A striking mountain from all angles it only rises to the very modest height of 486 metres above sea level. Striking because it is all alone to the east of the Hereford-Abergavenny road and rises out of an otherwise flatish area, striking because of the stories that surround it and striking because it is seemingly split down the western slope at it's northern end - and that is what we are interested in here.
History/Myths and Legends
Ysgyryd literally means 'shivered' or 'shattered' and Fawr is 'great' or 'big', hence the name (there is an Ysgyryd Fach just to the east of Abergavenny). There are many popular stories that surround this interesting mountain from God sending down a bolt of lightening to split the ground at the moment Jesus was crucified, to Satan stamping on the mountain and splitting it when St. Michael refused his evil temptations. What we know for sure is that the top of the hill was used as a hill fort during the Iron Age and that there used to be a chapel at the summit dedicated to St. Michael, remains of which can still be seen today around the relatively modern trig point. The modern mind though generally accepts that the distinctive shape, so loved by artists, photographers and walkers alike is due to a landslip - and this is or course the subject of this earthcache.
The Geology
The geology of Monmouthshire consists mainly of thick layers of sedimentary rock, and the Skirrid is made mainly of Old Red Sandstone from the Devonian period (about 416 to 360 million years ago), in common with the rest of the Black Mountains.
Old red sandstone is continental (rather than marine) in origin when thick layers of sand and mud were deposited, probably in basins and compressed over time to eventually form rock. Invariably animal, plant and fish remains are present (but not always) as was oxidised iron minerals that give its distinctive red colour, but again not always as colour can vary. The texture is normally gritty in nature.
The old red sandstone that forms the Skirrid overlays mudstone from the St. Maughan Formation. Mudstone is another type of sedimentary rock which was formed from laid down and compressed muds and clays and it actually has the look of hardened clay, it consists of incredibly fine particles that are indistinguishable with the naked eye. These clay particles can become aligned and give the appearance of layering in a parallel fashion and may break off in these layers. The stone is not particularly strong, is smooth to the touch and can be found in a number of colours.
This layer of mudstone below old red sandstone on the Skirrid has a lot to do with what we see today. The thinking is is that about 12,500 years ago, as the last Ice Age was ending and the glaciers were retreating, the ice-eroded and steeply-sloped ground that remained was no longer supported by the ice, and as it became water-logged and saturated (and therefore very heavy) the weaker mudstone below was no longer able to hold the heavier sandstone above. Together with the lubrication the water provided between the mudstone and old red sandstone above this resulted in a huge landslip. However the slide material did not all go fully down to the base of the hill with much of the material remaining at height and piling up on the western side against a still solid spike to form the distinctive 'split' shape we see today. Some of the material also went north and some south and piles of rubble and shattered areas remain along the western side of the majority of the mountain, although this isn't easily seen due to soil infill and vegetation cover.
Visiting
The stated co-ordinates take you to the middle of the landslip on the public footpath and this is where you will need to visit to be able to answer the logging questions below. You can reach this point from the north or south and via several other footpath approaches marked on Ordnance Survey maps, but I have given you a waypoint for the main car park to the south to help, and the walk from here is about 1 1/2 miles. From the car park follow the footpath up through the woods and after a bit less than half a mile you will reach a stone wall with a large gate, go through and then left onto the wooden walking boards. Follow this path around the base of the steeper ground and you will reach GZ. Once done you can make as much or as little of a walk as you like but I would encourage you to go to the top if you haven't before. Please remember though that, despite it's modest height, this is still a mountain and you will need to attired and prepared accordingly and with good, grippy footwear as it is often muddy all year round in places.
The Tasks
To log this earthcache you will need to visit the co-ordinates, as stated, and send the answers to me to the questions below:
1) Look to the top of the mountain and estimate the height difference between where you stand and the top. What is this height? (This will give you just an idea of the scale of the landslip).
2) You'll see plenty of rocks and boulders close by. Describe them, think of their texture, colour etc. Are these The Old Red Sandstone or the mudstone?
3) Take a look further around you. Do you see any evidence that the mountain is still unstable? The answer is yes, but please describe that evidence.
4) Is the mechanism of instability the same as the landslip described above, or is there something else going on here? If so, what? (A bit of extra research may be needed).
You do not need to wait until I reply to log your find, but I will come back to you to either confirm or clarify your answers as soon as I can. Photos are always appreciated too of course but are not a condition of logging.
| I have earned GSA's highest level: |
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Many thanks to the National Trust, for allowing me to place this earthcache here.
***Well done to Striguel for the First to Find.***