Llandrindod Lake

Llandrindod Lake was originally a marsh, hemmed in between the hills of the Ordovician inlier and the surrounding Silurian rocks. It was a boggy, rock-scoured hollow until the Victorian visitors desperately needed somewhere to shoot ducks during their relaxing rest cure... and so the Pump House Hotel duly obliged, by damming the end and making a lake.

The only outcrops around the edge of the Lake itself are on the north shore, where the road has been cut through the rocks. Many of the outcrops are covered by vegetation, although they can be seen more clearly in winter. The most critical outcrops are still visible, though.

Firstly, there is a shelter with a bench cut into a small knoll, just along from the parking area. This shaded little nook is carved into dolerite: a medium-grained igneous rock that forms when basalt magma cools moderately slowly underground. The outcrops at the back of the shelter are characteristically blocky and irregular, but the rock was readily cut into blocks and used for much of the early building work in the town – including the railway bridges that came before the town itself! At this alcove, be sure not to get confused between the dolerite and the Silurian stones that have been used to build out walls in order to finish off the shelter.

Further along the shore, there are lots of outcrops of dolerite peeking through the plants. The contours of the ground here suggest the extent of the magma chamber: small and domed. Technically, it was probably a laccolith: a small dome-shaped intrusion that pushed sideways and upwards from a central point, but never erupted.

When you come to the second shelter with a bench, you have reached the edge of the magma chamber. The area to the right has been cleared to show the transition from dolerite at the bottom, into dark, flat-bedded mudstone, and then a thick layer of paler-weathering volcanic ash. The ash breaks vertically, giving it a distinctive appearance compared with the originally flat layers in the mudstone.

Between the dolerite and the mudstone, interesting things happened. The dolerite, emplaced at around 1100°C, cooled rapidly as it came in contact with cold, wet sediment, and as a result it is finer-grained, and breaks more evenly. The mudstone, in contrast, was effectively fired: baked hard and crystallised to form a rock called a hornfels. The zone of baking fades out over a few tens of centimetres.

The mudstone itself has been dated by the presence of fossils: specifically the graptolite Didymograptus murchisoni, which dates the rock to around 461–462 million years, in the Middle Ordovician. These outcrops and their fossils were described by the famous palaeontologist Gertie Elles in 1939. The dolerite was intruded some time after, but its exact age is unknown; perhaps around 450 million years ago.

If you look upwards from here, the steep wooded slope above the Lake reveals what comes next. The paths through the woods occasionally run over hard, worn outcrops of more volcanic ash that forms part of the Cwm Amliw Tuff Formation: the last major eruptions of the Builth Inlier volcano.