Between 60 and 55 million years ago, in the Palaeogene Period, the North Atlantic Ocean started to rift open, causing molten rock to radiate out through underground cracks from volcanic areas in what is now western Scotland. It solidified into sheets of rock which cut across Scotland and northern England. One of these is the Cleveland Dyke, which stretches 400km from Mull to the Yorkshire coast and cuts across the Eden Valley north of the Westmorland Dales.
The Cleveland Dyke forming a natural weir across the River Eden at Armathwaite, between Penrith and Carlisle. Here the dyke cuts through Penrith Sandstone. © M. Byron