Norber Erratics are fascinating and unique geological formations scattered on the western flanks of Ingleborough in Ribblesdale near to the village of Austwick.
They are formed from over a hundred precariously balanced boulders and widely thought to be the best example of glacial erratics in the British isles, erratic referring to the fact that they are sited in locations that are unusual and seemingly random. The last time glaciers scoured this valley was only around 12,000 years ago and it was these ice sheets that deposited the boulders as they retreated at the end of the cold period.
Greywacke and Limestone
As you approach the boulders you can see that they are darker than the rock beneath them which appears almost white, this is because they are formed from a type of Sandstone dating from the Silurian period, which was over 400 million years ago, called ‘Greywacke’. The lighter rock they were deposited upon is another type of sedimentary rock called Limestone, the same rock which forms the peaks of Ingleborough, Pen-y-Ghent and Whernside, this is younger than the slate at around 300 million years old.
As you can see in this video and this 3D Computer Model the topmost boulders are very often wider and heftier than the Limestone plinths they are sat upon, this is because Greywacke is a much harder and impermeable rock than the softer Limestone which, although dissolved by rainwater over the years, has protected enough of it to leave behind a kind of pedestal to elevate it above the ground.
The Greywacke originally came from Crummackdale which is about a mile or two to the north, there strata of Greywacke were covered by Limestone over the course of about 100 million years but erosion of the softer Limestone in the following millenia exposed them to the elements again.
In the last Ice Age the movement of the immensely heavy ice sheet which covered this part of the country scoured clear the topmost layers of Greywacke and carried them with it, here a particular glacier carved out Crummackdale on its way south into Ribblesdale.
As the glacier spread into the valley and slowed down it lost momentum and, like a bored dog dropping a stick, deposited the boulders of Greywacke as moraine, on top of the younger Limestone.
Well worth a visit
The different types of rock also appear different colours for another reason, this is due to the types of Lichen growing upon them, with the Greywacke being more favourable to species of green lichen, mainly Rhizocarpon geographicum and the Limestone to a lighter coloured species, this accentuates the difference in the colour of the underlying rocks and makes it even more striking.
The erratics are well worth a visit even for those with no interest in geology, if not for the sheer, childish fun to be had exploring them then for the view, with the Bowland fells and the Three peaks both being visible from here. For botanists there is fun to be had peeking and peering into the nooks and crannies between and under the erratics, as there are species of ferns and plants here which also grow in the Limestone pavement of the nearby Yorkshire Moors.
To reach the Norber erratics it’s only a short walk from the pretty village of Austwick, which is about quarter of a mile from the A65, there’s a decent pub in the village; the Gamecock and the Traddock hotel is long established as a fine place to stay, often used as a base for geologists making field trips to the erratics to study their strange origins.
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Northwest Nature and History 