Parking for several cars is at O.S. ref SJ 234432.
From parking, take the path up the old incline, NW, and along the old tramway (You can see the occasional sleeper still.) to location 1. (SJ 231433) From here we get a good view of the local geology and the Dee valley. Castell Dinas Bran, facing you, is on Silurian (Ludlow) as is the lower ground below the road. The base of the Carb. limestone is shown on the map as faulted along the line of the road, which may be so, but the base is also a major unconformity, corresponding to it being above sea level during the Devonian.
The Carboniferous sea slowly transgressed over the eroded Silurian landscape during the Early Carboniferous (Dinantian) ~337Ma, covering S. Wales and later, N. Wales. The early Dinantian is therefore missing. Along the road below, "Basement Beds" outcrop sporadically in stream beds, these represent the "first washings" and are red shaley rocks.
The beds below loc 1 are the Upper Ty-Nant Lst Fm with the Eglwyseg Lst coming in at loc. 1 . Above the small scarp, in the gap, there is a sub-aerial eroded paleokarstic surface filled with a purple and green pebbly shale with Silurian pebbles showing imbrication.(Pics 1&2)
Following round to the east, higher beds can be examined as far as the end quarry, beloved by climbers. There are examples of colonial corals, burrows, etc. Above the quarry is an example of a limestone pavement.
A path can be followed back to the car parking for a cuppa and a sandwich, before taking a path to the ESE towards loc 2, (SJ 240428) Bron Heulog Quarry. This large quarry has a sheer face exposing nearly 200 feet of the Eglwyseg Limestone and shows really impressive cyclothems.
SAFETY NOTE Falling rocks from 200 ft are dangerous. Keep clear of the faces and wear your hard hat!
At the east end of the quarry impressive lime kilns are well preserved. (watch the unprotected edge!) Here lower beds of the Eglwyseg Lst are exposed (normally not seen as they erode easily) comprising green and grey marls and rubbly micrites with grey fetid biosparites and calcareous shales.
On the way back clean specimens of all lithologies can be found in the broken blocks, including examples from the Sandy Lst Fm (loc 3) and the overlying Cefn-y-Fedu Sst.
Location 3 (SJ 227455) is reached a mile or so east along the road but there is good parking so it is excusable to drive. This is in the Sandy Lst Fm and shows remarkably good large scale cross stratification, with conglomerates with quartz pebbles, nice deltaic structures. With a lens they are lovely oolitic sandy limestones and limey sandstones. Are the regular beds tidal? monsoonal? or what? This is a textbook face so, please, no hammers, but there is plenty more round the corner if you did not pick up some at loc. 2.
Brit Regional Geology North Wales. (At present O/P. Mine is prehistoric and dated 1948 price 3s.6d. but I think there is an issue about 1976.
A Pingo near Tylwch (March field trip)
In March we visited a pingo at Tylwch. (SN 946798)
"Pingo" is the Eskimo word for a hill and applies to the large number of almost circular hills found in the Canadian North West Territories round the Mackenzie Delta. They rise to up to 60 metres high by 600 metres wide and some have a central crater which may contain a circular pond. Within each there is a core of ice which builds up as a large lens pushing up a dome of sediment above it. Eventually the dome splits and the ice is exposed and can melt in the summer.
In Britain all we have left after the last ice age are circular ramparts filled with boggy sediment. They are rare in Wales. In the field at Tylwch we saw a good example with traces of another nearby.
Note that the pingo is on private land so do not traspass without permission, there is little point because there is nothing on the ground and it is best seen from the path which overlooks it to the north.
Do be careful because there are lots of farm ponds which are just that, Pic 6 shows one near Guilsfield!
Rheidol Gorge Walk and Temple Mine (June field trip)
Refs.
M.R.Dobson 1995 "The Aberystwyth District" G.A.Guide No. 54 ISBN 0900717785 pp77-80
Cave R. and Haynes, B.A. 1986 Geology of the Country between Aberystwyth and Machynlleth Mem. Brit.Geol. Surv. Sheet 163 pp119-120
Leaflet No. 7 "The Temple Lead Mine" available, with others, from the M.W.Mining Museum, Ponterwyd, Aberystwyth. This museum is well worth a visit and is 2km west of Ponterwyd on the A44.
