Between 394 and 384 million years ago during the Devonian Period, rift basins formed in and around the Caledonian Mountains as the landmass of Euramerica was pulled apart by tensional forces. Sediments eroding from the mountains collected within these basins to form the sandstones and conglomerates found in Shetland today. Three distinct groups of sedimentary rocks are recognised in Shetland. They formed at different times, in different sedimentary environments and were brought to their present relative positions by horizontal movements along strike-slip faults. The sedimentary rocks of Bressay belong to the youngest of the three groups. They were deposited on top of a thick sequence of metamorphic rocks, some of which contained deposits of ore minerals. ?Tens of millions of years after the sandstones formed, volcanic activity deep in Earths’ crust caused magma to intrude into the metamorphic basement rocks, fracturing them and tilting the overlying sandstones. Some of the basement rocks melted, producing large volumes of gas and carbonate rich fluids. The gases forced their way upwards creating vents in the sandstone by shattering the rocks into a breccia. The fluids formed orange/brown mineral veins within the sandstones. Breccia and carbonate veins surround one such vent at Muckle Hell.
These sandstone rocks were shattered by gases forced upwards at high pressure due to volcanic activity deep beneath the sandstone beds.
Directions
Bressay is opposite Lerwick and is accessible via a ferry which departs from beside the Esplanade (A969)
Drive across the island following signs for Noss and park at the end of the road
Follow the coastline to the north until you reach Muckle Hell Vent
Please note this walk contains two-step stiles.