A limestone pavement is the floor of a sea, raised and turned to the sky.
This is not a metaphor. It is the plainest possible account of the material. The grey rock that stretches in terraces across certain coasts in the west, pale, fissured, ringing underfoot like something hollow, is made almost entirely of the dead. Calcium carbonate, laid down grain by grain on the bed of a warm sea that covered this part of the world some three hundred and thirty million years ago. The bodies of crinoids and corals, the shells of brachiopods and the microscopic plates of creatures too small to name, settling through the water as they died, accumulating into a sediment that hardened, over unimaginable time, into stone.
To walk on limestone, then, is to walk on the compressed remains of an ocean’s worth of life. The fact does not announce itself. The rock looks like rock. But press your eye close to a weathered surface and the evidence is there: the coiled ghost of a shell, the segmented stem of a sea lily, fossils held in the matrix like punctuation in a sentence whose meaning has been lost.
The sea, withdrawn but not gone
What makes limestone strange among stones is that it never fully forgets what it was. Granite is born of fire, basalt of eruption, violent origins, sealed and final. Limestone is born of patience and of water, and water remains its master.
Rain is faintly acidic. Carbon dioxide dissolves into it as it falls, making a weak carbonic acid, and this acid does to limestone what no hammer can. It dissolves it. Not quickly. A human lifetime registers almost nothing. But across centuries the rain finds the cracks in the rock, the joints where the original sediment fractured as it lifted, and it works into them, widening, deepening, carrying the dissolved stone away in solution toward the sea it came from.
The result is karst: the particular grammar of dissolved limestone. The flat slabs are called clints. The fissures between them, scoured open by ten thousand years of rain, are grikes. From above, a limestone pavement reads as a kind of script, blocks of pale stone separated by dark lines of shadow, a text written by water into a surface that was once the bottom of the world.
In the grikes, out of the wind, a remarkable thing happens. Shelter and lime-rich soil collect, and plants grow that have no business sharing ground: arctic and Mediterranean species side by side, gentians beside orchids, ferns in the cool dark of the cracks. The dissolving of the stone makes the conditions for the living. The sea, withdrawn, becomes a garden.
What deep time does to attention
There is a way of looking at landscape that the geologist Tim Robinson practised on the limestone of the Aran Islands and the country behind them, a reading of ground so close it became a form of devotion. He understood that to know a place fully you had to hold its present surface and its deepest past in the same glance: the wall built last century and the seabed laid three hundred million years before it, occupying the same square metre of attention.
This is the gift limestone gives, if you are willing to take it. It collapses the scale we ordinarily live by. A person stands on the pavement and feels, briefly, the vertigo of it, that the solid thing underfoot is not solid in the way it seems, that it is sediment and time and dissolving water, that it was sea and will be sea again, that the firmness is a passage and not a state.
Heaney, who came from a different ground entirely, the bog country, wrote of the earth as something that kept things and gave them back. Limestone keeps differently. It keeps the dead as structure. It gives them back as scenery, as the pale theatre of the karst, as the gardens in the grikes, and, slowly, dissolved, as the minerals the rain carries seaward.
Stone that moves
We tend to think of stone as the opposite of water, the fixed against the flowing, the permanent against the passing. Limestone refuses the distinction. It is water made solid and then, by water, slowly unmade. The pavement is not a thing so much as a moment in a process: a seabed paused, mid-return, on its way back to the sea.
There is something steadying in this, though steadiness is the wrong word for a stone that is quietly dissolving. Call it perspective. The rain that falls on the pavement this winter is doing the same work it has done since the ice withdrew, writing its slow lines into the rock, finding the joints, carrying a little of the old ocean away.
The sea is not gone. It is only being patient, as it always was.