The creation of the tunnel in front of you exposed rocks formed 326 million years ago – a time when this part of the world was a reef in a warm tropical sea, close to the Equator. The Peak District climate and environment was similar to the Great Bahama Bank of today.
The layers of rock in Headstone Cutting are limestone and Longstone Mudstones, a type of shale. They show that this area was once a tropical lagoon with corals, plants and sea creatures. Like the Mississippi River at the Gulf coast today, the Peak District lay in the way of a large river delta that deposited sand, silt and mud into the basin. Gradual deepening of the seawater, either by a sinking sea floor or rising sea level, or both, allowed early inputs of mud into the marine basin, this forms the shales.
In 1860, when the Midland Railway was under construction to link Derby with Manchester, it is thought that gunpowder was used to blast the layers of rocks whilst men used simple drills to cut away the stone. Headstone Tunnel is the longest of the tunnels along the Monsal Trail at 490 metres. Shafts were sunk from the ground above and gangs of navvies would have dug each way from the shaft. You can still see outlines of the shafts inside the tunnel.