An Sgurr Some 58 million years ago the area we now know as the Hebrides (a group of Islands off Scotland) were a place of fire and smoke rather than the present windy sea lashed locale of the present era. The North Atlantic Ocean was opening, completing the bustup of Pangaea. This sundering of the European and North American plates was accompanied by huge outpourings of lava over a wide region, whose remnants still dot the landscape from Greenland to Ireland, Canada to Scandinavia. This 5km long ridge on the isle of Eigg is what remains of a hug pyroclastic flow from a large eruption, whose main vent was located at the present isle of Rum. An Sgurr is also a textbook example of what geomorphologists (those who study the surface shape of the Earth and how it came to be) call inverted topography, a process whereby what were once lowlands turns into highlands due to an interaction between the constraints imposed by the underlying geology and the forces of erosion. When the silica rich rhyodacitic lava erupted, the hot molten ash and rock flowed from the vent down a river valley, filling it to a height well above the 400 metres of the uneroded remnants that we see today. The rock is dark (unlike most silica rich volcanic rocks) because it is a quick cooled variety known as pitchstone with a high glass content, but coarser and with more crystallinity than the more familiar obsidian. It looks like a glassy matrix, filled with angular crystals. Our ridge is the largest exposure of such rock in the UK, and it displays typical lava cooling features (such as columnar jointing similar to the Giant's Causeway). Pitchstone also happens to be very resistant to erosion, the result of its peculiar part glass part crystal structure, and over the aeons since it was emplaced, the basaltic rocks (also from the growing continental rift) around it have melted away under the assault of wind, frost and wave, leaving the fossilised impression of a whole Palaeocene river bed standing proud as a tall ridge, complete with its rounded boulders and tree fossils at the base. Solitary mountains like An Sgurr (the notch) are known as inselbergs, but they form by a variety of processes, and most are not inverted. Loz Image credit: James Gray http://www.geograph.org.uk/snippet/11009 http://www.isleofeigg.net/an_sgurr.html http://www.summitpost.org/an-sgurr/195034