The walk starts at Yspyty Cynfyn, (SN752791) on the A4120, 2km south of Ponterwyd, where there is parking, (avoiding obstructing the church or barn entrance!). It follows a classic section described by O.T.Jones in 1909, defining the graptolite zones of the Cwmere and Derwenlas formations at the bottom of the Llandovery Series (Base of the Silurian). It includes the remains of Temple Mine which is a significant mine on the Castell fault. All the underground workings are dangerous and should not be entered. That said, hard hats are not required for the walk, just good boots and sandwiches.
Set off to the right of the churchyard, in the wall there are built in the standing stones of an old stone circle. This gave it's name to "Temple Mine". We follow the path a few hundred yards and down into a steep sided dingle where we cross Parson's bridge.
This structure replaces an earlier plank bridge which must have been a bit "hairy". It was used by the parson to visit his flock in Ystumtuen, which was a mining community above the gorge. The volume of water in the river was much greater before the power station took the majority of the flow, but it is still impressive. Note the enormous potholes. These are about as good as you get.
We bear right over the bridge, and 100 yards further on find the ruins of the dressing floors of Temple Mine. Dating from the 1880's, the ore bins, circular buddles, wheelpit and crusher base are clearly visible. The waste tips went straight into the river so there are few minerals etc to be found here. The ore used to be hauled up an incline to meet a track to Ystumtuen. The path up the slope is now overgrown and difficult to find. Continuing along the path, we are walking along the line of the tramway between the main adit and the dressing floors. The leat which fed the 14ft waterwheel is a few feet to the left.
Shortly we come to the main adit on the left with a very substantial wheelpit on the right below the path. This housed the waterwheel which ran plant, pumps and the winding gear.
50 yards on we cross a small tributary. This is an important location as both the tributary and the Rheidol follow the Castell Fault. This is a major feature along which mineralisation occurred. There is an old adit (Nantymoch level) about 50 yds up the tributary but it is quite a scramble to get to it.
The path now follows the old leat and graptolites can be found in rock exposures along it. You need to get your eye in for them as the slatey cleavage does not follow the bedding and the graps are on the bedding plane. The importance of these organisms is that they evolved rapidly and therefore make good zone fossils. In the Aberystwyth area in particular, we have a situation where, along the coast, we have a section along the cliffs where, in the north, the dip is to the south and, in the south, it is to the north. (i.e. basin-like) Because the sediment was deposited by north flowing currents, one stratum could well be a sandstone in the south and a mudstone in the north. In about 1909 graptolite zones were used to establish which Aberystwyth Grits in the south corresponded to particular Borth Mudstones in the north.
It was the noted geologist, O.T.Jones who mapped the area using graps for zoning.
(Ref: Jones, O.T. 1909 The Hartfell-Valentian succession in the district around Plynlimon and Pont Erwyd Qt. Jl. geol.Soc.Lond.,65, 463-537)
By a small oak, the path bears up and left. The leat continues to a steep cliff where it was carried in a timber box section, chained to the cliff face. The path approaches the river again before a scree slope and a stile into a field. Most of O.T.Jones' sections are in this 400m section south of the stile. Graps are mostly located in the rusty weathering rocks, rather than the pale ones.
It has to be said that there are hundreds of species of grap, so they are not easy to identify. However, some from here are quite pretty, being pyritised.
The public path goes north through Brynbras farmyard and bears right, following their track to the George Borrow Hotel. As the most interesting bits are in the south, we returned that way. An interesting feature in the north is that the river meanders. It was thought that these were "incised meamders" formed when sea level was higher and the river was mature. Sea level then went down and the river was rejuvenated, cutting the meanders deeper as it did so. It is now thought that the more likely explanation is that the river burst out above an ice blockage during the ice age. The meanders can be seen from the road, in various places.
The return path up the ravine above Parson's Bridge always seems steeper and longer going up than coming down!
Cwmystwyth Mine (April field trip with Simon Hughes)
Refs:
Simon J.S.Hughes, 1980 "British Mining, No. 17" Northern Mine Research Soc. (The most complete guide)
M.R.Dobson 1995 "The Aberystwyth District" G.A.Guide No. 54 pp89ff ISBN 0900717785
D. Bick 1991 "The Old Metal Mines of Mid-Wales" Pt 4 24-25
The Cwmystwyth mine (SN 805746) is at the west end of the old mountain road to Rhayader, just off the B4574.
Mineral specimens (Galena, sphalerite) can be found both sides of the road in the extensive spoil tips. Further to the north east is Copa Hill which has remains of uncertain pre-historic date with the best examples of the old "hushing" process. This involved damming water and releasing it to clear the topsoil exposing any signs of veins.
