The rocks of Jenny Twigg, the nearby Sypeland crags, and Brimham Rocks, some 20 miles away, are made up of millstone grit. This is a sedimentary rock that was deposited around 320 million years ago during the Lower Carboniferous period, a time well before the dinosaurs walked the earth. At that time Britain was part of a land mass called Laurasia and was attached to America.

The mill stone grit formed as a result of sediments being washed down from the mountains and deposited in the mouth of a large delta. Here the sediments were sorted and sifted by the actions of the river and sea into layers of deposits with differing characteristics. The courser materials tended to sink to the bottom near the mouth of the delta, whilst lighter materials were washed further out to sea. Over a period of millions of years these deposits deepened and formed distinctive layers. Eventually the depth of material that formed the millstone grit reached around 6000 meters.

Over time the deposits hardened and were heated and consolidated into a hard rock that preserved the ripple marks that were formed by the action of the sifting process of the original delta. Continental drift then separated Britain and America by plate tectonic action, leaving deposits of millstone grit on either side of the Atlantic. In Britain the grits were pushed up and buckled towards the surface and became outcropped over the area we now call the Pennines.

Once the rocks were exposed, erosion then acted to wear away the less resistant parts of their structure by chemical and mechanical means, leaving only the harder materials behind. These materials preserved the original layers created when they were deposited in the delta.

In the more recent times, (around 11000 years ago) the ice age accelerated the erosion process and contributed greatly to the rocks current shape and appearance. All over the Pennines millstone grit can be seen sculptured and contorted into strange and fascinating shapes.

At Brimham rocks there is a large outcrop of millstone grit that is owned by the National Trust. Here the stones have been weathered into the most bizarre and contorted shapes imaginable. Many have been given names such as ‘ The Dancing Bear’ and ‘ The Druid’s writing desk’. ' Idle rock' is a large chunk of millstone grit that weighs in the order of 200 tons and has been left suspended on a narrow plinth of rock just inches wide. It looks as if it might topple at any moment and one day it will become what's known as a rocking stone.

At the Sypelands the rocks are more desolate and remote than those at Brimham, they retain a quality of uniqueness, mystery and isolation that is hard to come by anywhere else in the country. It is a pity that such a location remains out of bounds to the public.