Our geology field school was very well attended. On our first day, Dr Joe Botting took us on a familiarisation walk, to see the geologically well known Builth Inlier. This area, to the east of Llandrindod Wells, is an internationally known volcanic island complex of Middle Ordovician Age. Joe took us to see outcrops of various shales, volcanic conglomerates and tuff and we began to recognise that the higher ground was usually made up of the harder tuff deposits. The area is famous for fossils including trilobites and brachiopods, and so we went fossil hunting. At the end of the afternoon we walked to Rock Park to sample the chalybreate spring that emerges there. This spring water is very high in dissolved solids and is thought to be the result of flow in a very deep circulation system that finds its way to the surface via the Rock Park Fault. We measured the TDS (salinity) of the spring to be 5500 parts per million.
On Day 2 we met at Cwmhir Abbey ruins, to look at the rock types that make up the remains of the Abbey. We could see that much of the rock that makes up the Abbey matches really well to the conglomeratic sandstone from Fowlers Cave, on the hill above Abbeycwmhir. We could also see that there are clear differences between the Abbey facing stones with their characteristic mica content and the modern desert sandstone that is being quarried at Grinshill in Shropshire (a proposed source for the facing stones). So the conundrum of the source of the facing stones remains……
From the Abbey we went to Bog Quarry to have a look at a marvellously exposed section of Silurian shales and grits. We were able to examine a number of sedimentary features and try to get our heads around how to recognise bedding from cleavage. This proved to be an important skill for the afternoon, when we examined the main spring at Ffynnongarreg – known as Saints Well – and did some field walking to see whether we could understand how the geology of the area influenced the spring. We measured the spring’s TDS, temperature and flow and made conclusions about the recharge area to the spring and its likely depth of circulation.
Joe’s description of the geology that we were seeing and the impact of the geology on the landscape was really well explained – and we all learnt a tremendous amount.
We managed to locate Fowler’s Cave today on our walk and possibly the tram line where they brought rock down from the the cave and possibly another quarry.
Well done for finding the possible tramway and other quarry. I had been told that there are other outcrops round the side of the hill, but I haven’t found either myself.
When we were at the Abbey we had a really interesting discussion about the mortar used in medieval buildings. I’ve subsequently found this link to a reference about earth mortars
https://www.buildingconservation.com/articles/earth-mortars/earth-mortars.htm
Talking to others at Abbey Cwmhir Heritage Trust, I gather that there is thought to be a link to a quarry near Old Radnor that worked Silurian limestone (and therefore could have been the source of lime for any lime mortar used in the Abbey construction). I have also been told that clay (earth?) mortar is present in the East End wall and near the North West door, but lime mortar has been used in other places. As CPAT completed the restoration work at the Abbey, I will ask them what they know about this.