Limestone: the language of pavements

This blogpost is part of my ‘limestone lockdown’ project. For an Introduction to the project, and a guide to the growing list of related posts, see Limestone in the Lake District: an Introduction – and the ‘categories’ list in the right-hand bar.

Clints, grikes, karst, karren and kamenitsas – they are evidence of the power of water in its liquid or frozen state, the power that sculpts limestone to form ‘pavements’.

Smooth surfaces, reflecting light from the sky when wet; slippery underfoot; tabular, raised above the grass, and deeply incised by intersecting lines of grikes, accentuated in the summer by burgeoning lines of ferns and dark whorls of willowherb. Or thin plates, jagged-edged; others intercut horizontally like the ragged-edged pages of an ancient book, edges that are sharp and unforgiving. Grikes that are long and straight, or truncated; shallow with overhanging edges, or deep and narrow, their dark interiors glowing with the red-purple leaves of Herb Robert. Surfaces are scooped and hollowed with channels and pools. There are ridges as smooth as sea-washed whale-bones, and delicate arches and whorls. Every rock invites attention.

To stand on the raised mound of Castle Fold at Great Asby Scar in the Westmorland Dales is to stand on a low island in a sea of ancient, Carboniferous limestone, now bared to the sky by erosion. But this wasn’t deposited as a single layer during that long, long era; it’s now known that there are many different layers, each corresponding to a time when the area – then part of a coastline South of the Equator – was a shallow, warm sea, the sea-bed teeming with invertebrate animals such as brachiopods, corals, sponges, crinoids and bryozoans; most of them sedentary, all of them filter-feeding on the organic matter and plankton brought in on the tides. The relative levels of the sea and land fluctuated throughout those millions of years, and during those periods when the seabed was exposed and became dry, its inhabitants were desiccated too. When the sea returned, so too did marine life, to re-colonise the sea-bed: the to-ing and fro-ing repeated over and over again. Microscopic planktonic animals drifted downwards when they died, their tiny skeletons contributing to the sediment. Creatures died, and their hard parts – the shells and tubes and minute cell-shaped box-y skeletons – accumulated over tens of thousands of years, being compressed and lithified to form thick layers of calcium carbonate. The fossilised skeletons of whole corals and brachiopods are scattered through the rocks at Great Asby. Occasional climatic events would have caused mass deaths, as evidenced by the fossil-filled boulders at nearby Little Asby: a rich graveyard of former brachiopod lives.

‘Carboniferous sea’: (C) Elizabeth Pickett (my grateful thanks to Elizabeth for allowing me to use this image)

The beds of sediment are not intact sheets of material because, in drying, they crack and ‘joints’ open up through which water can percolate. Horizontal gaps between the beds are filled with muddy shales or even volcanic dust. These layers or bedding planes, sometimes a metre or more thick – and therefore tens of thousands of years in the making – are clearly seen in many outcrops and scarp edges but more especially in quarries. As you would expect, their sequence and age, and of course their naming, has been much debated by geologists. In the Westmorland Dales local places like Ashfell, Potts Beck and Breakyneck Scar have given their names to these layers of the Asbian or Great Scar Group.

Layers of limestone, Pickering Quarry, Great Asby

Probably the majority of people would not think, if they ever even wondered about it at all, of geology as being a ‘research’ subject, but the ideas about how the limestone pavements have formed are various, and it’s a continuing subject for investigation: it’s accepted though, that limestone pavements arise in different ways, they are ‘polygenetic’ (of many origins, a good weasel word). It can be hard to comprehend the length of time, those tens of millions of years, during which the hard parts of marine animals accumulated and were eventually transformed into rock. What is slightly easier is to think in thousands of years, and consider the time when glaciation reached its maximum about 22000 years ago. At that time the great limestone beds of the Westmorland Dales would have been (roughly) where they are now on the face of the globe, in a land that was more recognisable as ‘Britain’ (although not yet an island archipelago). They were there, but only gradually becoming visible as glaciers crept over the surface of the land, filing down the surfaces and gouging out corries and valleys. Overlying rock was scraped away to expose the limestones to the rain and sun and freezing temperatures.

Water trickled through the joints and between the beds and, because it was made slightly acidic by dissolved carbon dioxide from the air, it reacted with the calcium carbonate and dissolved it, in the same reaction as dripping vinegar onto a cockle shell. The limestone was re-modelled both where it was exposed, and where it was hidden beneath soil and vegetation. Water scooped out hollows and runnels, and joints were widened into grikes. This became the typical karst landscape, with the names that define the intimate details of the pavement’s anatomy: hollows on the top faces of the clints are kamenitsas, a caressing kind of word; karren are stronger, the runnels and grooves. There are rundkarren, smooth-edged grooves sculpted by water running across a pavement that is covered by soil; the rinnenkarren are ridges; trittkarren are, as their names suggests, grooves that are ‘stepped’ to form different levels; and centripetal runnels seek the centre.

Every limestone pavement is unique in its design, the shapes and thicknesses depending on small differences such as the rock’s chemical composition or its angle of tilt to the horizontal. At Great Asby the patterns are diverse, differing each side of the Castle Folds valley that forms a perfect but shallow U-shaped syncline. In 1995, P Vincent wrote (quoted by Sylvia Woodhead in a very useful webinar about the Westmorland Dales) that at Asby Scar “there is probably more variety in pavement form than anywhere else in the British Isles.”

And so it is that walking around Great Asby Scar becomes an exercise in leaping, striding and tip-toeing; balancing, bending, crouching and touching. The pavements are neighbourhoods that are home to hundreds of plants and insects; blue-green lizards spread themselves on the rock on sunny days; curlew-calls bubble and wheatears flit and chak. And in the winter when the freezing wind and rain blast across the moorland, water seeps and trickles over the stone, and ice prises rock flakes apart, as the pavement changes inexorably and slowly.

Sun-bathing lizard
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