This EarthCache will take you to a sinkhole in the middle of an upland peat bog, and will teach you about the underlying rock types, the 'karstic' landscape, and the peat bog. It is a remote area, about 1km east of the road and it is easy to get disorientated in bad weather: prepare properly and wear suitable clothing and footwear if you decide to visit.
Background Geology
Mynydd Llangatwg (Llangattock Mountain) is a wide, relatively flat upland plateau at around 500 m altitude to the north of the Heads of the Valleys Road. The plateau is mostly formed by rocks that used to be called ‘Millstone Grit’, but which are now named the Twrch Sandstone. They were laid down during the Carboniferous Period (359-299 Million years ago), and are about 320 Million years old. The Twrch Sandstone is a sedimentary rock laid down in an ancient river delta and is formed of medium-sized grains of silica-rich sand (0.05 to 2mm in diameter) bound with a cement. Sometimes these medium-sized grains are joined by larger (>2mm diameter) quartz pebbles in what are termed coarse-grained rocks. The rocks tend to be rather dark grey, and often have rust staining because of high iron content.
Twrch Sandstone rocks have a very low calcium content, so plants which avoid lime-rich ground dominate the sandstone plateau. These plants include Sphagnum moss, which has built a blanket of peat across much of the plateau over several thousand years.
Peat
Peat is formed in waterlogged conditions where a low pH and cool temperatures slow down vegetation decay. Plant remains build up over thousands of years to form a very dark brown ‘soil’ that holds water like a sponge. A high altitude plateau like Mynydd Llangatwg, with impermeable rocks, is the perfect place for peat to form and large areas of the plateau are covered by peat bog. A peat bog is somewhere with peat at least 50 cm thick over bedrock; much of Mynydd Llangatwg has one metre or more of peat. Some bogs in Wales have peat more than 10 metres deep! When a peat bog is damaged by erosion or drying out, the peat can collapse in some areas to leave low ‘cliffs’ of peat known as haggs.
Karstic Geomorphology
The Twrch Sandstone rocks that form the plateau sit above older rocks belonging to the Carboniferous Limestone group. These are between 328 and 359 Million years old. Unlike the Twrch Sandstones, these older limestones are fine-grained (fragments less than 0.05 mm in diameter) to medium-grained sedimentary rocks with a high calcium content. They are light grey in colour, were laid down in a shallow sea and formed of the lime-rich shells and hard parts of millions of sea creatures. On the north side of Mynydd Llangatwg there is a long series of limestone cliffs 20 to 30 metres tall.
Carboniferous Limestone dissolves in water over thousands of years. This allows cave systems to form within blocks of Carboniferous Limestone. More than 50 miles of caves have been explored under the Mynydd Llangatwg plateau, in one of the most extensive cave systems in Britain. Associated with the caves are various other features associated with dissolving rocks, in what is called a Karstic landscape. As well as the caves, typical features of a Karstic landscape include sinkholes, sinking streams, springs and underground rivers.
This EarthCache visits a sinkhole that has formed where rocks have collapsed from above into a cave. This particular sinkhole has a stream flowing into it from the surrounding peat bog, but no stream flowing out: the stream disappears into the sinkhole, pouring down into the cave system below. The hole down which the stream flows appears to be too small for a person to fit through, but you must not try because you could fall 10s of metres into the cave below! Water flows through the cave system in a southerly direction, eventually emerging in the Clydach Gorge. Sinkholes are also sometimes called shake holes, swallow holes or dolines.
To claim a find you need to answer the following using the Message Centre or by emailing me. I will reply promptly, but you are welcome to log a find before you hear from me.
At Waypoint 1 N51 49.788 W003 11.746 answer the following questions:
- Estimate or use your GPSr to work out the depth of the sinkhole, to water surface level (don’t risk wading).
- Describe the rocks on the sides of the sinkhole, including their size, shape, colour and grain size.
- Is any Carboniferous Limestone visible in the bottom of the sinkhole?
At Waypoint 2 N51 49.820 W003 11.713 answer the following questions:
- What depth of peat is exposed on the hagged bog edge?
- What features of peat formation are visible on the exposed vertical peat?