This extension of the Great Glen fault that cuts through the Scottish Highlands is the best ‘hands-on' exposure of a shear fault to be seen in Britain.
Between 480 and 390 million years ago the Iapetus Ocean closed as continents on either side collided and forced up the huge Caledonian Mountain chain. Sandy sediments were metamorphose to form psammite and tilted almost vertically. Rocks melting beneath the mountains generated magma that forced its way up into the crust and solidified into bodies of granite. After the Caledonian Mountains formed, tensional forces pulled blocks of crust horizontally past each other for hundreds of kilometres along a series of strike-slip faults. At Back Sand Dalradian psammites lie side by side with the Graven Granite, separated by the Walls Boundary Fault – an extension of the Great Glen Fault that runs through mainland Scotland. The fault is not currently active but significant movement occurred around 370 million years ago and again 200 million years later causing massive earthquakes and moving the rocks on either side about 100km past one another. Fault movements crushed and folded the psammites against the tougher granite.Folding varies in scale from centimetres to several metres producing the beautiful features at the west end of the beach. The sandy bay formed as the weaker crushed rocks eroded exposing the 20m high granite wall in the east.
