The Rock Underneath

Read the rest of this site and the same explanations keep recurring: this stone, that soil, this kind of farming, that industry. All of it comes from a few hundred million years of geology, and the county divides into three or four blocks that behave completely differently.

The middle: Silurian and Ordovician

Most of Powys, from Machynlleth across to Knighton and south to Builth, is Lower Palaeozoic mudstone, siltstone and grit — deposited in a deep marine basin, subsequently folded and hardened.

It produces thin, acid, poorly drained soil. It weathers to a grey stone that splits into thin uneven slabs, which is why building in Radnorshire and central Montgomeryshire is grey, roughly coursed and mortar-heavy.

It carries mineral veins, which is where the lead came from: Van, Dylife, Llangynog and the smaller workings.

And it makes rounded, uniform hills without much drama. The central Welsh uplands look the way they do because the rock is all much the same and has been eroded evenly.

The south: old red sandstone

South of a line running roughly through Builth and Talgarth, the rock changes to Devonian old red sandstone, and everything changes with it.

It weathers to a deeper, warmer, browner soil that farms considerably better. The Usk valley is the best agricultural land in the county for this reason.

It produces a brown-purple building stone, easy to work and handsome, and Brecon and Crickhowell are built of it.

And it makes the Bannau Brycheiniog: the sandstone lies in near-horizontal beds, which erode into the flat-topped ridges with steep north-facing scarps that give Pen y Fan and Corn Du their characteristic profile. That shape is a direct consequence of horizontal bedding and it is why these mountains look nothing like Snowdonia.

The southern rim: limestone, grit and coal

Along the southern edge, from Ystradfellte west to the Tawe and east to Llangattock, the sandstone is overlain by carboniferous rocks in sequence.

Limestone first, which dissolves. That gives the caves — Dan yr Ogof, Ogof Ffynnon Ddu, Porth yr Ogof — the shakeholes and swallets on the moor, and the waterfalls where rivers cross from one rock type to another. It also gave the quarries and the lime kilns, and limestone flux for the ironworks.

Millstone grit above it, hard and resistant.

Coal measures above that, in the far south-west corner around Ystradgynlais and Abercraf. This is the western anthracite coalfield, and it is the reason there is a town of over eight thousand people in the corner of Powys that most accounts forget.

The ice

All of it was worked over by glaciation.

Ice deepened and straightened the valleys, left moraines that dammed lakes, and dumped boulder clay across the lowlands. Llangorse Lake sits in a glacial hollow. The Elan and Vyrnwy valleys were suitable for reservoirs because ice had already given them the right shape.

Gravel terraces along the Severn and Wye are glacial outwash, and they are why Four Crosses has both a Bronze Age cemetery and a gravel quarry — light, well-drained, easily worked ground attracted early settlement and modern extraction to exactly the same places.

What follows from it

Buildings. Grey Silurian rubble in the middle, warm sandstone in the south, timber frame along the eastern border where good stone runs out and the tradition changes.

Farming. Sheep on the acid uplands, mixed and dairy on the sandstone and the lowland gravels.

Industry. Lead where the veins are, limestone and coal on the southern rim, and nothing much anywhere else.

Water. High rainfall on impermeable rock in narrow glaciated valleys is precisely what a reservoir engineer wants, which is why Birmingham and Liverpool came here.

Where to see it

The Fforest Fawr Geopark, in the south-west, is designated for exactly this and has interpretation across the waterfall country and the limestone escarpment.

Anywhere the rock is exposed: quarry faces, river gorges, road cuttings, and the scarp of the Bannau above Storey Arms.