During the Carboniferous period 350 million years ago, the whole area was the bottom of a warm and shallow sea. The remains of coral and shells fell to the seabed, and coastal waters dumped sand and silt on top of these lime deposits. Time and pressure turned the layers to stone, with limestone below and shale and sandstone above.

Massive rumblings in the earth’s crust some 270 million years ago buckled the edges of Europe and forced the seabed above sea level, at the same time bending and fracturing the stone sheets to form, long deep cracks. Wind, rain and ice have since removed most of the overlying shale, leaving these mountains of limestone.

Being slightly acidic, rainwater dissolves the limestone, widening the vertical cracks known as grikes. Springs, rivers and even lakes, known by the Irish appellation turloughs, around Corofin appear and disappear, seemingly disappear into the underworld. The water follows weak points (sinkholes) in the rock, carving out underground rivers and caverns. The calcium bicarbonate from the dripping water below creates stalactites (the ones that hang down) and stalagmites (the ones that shoot up). When these underground caverns collapse- which they do periodically- they form a depression. Rainwater is caught on top of the shale and eventually drains off at the edges into the limestone, which it erodes.

During numerous ice ages, glaciers scoured the hills, rounding the edges and sometimes polishing the rock to a shiny finish. The glaciers also dumped a thin layer of rock and soil over the region. Huge boulders were carried by the ice, incongruous aliens on a sea of flat rock. Seen all over the Burren, these ‘glacial erratics’ are often a visibly different type of rock.