Alan Underwood’s walk challenged us to see how today’s Castle Meadows were influenced by events that happened as long as 400 million years ago, and as recently as two hundred.

The earliest of these is actually the most obvious – the rocks that are now Sugarloaf, Skirrid and the lower slopes of the Blorenge were formed in the Devonian period. That’s so long ago that what is now Abergavenny was south of the equator, in a dry tropical climate, where a river – think something the size of the Mississippi or Ganges, not the Usk – deposited fine sediment that was then compressed into sandstone. Fifty million years later, in the Carboniferous, oceans and land had moved, and we were now close to the northern shore of a warm shallow sea, with corals whose fossils can still be found in pebbles on the riverbank. The remains of plankton from that sea were deposited as a layer of limestone on top of the sandstone, now high on the south side of the Usk valley. The northern edge of that layer is thinnest as that is where the advancing sea was last to reach.

Though humid swamps with tree ferns later appeared on top of the limestone, producing the South Wales coalfield, little from this period remains visible from the Meadows. Rocks were still being deposited and eroded – a dome of chalk covering the whole of Wales was later formed and completely worn away – but the only hint of this is the course of the Usk itself. The river doesn’t follow any of the current surface geology, so seems likely to have developed on a higher land surface that no longer exists.

The next visible features formed as recently as twelve thousand years ago, at the end of the most recent Ice Age. With a great deal of the Earth’s water frozen, sea levels were much lower, so rivers like the Usk and the Gavenny descended more steeply and flowed more rapidly. Water from melting glaciers brought gravels (as the Usk in winter flood still does) that were deposited where valleys joined – Abergavenny town centre is on one of these banks, Crickhowell on another. As sea levels rose around six thousand years ago, the rivers slowed and started to meander from side to side, depositing silt along their banks, forming the flood plains of which the Meadows are such a good example.

More recently, human activity has brought further changes. Over the past two centuries, floods have become more frequent and more severe as woods upstream have been replaced by farmland, from which rain runs off more quickly; ploughing has exposed more soil to be washed into the rivers by those floods. It seems very unlikely that the Romans would have chosen to build a town on a site that is flooded as often as modern Usk.

Sea levels are now rising again, as a result of global warming, and land use continues to alter. Far from being unchanging, geology and landscape can be very dynamic.