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

Wednesday 27 March 2019

Nappa Scars - Awesome Geology

A huge slice of the basement rocks of the Dales has been brought to the surface by the North Craven Fault, and further exposed by erosion.  Nowhere is this better seen than at Nappa Scars, just north of Austwick.  Here, the 350 million year old beds of Great Scar Limestone rest on the basement rocks - the eroded stumps of a former mountain range,  formed under the sea over 400 million years ago.  It is one incredible place!

A lovely walk up from the village sees the scattered erratic boulders of the Austwick Formation (from the Silurian Period so slightly younger than the Nappa Scar basement outcrops), providing convenient short-cuts to drystone walling.  These famous boulders were plucked from the valley sides about 1 kilometre to the north at SD770704) and dumped both here and on Norber Brow, where they lie as one of the most famous geological locations in the UK.

Many lie scattered on the emerald green turf.  This view looks eastwards along the line of the North Craven Fault, with all the land to the left having been lifted in relation to the 'downthrow' on the right, effectively exposing this wonderful slice of undulating basement.  Let's have a closer look at this baffling series of rocks ...

The 'rollercoaster' nature of the basement can be seen in this image - where the basement rocks here dip from a crest or 'anticline' that was weathered away long before the limestones on the top (visible top right in the picture)  were laid down.  What we are seeing here then, are the stumps of ancient mountains beyond our imagination ...

At Nappa Scars (SD 768697) we add detail to the picture.  The cliff is of Great Scar Limestone of course, but the  footpath actually crosses slabs of basement slate, which once lay at a higher level BEFORE the limestone was laid down and are probably the remains of a former landslide onto an ancient beach!!  This isn't the actual basement - as some people think - as we have to scramble down a couple of metres for that. And the beach? Well - it's there for all to see in the form of a band of 'conglomerate' .. you see that pale looking 'mushroom' stem that appears to be holding it all up? ...

Here it is - a mass of pebbles, shells and bouldery bits all cemented together in a muddy limestone 'matrix'.  It almost appears to be man-made.  We call it conglomerate - and  you can't get a better sample of it than here at Nappa Scars.

About 350 million years ago, the 'Dales' were south of the equator - and had been dry land for millions of years during the Devonian period.  Rising sea levels saw every bit of the land becoming a beach for a period as the waters gradually invaded ... 

That's what we see here.  Shallow water, allowing shelly organisms to flourish - caused a muddy mixure of their remains to cement the beach material together ...

It's amazing to touch it and consider that - it really is!

As the waters deepened, the conglomerate gradually gave way to limestone - as bed after bed of shelly creatures deposited their remains on the sea floor.  You can see the gradual 'fade out' to limestone in this picture.

There are some pretty hefty pieces of rock cemented into the conglomerate.  

Here's a classic view of Nappa Scars - with the conglomerate bed being very slowly weathered beneath the limestone.  

If we scramble down below the path we find the astonishing 'unconformity' between the 480 million year old basement rocks and the 350 million year old limestone - a whopping time gap of 130 million years!!!

Let's touch the time gap.  What's a human life span - or even human life on earth in comparison to this?

Even on this picture - the steeply dipping beds of a former mountain range can be clearly seen!

You can see that the limestone - above right - is still at this stage very pebbly ...

Why is there less conglomerate here? Well - because the limetone was probably deposited on a rocky shelf free of debris at this spot ... 

The steeply dipping beds of the basement - caused by the folding as two continents came together to close the Iapetus Ocean ...

From the crest or mountain top that would have existed beyond the left of the picture (anticline) - the beds would have plunged into the dip (sincline) now lying beneath Crummackdale (to the right).

Another view of the unconformity - with the pebbly beds of limestone above. 

Many gastropods and other fossils can be found in the conglomerate.

Tearing yourself away from a place like this can be tricky.

What went on along the scars of this hillside makes you question the significance of human life on earth.  It's an awe-inspiring place to sit and ponder it all!

