Beneath the Radnor Forest: Reconstructing a Lost World

The Radnor Forest as we know it today is a beautiful range of open hills, with many layers of history to unravel. Beneath that, though, is another layer entirely. The outline of the hills is geologically modern, shaped by the ice over the past few million years. That's nothing, really; geological forces work their magic over much longer timescales...

            The hills of Wales, Scotland and the Lake District are the last remnants of one section of a mountain range that stretched from Norway to the Appalachians (before the Atlantic opened, of course)... and those mountains have been eroding for some 400 million years. At the time they were pushed up, plants and invertebrates were getting properly established on land, and certain fish in the shallow rivers were taking the odd gulp of air... if, as they say, the rest is history, then we're looking here at a time before it. That, though, is just when these rocks were pushed upwards, driven by a continental collision between England and Scotland. The rocks themselves already existed... and it's to the world of their formation that we now need to look.

Fig. 1. A typical outcrop of thinly bedded Silurian siltstone, at Fron-Las Dingle

            As you walk around the Radnor Forest, you'll see quite a few outcrops (e.g. Fig. 1 above), in old quarries and on the sides of ravines, and in most of them the rock is all fairly similar. It's grey, fine-grained, and obviously layered, usually breaking along beds a centimetre or so thick. Look closely, and within those layers you will sometimes see finer layers, on the scale of a millimetre or so; in other parts of the sequence, those fine layers aren't there. The sequence of rocks is hundreds of metres thick, but represents only a few million years... but that is still long enough for there to have been some changes. The oldest rocks (on the western slopes) are the finely laminated ones (sometimes with thick sandy and limey beds interspersed as well), and the more continuous grey rocks above represent a slightly different environment. With the right eyes, we can read these changes, and understand the world of that time.

            Some of the biggest clues we have are fossils (Fig. 2 below). The remains of animals and plants tell us what the environment was like, because each group has habitat preferences and limitations. In the Radnor Forest, there are fossils all the way through the sequence, although they are much more abundant at some levels than others... and they all tell more-or-less the same story. Among the most common fossils are graptolites: small colonial animals whose skeletons look like hacksaw blades. These were planktonic, drifting through the surface waters of the ocean, and the species present can be used to identify the age of the rocks. After identifying the species in various places, we can work out that the rocks of the Radnor Forest were laid down as sediment on a sea floor, between 423 and 428 million years ago: the later part of the Silurian Period. These are not the only fossils, though...

Fig. 2. A selection of fossils from the Radnor Fforest; from left to right: a graptolite, Monograptus sp.; an orthoconic (straight-shelled) nautiloid; small Eoplectodonta brachiopods and crinoid fragments; a complete crown (body and arms, lacking the stalk) of a crinoid, possible Periechocrinites. Scale bars 1 cm. 

            Perhaps the easiest fossils to find here are the long, conical shells of nautiloids: ancestors of squid and ammonites that swam through the water hunting for prey. Most of their food would have been soft-bodied or delicate, and was never fossilized; sadly, we only ever see a small part of the total ecosystem. Other fossils are common at different levels in the sequence, though, including the ribbed shells of brachiopods, stick-like colonies of bryozoans, and rare fragments of trilobites. At some places (particularly in the older rocks), there are thin limestone beds made mostly of dense, winnowed layers of these skeletal remains. There are also rare examples of much more unusual fossils, including sponges and sea lilies (a delicate, stalked relative of starfish).      

            The story is complicated, but combining the fossils with the details of how the sediments were laid down, we can reconstruct a picture of this lost world. Imagine a warm sea, perhaps 20-30 miles from land, with a quiet, practically dark sea floor. The remains of animals gently fell from the water above, and sometimes storms at the coast washed in layers of shelly detritus from the shallows, but little lived on the sea bed itself... oxygen levels were too low for comfort. As the ages passed, the sea became shallower, and the sediment built up more quickly. Animals colonised the sea floor as the oxygen levels increased, and worms started burrowing through it. The sequence ends before it became so shallow that it disappeared... but that story is also told nearby, and can be revealed around Kington and Ludlow.

            It was a different world, in many ways, even though there are hints of familiarity. Our little continent was in the southern tropics, and the first, tiny land plants were getting a foothold in the damp river valleys. We could make up more stories about the details, but we don't need to: the real history is revealed in the layers of sediment... which are far older than the hills themselves.