Formed under the sea then weathered over millennia by glaciers, Britain’s distinctive limestone outcrops are spectacular, natural curiosities
High above a seemingly barren landscape, a skylark sings as it rises into the air. Under the cloudless blue sky, flat slabs of shadeless, bare limestone are baking. Stunted bushes, few in number, that appear contorted in pain, and a solitary tree are an incongruous sight in such a hostile environment.
This is the countryside of the north of England’s limestone pavements, dominated by rock, with only a scattering of vegetation to soften it. Deep incisions run across a wounded land as if cut by a gigantic knife. Almost geometrical in their layout, these scars are not the work of man; rather, they are wonderful curiosities of natural processes. The product of both tropical seas and subarctic glaciers, they encapsulate evidence of the northerly drift of the British Isles over millions of years.
The formation of limestone pavements began some 350 million years ago, during the time that geologists refer to as the Carboniferous period. Britain was on the equator, and what are now the rugged hills of northern England lay under a warm tropical sea. The bodies of microscopic sea creatures accumulated on the sea floor. Over millions of years, they became consolidated into the sedimentary rock now known as limestone. Pushed upwards by tectonic forces within the earth’s crust, today, limestone forms the bedrock of much of the English Pennines, extending from Derbyshire to Yorkshire and Cumbria. These mountain-building episodes also flexed the limestone, causing it to fracture and creating joints in the rock that run at angles to the original bedding planes.
Far more recently in geological time, northern Britain was glaciated. For much of the last two million years, a succession of huge ice sheets covered the uplands. Glaciers ground their way across the limestone, stripping soil and scouring the underlying rock. The pressure of ice fractured limestone along its natural bedding planes. Then, as the climate warmed and the ice melted, it left a flattened and polished rock surface.
Further evidence of this ancient glaciation is the presence of boulders scattered across the scoured limestone. Known as glacial erratics, from the Latin errare to wander, these are rocks not indigenous to the locality. Instead, they were transported long distances inside moving ice sheets before being dropped when the ice melted.
The melting ice is also responsible for the spectacular Malham Cove in North Yorkshire. Here the limestone pavement is located on top of a curved cliff face, 260ft (80m) high and 980ft (300m) wide. The cliff and cove below was formed by a huge ancient waterfall carrying glacial meltwaters.
Shaped by rain
Limestone is an unusual rock, being highly porous and also soluble. The characteristic appearance of limestone pavement is the result of water reacting with ice-fractured rock. Rainfall seeps through the joints, and being slightly acidic, it slowly dissolves the limestone. Where the fracture joints are approximately perpendicular to the original bedding planes, a curiously regular pattern of clints – blocks – and grykes – deep vertical fissures – is created. It is this that creates the characteristic ‘pavement’, which resembles paving slabs.
Most grykes are from 3-6ft (1-2m) deep, but occasionally they extend to as much as 20ft (6m) below the surface. Their width is anything from a few fractions of
an inch to 18in (0.5m), while they can be hundreds of feet long. Grykes that formed under a covering of post-glacial soil are typically more rounded in form. The longer the limestone pavement has been exposed
to the air, the sharper its edges will have been weathered.
Surface weathering of clints has also given rise to a range of other characteristic features of the rock. Runnels are shallow channels draining rainwater into grykes. Pans are shallow depressions which hold water, while pits are deeper, draining rainwater directly underground.
Home to rare fauna
This strange landscape may appear to be inimical to life, but it is not. A number of scarce butterflies can be seen on limestone pavement, including Britain’s most threatened, the high brown fritillary, Fabriciana adippe. Another resident is the northern brown argus, Aricia artaxerxes. Best adapted to the specialised conditions is the warmth-loving grayling, Hipparchia semele, that basks on bare rock.
As well as the skylark, birds such as the wheatear, meadow pipit and chiffchaff frequent the pavements. Back on the ground, another very rare inhabitant is the narrow-mouthed whorl snail, Vertigo angustior. At just 2-3mm in length, this minute creature lives in moss on the clints at Gait Barrows in Lancashire.
Nestling in the grykes
Bare rock exposed to wind, rain, frost and baking sun seems to be no more a promising environment for plants than animals. However, the pavement’s grykes offer a very different microclimate to its clints. Most grykes contain a little soil. This may be derived from boulder clay left by a melting glacier, fine debris washed or blown in over millennia and humus from plant decay. Furthermore, sheep and other grazing animals cannot get into grykes, with the result that plants within are protected from browsing.
The shady, humid environment of a gryke is perfect for moisture-loving plants such as ferns. Maidenhair spleenwort, Asplenium trichomanes, and harts-tongue fern, Phyllitis scolopendrium, are frequently encountered. No less than 85 per cent of the entire UK population of Dryopteris submontana, the rigid buckler fern, is found on limestone pavements.
An extraordinary diversity of flowers thrives in grykes. Some are ones more typically found in woodlands, such as herb Robert, Geranium robertianum, ramsons, Allium ursinum, dog’s mercury, Mercurialis perennis, herb Paris, Paris quadrifolia, and angular Solomon’s seal, Polygonatum odoratum. The shady conditions within grykes obviously suit these species, although there are still questions about how they got there in the first place. Evidence suggests that some limestone pavements were once not only covered by soil, but also wooded.
