Travelling back in time a few millenia, Brian Spencer describes what the Peak District looked like when land that became the British Isles was part of a tropical island situated near the Equator.
A series of small inter-connected abandoned limestone quarries off the road to Ashbourne, near the top of the hill marking the boundary between Wirksworth and Cromford, close to Steeplegrange miniature light railway, hold a unique record of the limestone rocks formed in an ancient sea, long, long ago. This record has been preserved as the National Stone Centre. Access is free and a short, easy walk of less than a mile, takes us back to a time before we humans walked on the earth.
What eventually formed the Peak District would have looked not unlike today’s Bahamas. A shallow lagoon surrounded by jagged reefs and dotted by small active volcanos, stretched from what is now roughly on a line between Ashbourne, Wirksworth, Matlock and Ashover in the south and a similar line from Buxton to Castleton in the north; its other boundaries followed what is now the Derwent Valley and the Dove and lower Manifold Valleys. This is now called the White Peak in acknowledgement of the colour of limestone laid down millions of years ago. Standing in the Stone Centre, facing the direction of Wirksworth, to your front would have been a deep ocean, whilst to your rear would stretch a roughly oval shallow lagoon teeming with corals, shell-fish and creatures similar to water lilies. The one unique feature of all these plants and animals was that they had shell-like structures and when they died in their countless billions became limestone, the raw material of a major Peakland industry.
rom the car park above the High Peak Trail walk down the Stone Centre access drive, past a restored limekiln on your left near the railway. Pass Geosteps an amphitheatre of stone benches, each level made of stones ranging from the oldest at the bottom, gradually moving in time to the youngest at the top. Although the trail proper starts at the Visitor Centre, the track passes the blocked-off top of a lead mine, a now dead Peak industry exploiting the result of minerals deposited during the time of volcanic activity.
There is a small, but easy to understand set of details, mostly posters fixed around the walls of the Information Centre café, browse on them, building up your knowledge before setting out on our voyage back in geological time. Plaques on the site of features mentioned below also explain what you are looking at.
Looking out to sea.
To the right outside the café entrance, a short hop past a line of parked cars leads to the first viewpoint. Here and with a little imagination, you are standing on a tropical island in mid ocean. This was a narrow strip of land forming a wave-washed reef, the edge of a shallow lagoon. Warm water was to the rear, fronting deep sea in front.
Moving on to the trail proper, the next feature is a series of rock shelves topped by a rock tower created by a local artist. What at first looks like nothing more than whitish rock is, after a closer look, made from the fossilised remains of billions of creatures that once lived on the surrounding sea bed. Most are easy to identify, from hollows made by the two halves of clam-like brachiopods. Once as plentiful as today’s mussels, they more or less died out. They lived on the muddy sea bed with their convex sides face-down. Nearby there is the circular top of a coral, a plant-like creature that thrived in water heated by strong sunshine. Looking at the lower sections of undamaged fossilised remains, it can be deduced that the sea bed was calm, but higher up the fossils are damaged and jumbled, a sure indication that this was where waves came crashing in from the deeper ocean.
Limestone beds above the lagoon
Moving round to the left, following the quarry face, the path leads into a cul-de-sac above a worked-out part of the quarry. Here are limestone beds that were deposited in the bottom of the broad, shallow lagoon. The debris is composed of the remains of shells and even faecal pellets and parts of skeletons, suggesting that larger creatures fed on lagoon life. This was the lagoon that eventually became the White Peak of Derbyshire. If you look carefully, you may notice that the beds tilt very gently to the east, a feature caused by much later earth movements also left vertical fractures or joints between the beds.
A mineral vein and crinoid beds
Turn back along the track leading to the limestone beds and go down a short flight of steps in order to stand beside a rock face to your right. You are now looking at the side wall of a fracture in the limestone face. These fractures were later filled by mineral rich, hot liquids that interacted chemically with the limestone to form lead-based compounds such as galena, barite, fluorspar and calcite. The vein you are looking at was once part of a worked out mine, but you can still see tiny grey flecks of galena (lead sulphide) and barite (barium sulphate). This vein, incidentally, is aligned east-west, a feature typically prevalent to Peak District veins or rakes.
Turning round to face the other direction, you will see that instead of a vertical rock face you are now in front of a sloping mass of dark grey rock. This is all that remains of the back slope of the reef sheltering the lagoon, away from the open ocean. Towards the base there are the fossilised remains of creatures called crinoids which are related to modern starfish. They fed by filtering seawater through their frond-like arms to catch microscopic plankton, whilst attached to the seabed by long stems. The stems being the strongest part means they were more likely to be preserved. Unlike other places where the stems are broken into fragments, many of the stems hereabouts are almost intact; indicating that conditions at the back of the reef were calm and sheltered.
Point 5. Standing at
the top of the reef mound.
A few yards gently uphill and further on you will be standing on top of the reef forming a protective barrier between the ocean and the lagoon. This was once limey mud strengthened by microscopic algae. The surface of the mound slopes away evenly in all directions – something that probably happened in the later times.
Following the path, you will pass the Millennium Wall. Using appropriate rocks, sections of it are made in the dry stone walling styles from all over the UK.
Inside the reef.
The path curves round to the right beyond the Millennium Wall, making its way to the bottom of the remains of part of the quarry. Because it has been quarried in the past, you will be standing inside the reef. Notice how the limestone rather than be in neat beds as in the lagoon, instead forms a solid mass of limestone, similar to what we saw at the last stop. Look to the east and into the main part of the quarry, here the dark grey limestone at a lower level gives way to paler brown rock. Look for the junction of the two rock types – it has a zig zag shape, an indication that this part of the reef was once exposed to the waves. Later the reef sank, possibly due to changes in sea level, or earth movements and the eroded surface of the reef was filled by another type of limestone.
Looking into the deep blue sea.
At the bottom of the quarry you are at the edge of the reef and gazing towards a deep pre-historic sea. Looking into the quarry, you can see that the now un-beded limestone is much thicker to the left with thin layers on top – these are made of fragments of rock from higher up the reef that rolled down the slope.
This point marks the southerly end to limestone in the Peak, beyond this point you will find only mud and sand, the bottom of a once mighty ocean.
What happened next?
Steady movements of the tectonic plate on which Britain stands moved colliding with another land mass, this in turn became the rock of the Dark Peak, with a nearby outlier called Black Rocks.