General Information and Warnings
Do not attempt to cross the closed off bridge to reach this earthcache. It has been dangerously undercut and is partially collapsed. Please walk up to the dam (where you will fine #1 on the trail) and walk back on yourself to reach this earthcache.
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 Glacial Movements
The movement of a glacier through a valley often leaves evidence behind in the form of channels, grooves or striations cut into bedrock by glacial abrasion. These scratches and gouges were first recognised as the result of a moving glacier in the late 18th century when Swiss alpinists first associated them with moving glaciers. There are some particularly good examples of these scratches in the rocks in front of you here. These scratches are known as 'striations'. In glaciated regions such as here, the rock surfaces will show signs of ice movement over them because the surface has not been eroded in the time since glaciation. Signs of movement could be channels, grooves or striations depending on the size of the sediment contained in the glacier.
In simple terms, the bottom of the glacier picks up sediment and rocks as it moves along by rubbing against rocks (abrasion) or by pulling bits of rock away from the land (plucking). These rocks are now under great pressure from above by the glaciers weight above and these rocks then gouge the valley floor by grinding against the bedrock underneath as the glacier moves.
An good example of a Glacial movement elsewhere, note the parallel lines in the rock.
The example of glacial movement we can see here is striations; these are scratches in the rock (rather than grooves or channels). Glacial striations are usually multiple, so you would expect to see quite a few in the same area. They are also straight and parallel. These lines show us the movement direction of the glacier. Striae can range from microscopic size on a seemingly polished rock to obvious markings millimeters deep and nearly a meter in length. So how does ice cut grooves into a hard rock like you see here? Well, the ice doesn't directly, because ice isn't harder than rock and in a war, rock would beat ice. However, that doesn't mean ice doesn't find a way to do some damage. The ice gathers rock fragments and sand grains as it moves along and these become embedded in the bottom of the glacier and they then work like cutting tools. So these large amounts of coarse gravel and boulders carried along underneath the glacier provide the abrasive power to cut trough-like glacial grooves into the rocks beneath the glacier - the rock fragments might only be small, but imagine the weight of the glacier on top of them. Finer sediments also in the base of the moving glacier further scour and polish the bedrock surface, forming a glacial pavement.
So, let's take a look at the main things that affect the formation of these 'lines' scratched into the rock.
- If there is no rock in the bottom of the glacier there will be no abrasion.
- The fragments must be harder than the bedrock. Something really hard like Quartz fragments will abrade a soft rock like sandstone, but sandstone fragments would not do any damage to a quartz rich bedrock.
- Speed of the glacier. The faster the glacier moves, the deeper the bedrock will be eroded.
- Thickness of the ice. Thicker ice is heavier ice which causes more downward force and increased pressure between the abrading fragments and the bedrock. There is a limit to how much ice will enhance abrasion. If the friction force between fragments and bedrock is too great the ice will flow around the fragments.
- Shape of the fragments. Larger more angular fragments will scratch and scour more effectively than small and round fragments will, which leads to the next point.
- As the bedrock is being worn away the abrading fragments within the glacier are also being worn. Similarly to sandpaper being worn away with use. A continued supply of abrading fragments is required to uphold a similar level of abrasion.
- A constant flow of melt water between the basal surface and the bedrock speeds abrasion. The meltwater constantly rinses away the rock flour allowing the coarser fragments to abrade bedrock.
The forms of striae can vary greatly. Variances can be caused by a change in topography, bedrock type, pressure on the debris making the inscription and the orientation of the debris to the bedrock. Most striae are blunt and deep at one end and then taper to a point. These are known as nail-head striations.
Freshly exposed striations have a preferred orientation of rock grains. By lightly running a finger along the striation it is possible to discover that when moving one way along it, the rock feels smooth, but when moving the other way it feels more coarse. The moving ice leaves the rock grains aligned with the direction of movement, so when the striation feels smooth, your finger is moving in the direction of ice flow. When the striation feels rough, you are moving against the ice flow. This test doesn't always work, and wont work on striations that have been exposed for a long period of time.
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. Roughly, how many notable glacial striations can you see on the rocks in front of you here? (approximately)
2. Look closely at the biggest glacial striation you can see. How wide and how deep is it ? (approximately)
3. You will notice that there are striations on more than one rock. What is the obvious evidence that these striations were created at the same time by the same glacier?
4. If you run your finger along a striation in both directions, does it feel smoother when you move it in one direction than the other? The smoother direction would indicate the direction the glacier was moving in - which direction do you think it was moving? (If you have completed Cwmorthin Geology Trail 2 about Roche Moutonnée you may already have an idea of this answer.)
5. Now look at the angle of the striations compared to the path. What additional information does the angle of the striations tell you about the movement of the glacier?
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
