than the layers you saw at the previous earthcache, so there is obviously a difference in rock type here. However, it is not the rock type we are interested in here, it is the angle of the rock.
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 will usually be older than any bed on top of it. (Very occasionally a fold may have turned the layers right over.) 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
Information about the final Freeze-thaw
We also want to briefly consider the effect of the last ice occupation of the valley, as sometimes it doesn't need a huge glacier to make changes to a valley. Although you have got used to thinking that all the piles of rocks in the valley are abandoned by the slate industry, this isn't the case as there are areas of scree that pre-date the industry by over 11,500 years. By about 15,000 years ago the entire area would have been ice-free - all the major glaciers were gone. However, during a brief cold period between 13,000 and 11,500 years ago, there came a time when we know small glaciers re-occupied many of the cirques basins of Snowdonia (a cirque basin is a theatre-like valley formed by glacial erosion). It is likely that a small glacier, or at least a permanent snow-bed, would have re-established itself in the shaded, north-facing basin here.
This glacier, or snow-bed, would not have moved from the valley. However, freeze-thaw action would have shattered some rocks, resulting in the formation of screes on the lower, valley-side slopes. The flat ground between the current head of the lake and Plas Cwmorthin was once occupied by the lake Llyn Cwmorthin, but is now filled with flood deposits (gravel, sand and silt), caused by the ice melting into the abundant streams. So, at this time and during the post-glacial period (i.e. following the warming of the climate post-11,500 years ago), stream erosion led to the partial infilling of the floor of the Cwmorthin valley.
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. Estimate the approximate angle of the rock layers.
2. Please move to waypoint 2. Although you are still very close to the previous rocks, this rock is very smooth and the layers less obvious. Look more closely. Are the layers still aligned in the same direction, as the rock above? Why is this rock so smooth when the rock just slightly higher on the hillside is so rough? Knowledge gained in previous earthcaches on the trail will help you here.
3. Have a look around the valley (this can include looking as you walk back from here). Can you find any evidence to confirm the presence of the freeze-thaw that is believed to have taken place during the final ice occupation of the valley? Look for scree on the lower slopes not connected to the quarries. You can identify this easily by looking for scree where made from rock types other than slate. Also look for areas containing sand and silt. If you spot anything, tell me what you saw, and record roughly where you spotted it.
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
