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 Valley Types
A valley is an extended depression in the Earth's surface that is usually bounded by hills or mountains and is normally occupied by a river or stream. Because valleys are usually occupied by a river, they can also slope down to an outlet which can be another river, a lake or the ocean.
Valleys are one of the most common landforms on the Earth and they are formed through erosion or the gradual wearing down of the land by wind and water. In river valleys for example, the river acts as an erosional agent by grinding down the rock or soil and creating a valley. The shape of valleys varies but they are typically steep-sided canyons or broad plains, however their form depends on what is eroding it, the slope of the land, the type of rock or soil and the amount of time the land has been eroded.
There are three common types of valleys which include V-shaped valleys, U-shaped valleys and flat floored valleys.
V-Shaped Valleys
A V-shaped valley, sometimes called a river valley, is a narrow valley with steeply sloped sides that appear similar to the letter "V" from a cross-section. They are formed by strong streams, which over time have cut down into the rock through a process called downcutting. These valleys form in mountainous and/or highland areas with streams in their "youthful" stage. At this stage, streams flow rapidly down steep slopes. An example of a V-shaped valley is the Grand Canyon in the Southwestern United States. After millions of years of erosion, the Colorado River cut through rock of the Colorado Plateau and formed a steep sided canyon V-shaped canyon known today as the Grand Canyon.
Flat-Floored Valley
The second type of valley is called a flat-floored valley and is the most common type in the world. These valleys, like V-shaped valleys, are formed by streams, but they are no longer in their youthful stage, and are instead considered mature. With these streams, as the slope of a stream's channel becomes smooth, and begins to exit the steep V or U-shaped valley, the valley floor gets wider. Because the stream gradient is moderate or low, the river begins to erode the bank of its channel instead of valley walls. This eventually leads to a meandering stream across a valley floor.
U-Shaped Valley
Due to their large size and weight, glaciers are able to completely alter topography, but it is the alpine glaciers that formed most of the world's U-shaped valleys. This is because they flowed down pre-existing river or V-shaped valleys during the last glaciation and caused the bottom of the "V" to level out into a "U" shape as the ice eroded the valley walls, resulting in a wider, deeper valley. For this reason, U-shaped valleys are sometimes referred to as glacial troughs.
A hanging valley is a smaller side u-shaped valley left 'hanging' above a larger u-shaped valley. A waterfall can often be seen. During glaciation the smaller side valley contains less ice than the main glacial valley, which is why it is not as deeply eroded.
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. Please look at the valley around you and describe what you see. From this point you are at a disadvantage, as the river above is hidden by the lake, and below mining has changed the look of the valley, however, please feel free to consider this earthcache from other locations in the valley. Describe the shape and explain which of the three types of valley you believe the valley to be.
2. At the end of the listing we explain about hanging valleys. The Cwmorthin Valley sits at an altitude of about 330m near the lake shore, which is about 150m above the floor of the main valley at Tanygrisiau. Also consider the evidence you saw as you climbed up into the Cwmorthin Valley. Do you think this valley is an example of a 'hanging' valley?
3. Based on your answers to the previous question, what does this tell you about how the valley Cwmorthin connects onto (e.g. the valley where Tanygrisiau and Blaenau Ffestiniog are) was 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