Stephen  x

Saturday 9 March 2019

Aysgill Force - A Sandstone Sandwich

Just above the village of Gayle, to the south of Hawes in Wensleydale, we have another fine insight into the Yordedale Series of rocks.  The rocks began to be laid down about 320 million years ago.  Fluctuating sea levels saw limestones being laid down in deeper conditions - built up from the remains of shelly creatures in a warm sea, while shallower conditions saw rivers washing in sands and mud to form sandstone and shale - and so the cycle repeated.  There are 11 named limestones in the sequence, the highest being the Main Limestone found just below the summit of the Three Peaks.  In Wensleydale, however, we are concerned with the lower limestones in the sequence - in this case the Gayle limestone, named after the cute little village that stands on it as a foundation!

My little drawing of the Yoredale Series, much simplified, shows the limestones in Wensleydale, the lowest of which is the Hawes limestone on which the town sits.  As you can see, the next are the Gayle and Hardraw Scar Limestones - both very distinctive in terms of their fossils  - and both, of course, associated with famous waterfalls!  This diagram is, in effect - the 'cone' of Penyghent or Ingleborough - or of Pen Hill - lower down the dale.

Gayle Limestone is seen in the village of the same name where we see a miniature version of Upper Aysgarth.  The beck tumbles over steps of this distinctive dark grey limestone - the same bed as seen lower down the dale at  the more famous falls.  There are many crinoid and brachiopod fossils visible in this rock - and these can be seen easily under the bridge.  Note the shale here being undercut in the dark slit just below the wall on the opposite side.  Eventually the undercut will be so great that the slab of limestone will snap off causing the waterfall to retreat upstream.

Lovely view of Gayle Beck - with the Gayle Limestone of the Yordedale Series clearly displayed.

The distinctive dark grey hue of the Gayle Limestone.  A close up inspection reveals the hive of activity that went on in the warm sea when this rock was laid down.  It is teeming with fossils.

We move upstream and leave the Gayle Limestone behind, as we reach a very thick bed of shales, laid down when the sea was very shallow and muddy rivers dominated the deposition.  Being so fragile, the shale erodes easily compared to the limestones and sandstones of the series.

The gap here between the Gayle Limestone and the next limestone in the sequence - the Hardraw Scar  - is very wide; so much so that the shales and sandstones dominate for several hundred feet.  Here at the lovely Aysgill Force the beck has cut through a strong lip of sandstone in the Yoredale Series and falls down a shale screen into a plunge pool gradually deepening.  The Hardraw Scar Limestone is hidden on the hillside far up on the left.  The Wensleydale waterfalls never fail to impress - and each as its own personality.

Take a stroll up to the force from Gayle - then combine your visit with a trip to Hardraw or Aysgarth.  You are spoilt for choice in old Yoredale!

Stephen x

Thursday 28 February 2019

Wensleydale Waterworks!

Waterfalls of Old Yoredale

Wensleydale was not scoured just as deep by its glacier as neighbouring Bishopdale, and so a 'step' of limestone was left along the valley foor.  This step, part of the Yoredale Series of limestones, sandstones and shales, is the major reason for the abundance of waterfalls .. some of which are spectacular indeed!

The 'Yoredale' series are named from the old name for Wensleydale - 'dale of the Yore or Ure' and are superbly seen along the length of this fine glaciated valley.  About 320 million years ago in the late Carboniferous, fluctuating sea levels on a regular pattern produced this lovely 'sandwich cake' of rocks, well seen here at West Burton Force.  At the top here can be seen Gayle limestone, one of the lowest limestone bands in the sequence, with sandstone and shale beneath.  The sandstone and shale were formed when sea levels lowered to allow river deltas to wash in sands, gravels and muds.  Once the sea level rose again for marine life to flourish, limestones were once again laid down.

The Yoredale series is well seen here at Penhill, as we drive from Bishopdale into Wensleydale.  Each 'step' is a resistant layer of limestone, marking where shales and sandstones have been undercut beneath.  The layers of limestone were named by lead miners in the past.  The highest step here - the 'main' limestone, is the same as that seen just below the summits of the Three Peaks.  The massive plinth of Great Scar Limestone on which Ingleborough sits is nowhere in sight here, being buried far beneath our feet, as the Dales rocks all rest on a foundation, the 'Askrigg Block' which dips to the north and hence hides the Great Scar Limestone in all but a few places.  

