Silurian Shelter - The Norber Erratics EarthCache

The Norber Erratics

The Norber Erratics are a series of hard sandstone boulders that cover the hillside above Nappa Scar near the village of Austwick, and are of national importance to geologists as a textbook example of glacial transportation and weathering.They have been the subject of great geological interest for nearly 200 years as they are distinctly different from the limestone bedrock that forms the pavements for which the Yorkshire dales are so famous.

The term 'erratic' refers to the fact that these rocks were formed over 100 million years before the bedrock upon which they now sit, so they are out of place geologically speaking. The Norber Erratics also tell us much about the limestone features that surround them.

So where did they come from and how did they end up here?

The boulders originate from Crummack Dale, about a mile to the north. They were formed during the Silurian period over 450 million years ago, and were overlain by Carbonaceous Limestone 100 million years later. Subsequently, erosion that gave rise to the Crummack Dale Valley cut through to expose and fragment the older rock beneath.

Fast forward to the most recent ice age, between 20,000 and 70,000 years ago, and the entire area is now underneath a huge glacier. This glacier has pushed its way south, carving out the valley of Ribblesdale, picking up and transporting material (deposited as morraine) including the boulders as it moves. The map below shows the movement of ice through the local area during the most recent ice age.

By about 17,000 years ago, as the climate begins to warm towards the end of the ice age and the glacier retreats, the boulders are left stranded on the hillside at Norber, the older boulders sitting on top of the younger limestone bedrock.

This is an example of glacial transportation, which is interesting in its own right, but is only part of the fascination of the Norber Erratics.

The headline coordinates will take you to a good example of a dark sandstone boulder perched precariously on a limestone 'pedestal'. The harder Silurian sandstone is far more resistant to weathering than the underlying limestone, so over thousands of years the surrounding limestone has been lowered by chemical weathering, except where the boulders have protected the limestone beneath.

Tasks

1. Measure the height of the limestone pedestal beneath the boulder, and compare this to the height of nearby exposed pavement. What is the difference in height? (hint; if you express your answer as mm, it makes it easier for the next task!)

2. Assuming that the ice sheet retreated 17,000 years ago, using the difference in height, could you suggest the average rate of limestone erosion in this area? (express your answer as mm per thousand years).

This differential helps to tell us about how the local limestone pavements would have formed and changed over time.

3. Do you think this rate has always been constant? If not what factors might have influenced the rate of erosion? Looking to the future, how do you think this rate might change and why?

4. How does the surface, particularly the grikes (cracks), of the exposed limestone compare to those of the limestone pedestals? Can you suggest a reason for any differences in appearance?

Lichenometry - measuring time with Lichens

The clean air of the Yorkshire Dales supports the growth of lichens. Looking around the headline coordinates you will see at least two common species; the bright green Rhizocarpon geographicum and the bright yellow Xanthoria spp.

Both of these species may be used for lichenometry, a technique that uses the diameter of common lichens to estimate a period of time that a rock face has been exposed. This technique is a useful field tool to help to estimate dates of exposure of rock surfaces as far back as 500 years. This can be useful in an area of instability such as a boulder field to help determine which rocks may have been in place since glaciation, and which are more recent arrivals due to landslip.

   Rhizocarpum
geographicum      Xanthoria spp.

Bonus Task (optional!)

Looking at the boulders and the surrounding geology, which lichen species favours which rock type, and can you suggest a reason why?

If you would like to upload a photo then that would be welcome. Please just remember not to post any spoilers or give any of the answers away in your log. Thanks for taking the time to visit this Earthcache and I hope you enjoy your visit.

Earthcache placed with kind permission of English Nature.