Geology and Landscape

The area of the Heart of Wales Geopark includes a very wide range of rock types in a small area, and this gives rise to the complex landscape. The edges of the inlier are readily visible as you travel around it, because the hills inside it become lumpier, rockier and more irregular. Compare the smooth lumps of the Radnor Forest with the crinkled Llandegley Rocks, and the contrast is obvious. There is much more information available here about the geology and fossils, and this section is only a brief introduction.

The surrounding area of rounded hills and wide valleys has been carved by ice into Silurian sediments, which were laid down in a quiet, stable sea some 440–420 million years ago. These mudstones and occasional limestones contain a record of the ancient environment, with occasional storm-washed layers of debris in a finely layered, muddy background. Some of the creatures that lived there can also still be found as fossils, from the drifting colonies of graptolites to the shells of predatory nautiloids.

The Ordovician rocks (around 455–465 million years old) that form the centre of the geopark are the remains of a volcanic island system. You can't see the volcano any more; it has been eroded, faulted, twisted, chopped up and worn away by ice. In general, the rocks are tilted down towards the east and southeast, where the oldest rocks are preserved, with the youngest layers now cropping out in the extreme west and north—but that pattern is very inconsistent. However, the landscape still holds clues, in that the high, rocky hills are generally made from volcanic rocks: either quartz-rich ash, or igneous intrusions (magma bodies that solidified underground).

These hills are not just volcanic, though. Around that ancient island sand and mud was deposited in thick layers, preserving the remains of the animals that lived there. Over roughly ten million years, the environments changed dramatically, from shallow, pebbly coastlines to quiet, dark, muddy sea floors. These changing environments can be understood as part of the evolving volcanic history, and a great deal of information can be teased out from even the most ordinary-appearing of rocks.

The area was an extremely rich location for Ordovician life, and the evidence is still there today. Hundreds of species are preserved as fossils through the sequence, and we can trace the patterns of their appearance and extinction through the rocks. We can even understand their ecology, based on the distribution patterns and environments that they are found in. Many of these species are unique to the area, having never been recorded from anywhere else.

Most importantly, the area contains several fossil sites that preserve “exceptional” fossils: the remains of animals and plants that normally cannot be preserved. The most spectacular is the recently discovered Castle Bank Biota, which preserves amazing fossils of (often very small) animals and seaweeds, including entirely soft-bodied animals and their internal organs. Many entirely unique species are known from here, with much more yet to be discovered.

The completeness of the rock record in the geopark, and the richness of its fossil deposits, makes the area critical to understanding Ordovician life and how it adapted to the warm, volcanically active world of the time.