Here at the lovely Cauldron Falls, at West Burton - we see two of the lower layers of limestone in the Yoredale Series well displayed.  The beck falls over Gayle Limestone (named from the area of the dale in which it is prominent) and onto Hawes Limestone.  The undercutting of thin shales and sandtones is forming a small cave to the left of the force.  There are many fossils in both the limestone and shale beds here.  The Hawes Limestone is famous as being the lowest in the Yoredale Series.  The Great Scar Limestone (Forming the familiar Malham Cove and Gordale Scar) can't be too far below us here!  

Another view of the Cauldron Falls, showing the cliffs of Gayle Limestone and the undercutting of the shale/sandstone beds.  

Beneath the overhang we have an interesting sandwich ....

Thinly bedded sandstones and shales that seperate the Gayle Limestone (top) from the Hawes Limestone beneath my feet. 

Regardless of any geology - the Cauldron Falls are there to be enjoyed.  Turner loved them!

Nearby lie the most famous falls in the Dales at Aysgarth.  These are the beautiful Upper Falls - best seen after heavy rain. This time the steps were formed in one kind of limestone - in this case the Gayle Limestone - when shale beds in between the limestone were cut back by the swirling water so the the limestone wedges above collapsed to to form the steps.  Hope that makes sense!  This is the major 'step' left by the Wensleydale glacier.

Here, the Ure falls over a major step caused by collapse of a block of limestone caused by undercutting of shale beneath; a splendid sight.

The Middle Falls at Aysgarth tumble over steps between the Gayle and Hawes limestones ...

While finally - at the Lower Force ...

The Ure cascades spectacularly over the same Hawes Limestone we stood on at The Cauldron Falls.  We reach the very base of the Yoredale Series.  Hundreds of thousands of years from now this will be a great gorge worn down so that the hidden Great Scar Limestone beneath forms the valley sides.  But we'll all be dead - so forget that!

Further west along the dale at Askrigg, we meet the beautiful Mill Gill Force.  We are also in the higher layers of Yoredale limetones.  In this case, the beck tumbles over Hardraw Scar Limestone, over sandstone and into a shale plunge pool.  The complete cycle of three rocks in the sequence - formed by sea, river and thick mud  - is called a cyclothem.  This is one of the very best examples in Yorkshire.

Mill Gill certainly deserves a visit along the well-marked path behind Askrigg church.  In flood, it is one amazing wall of sound! 

Wensleydale abounds in stunning views.  Here the stepped Yoredale Series is seen well below the legendary hill of Addlebrough.

In close up - it's an impressive sight - and one that beckons you to climb it!

Wensleydale does have its gentle side - as this view of the glaciated dale shows.

It also has its aggression - as any trip behind the Green Dragon pub at Hardraw quickly reveals ...

Here Hardraw Beck falls as the highest surface waterfall in England - Hardraw Force -  nearly 100 feet over a single cyclothem of Hardraw Scar Limestone, sandstone and shale.  The people to the right of the fall are standing on the soft shale bed.

The overhang of Hardraw Scar Limestone is well seen here, with the sandstone gradually becoming shale at the plunge pool.  This is a wonderful place, no matter what the water conditions.

Classic view of Hardraw Force: worth every penny of the entrance fee.  It lies on private land.

In winter, without the leaf cover - the rainbow in the spray is worth the trip alone.  I was lucky today.

Paint the whole world with a rainbow!

The classic 'cyclothem' of limestone, sandstone and shale.  You couldn't see it better!

And to finish the day - a visit to the stunning Cotter Force, just outside Hawes.  This follows the same cyclothem as Hardraw Force, but the beds are closer together so we get an impressive series of steps in the river bed.

The steps in the beds of shale are well seen.  This is a place that has to be seen.  Watch out for kingfishers!

And that concludes our tour for now.  Later in the year we'll complete our Wensleydale Waterworks survey with a visit to two more classic waterfalls that lie in this enchanting dale.

For more information on geology - please visit my Dales Rocks website  for the Yorkshire Dales National Park.  

Stephen x