“Woodland was present from 8,000 years ago until maybe 5,000 years ago, when there was a general clearance by Neolithic and later peoples,” explains Rob Petley-Jones. He is Natural England’s national nature reserve warden for south-east Cumbria and north Lancashire. “Extensive grazing probably followed, which inhibited re-establishment of woodland.” With trees removed, soil washed or blown away, the woodland ground flora retreated into the grykes.
Among the most spectacular flowers found on the limestone pavement is bloody cranesbill, Geranium sanguineum, flowering through the summer. Its rich purple is especially striking when set against the pale limestone.
On the exposed surfaces of the clints, even tiny amounts of soil are sufficient to sustain drought-tolerant thyme, Thymus polytrichus, and biting stonecrop, Sedum acre. These plants are typical of very well-drained, sun-baked limestone grassland. Their presence, adjacent to typical woodland flowers in the grykes, emphasises the diversity of ecological niches present in this environment.
Other plants are very much pavement specialists, rarely found elsewhere. Among them are several orchids, such as dark red helleborine, Epipactis atrorubens. Flowering in July, the British distribution of this charming flower is confined to limestone in northern England, North Wales and Scotland. Its gorgeous flowers may be seen protruding from grykes, within which its roots are anchored.
Of all their diverse flora, the most surprising to find on a limestone pavement are trees. Solitary, wind-sculpted hawthorns dot many limestone pavements, such as above Conistone in Wharfedale.
A gryke seems hardly sufficient to sustain a mature ash tree, but several do exactly that. Examples can be seen at Twistleton Scar, near Ingleton, and at Malham Lings, about a mile east of the cove. Some of these trees may have become established when soil still covered the clints. As the soil diminished, their roots penetrated cracks and fissures in the limestone, giving access to just enough nutrients and water to sustain them.
Ash is an easy tree to propagate, and seedlings protruding from grykes show that the establishment of new trees on areas of bare pavement is also possible. Even when mature, however, these trees are dwarfs. They grow exceedingly slowly and reach a height of just 6-10ft (2-3m), after 1,000 years. This may be compared to woodland ash, which might reach 100ft (30m) at maturity and would be a veteran tree after 300 years.
Rob has studied how limestone pavement ash lives so long. “Continual drought stress modifies their growth form,” he says. “They have very slow rates of growth, often dropping their leaves early in dry summers to save water. In extreme drought, the aerial stems may even die back, leaving just the ancient rootstock to survive for future years. It’s a form of natural coppicing that allows the rootstock to live for maybe 1,000 or 1,500 years, though the stems may be much younger, at 200 years.”
Occasionally, a solitary limestone pavement yew can be found, eking out existence at the limits of endurance. Frost, drought and wind all take their toll on growth, and there is little space for roots in the confines of a gryke. These yews are stunted by their stressful environment into modest bushes, barely 3ft (1m) high.
Scenically unique, wildlife-rich and scientifically fascinating, the limestone pavements of northern England are one of the world’s rarest habitats. Although much has been destroyed by decades of extraction for use as garden features, this unique environment is now protected by laws which prohibit damage, or the removal of any stone.
“There are wonderful rock formations which you don’t get anywhere else,” says Rob. “I am endlessly looking down grykes to find something new. It’s fascinating.”
Having survived for thousands of years, these curious, hauntingly beautiful landscapes will continue to intrigue future generations for years to come.
Britain has approximately 10sq miles (26sq km) of limestone pavement, representing a large proportion of the habitat globally. Northern England has the most extensive and best-known limestone pavements in the UK, but Wales and Scotland each have their own. In Wales, there are two arcs of limestone pavement formed on Carboniferous limestone. One extends from Denbighshire along the north coast to the Isle of Anglesey. The other is in the south, mostly within the Brecon Beacons National Park. Scotland’s limestone pavements are scattered across the Highlands and Inner Hebrides. The rocks here are older and belong to the Cambrian and Ordovician periods. This limestone was formed between 100 and 200 million years earlier than that of England and Wales. Although there are differences in climate, limestone pavements in Wales and Scotland support similar flora and fauna to those of northern England. For example, the distribution of dark red helleborine closely mirrors that of limestone pavement across the whole of the British Isles.
One very rare plant found on the limestone pavement is the sensuous lady’s slipper orchid, Cypripedium calceolus. Although once fairly widespread in northern Lancashire and Cumbria, collection by Victorian gardeners led to its virtual extinction in Britain. By the 20th century, only a single plant remained, growing in the Yorkshire Dales, dubbed “Britain’s rarest flower”. Since 1989, however, a concerted effort has been made to introduce lady’s slippers to several of their former haunts. It takes between eight and 15 years for the exotic-looking bloom to be produced from seed, but the project has been successful. Limestone pavement at Gait Barrows nature reserve in northern Lancashire now has a thriving colony of lady’s slippers, flowering in late May and early June. “Gait Barrows has a long history of ancient plant communities,” says Rob Petley-Jones. “The mycorrhizal fungi required to sustain the orchid are not fully understood, but we thought they were likely to be present. Each orchid seed pod contains up to 50,000 seeds, and only one of those might make it through to flower.” Public access was another objective of the re-establishment programme, and Gait Barrows welcomes visitors to see this spectacular native orchid.
Words and photography: Robert Harvey