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240508-05. © NRT, 2008
Glacial erratic, Norber, Yorkshire Dales National Park, above Austwick, N.Yorks., UK

About 15,000 years ago, the glacier occupying Crummackdale scoured blocks of Silurian greywacke from nearby low cliffs, transporting them about a kilometre before redepositing them here at the end of the Ice Age. That in itself is of interest to geologists as the displaced boulders, or 'erratics' are ~100 million years older than the Carboniferous bedrock beneath them.

Greywacke, a coarse sandstone, is far more resistant to erosion than the underlying limestone. Over millennia, the ground level has been considerably lowered by solutional weathering – except where protected by overlying greywacke canopies. The result can look startling, with large boulders 'precariously' balanced on half-metre high pedestals.

Even when not on the famous pedestals (and, frankly, few are), the Erratics are quite distinct from the native bedrock, as the the Silurian greywacke has been colonised by darker and greener species of lichen than the Carboniferous limestone (favoured by grey and white species), thereby amplifying the colour difference between the bare rocks.

It's no wonder that the Norber Erratics are of national importance to geologists and geomorphologists, not only as textbook examples (literally!) of the processes but also as dramatic inspiration to students.

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