Thursday, 4 April 2019

Down in the Basement: 

The Oldest Rocks of the Yorkshire Dales

The oldest rocks of the Yorkshire Dales form a substantial basement underneath the landscape we see and enjoy today.  They are mostly hidden from view, but thanks to being lifted close to the surface by the movements of the North Craven Fault, they have been exposed in Chapel-le-Dale, parts of Ribblesdale - and in Crummackdale.  

The rocks of the basement were laid down in the periods we call the Ordovician (named after the Celtic Ordovices tribe who inhabited landscapes composed of these rocks in Scotland) and the Silurian (named after the Silures tribe of Wales) between 480 and about 415 million years ago!!  'England' was then part of the continent of Avalonia, beneath the equator - while Scotland, Ireland and North America lay in the continent of Laurentia, across the Iapetus Ocean.  Imagine the Iapetus as being an Atlantic - but separating two vastly different continents in a different location on the globe ...

Huge rivers washed sands, silts and muds into this ocean  - and later 'landslides' on the sea bed meant the particles rose up and clouded the sea water (hence the alternative name for these rocks as 'turbidites' ) - and then settled in size order - with the coarser grains becoming sandstones and the finer muds becoming shales.  Nature wasn't quite finished yet though ...!

Tectonic movements began to push the continents together and fold the rocks like a huge sheet of corrugated iron.  These folds not only shaped the rocks so that some of the beds now stood vertically - but the immense pressure created heat which partly 'cooked' or 'metamorphosed' the rocks into a different state.  Therefore the sandstones became a hard, coarse grained rock we call 'greywacke' while the shales and siltstones became 'slates'.  The crests of the folds we call 'anticlines' and the dips we call 'sinclines'.  The folding effectively created massive mountain ranges on the sea bed, much bigger than the Alps or Pyrenees.  You can see the dip of the beds from the folding in the photo above.  Here the crest of the anticline - a former mountain top - has been sliced away by erosion and glaciers over millions of years - but you can still imagine it up there on the left.  The much younger Carboniferous limestone is seen laid on top of the old mountain range!  

The very oldest rocks of the basement are known as the 'Ingletonian' and here is an example - of a greenish hue and with mineral veins (quartz and micas).  These rocks were folded particularly steeply and contain no fossils - which makes them unique in comparison to the younger layers of the basement.  This is a sample from the old 'granite' quarry in Ingleton.  The rock here is a mixture of slates and greywackes - the latter containing pink feldspar chunks which give it the appearance of granite.  It is one of the very best places to look at the Dale basement!  Not only were these rocks folded, but a massive amount of real granite bubbled up around them and helped with the cooking process! It also helped form the foundation block of the entire Yorkshire Dales - a feature known as the Askrigg Block.  This quarry is a window into the block itself!

Slate beds can still be seen in the quarry in their natural state.

This photograph contrasts the greywackes and slates in the 'granite' quarry.

The Ingletonian are also famously exposed in the old Pecca Slate Quarry on the waterfalls walk.

Note the vertical bedding here - and think of the forces that must have been involved.  Incredible!

The Ingletonian are most memorably exposed at Thornton Force - where Great Scar Limestone (350 million years old) lies horizontally on vertically bedded Ingletonian rocks 130 million years older!

As we move east from Ingleton the beds are slightly younger.  These are dipping beds of Upper Ordovician rocks of the 'Norber Formation' - containing some fossils and limestones  and being much 'muddier' in composition - with less evidence of any 'cooking' by pressure and heat.   Here again, at Nappa Scar - you can clearly see a 'sliced off' anticline - the stump of an ancient mountain - with the limestone bedded on top.  During the Devonian period (416 - 360 million years ago)  the entire mountain ranges were lifted out of the sea by further tectonic activity and exposed to erosion, before the early Carboniferous seas drowned the mountain stumps allowing limestones to bed down on top!  You can see this clearly here.

Here at Studrigg Scar in Crummackdale we see a still younger formation, known as the 'Austwick Formation' from the later Silurian Period, with their beds rising steeply to the former anticline - and the Great Scar Limestone bedded down on top.  This is a classic 'unconformity' with a massive time distance between the two rocks.

And younger still - we have the very smooth Silurian rocks of the Horton Formation - or Horton Flags - exploited here at Dry Rigg Quarry.  Each rock represents the grading of sediment on a sea bed in turbid conditions ....

Here's the Ingletonian slate - 480 million years old ...

'Norber Formation' siltstone from Nappa Scars - about 420 million years old ...

And the slightly younger 'Horton Flag' sandstone of the Dry Rigg quarry.

The basement of the Yorkshire Dales is a fascinating roller-coaster of rocky surprises and variations which never fails to capture the imagination.  

Stephen x