Saturday, 29 June 2013

The Limestone Benches of Ingleborough

Yorkshire's most famous mountain has a fascinating geology.  The actual 'cone' of Ingleborough, seen in the background, is made up of a great 'sandwich' of rocks known as the Yoredale Series (Yoredale being an old name for Wensleydale where these 'sandwiches' are prominent).  The actual Great Scar Limestone, seen in the foreground, and for which Ingleborough is internationally famous, was formed in a clear tropical sea about 340 million years ago from the remains of shelly creatures and corals.  Changes in sea levels caused the 'Yoredales' to form on top, made up of alternating bands of limestone, sandstones and mudstones.  The thin limestone layers of the Yoredales were formed from living remains when the water was very shallow and muddy.  When land was exposed altogether layers of sandstone were laid down by rivers washing in sands and gravels, and when conditions allowed plants to flourish, thin seams of coal were also formed.  

All the water flowing off these steep Yoredale slopes eventually meets the 600 feet thick layer of Great Scar Limestone and sinks into it at the earliest opportunity - finding a handy joint or bedding plane and slowly dissolving the rock into a cave passage or pothole.  This is Great Douk Cave, formed on a fault.  The water enters the limestone through a pavement half a mile to the east and wanders through an exciting cave passage before emerging here in this huge collapse crater.  The water has no sooner emerged from the cave than it sinks once more into the limestone floor!

Emerging here at Great Douk Cave, the stream can be seen to have cut out a major joint beneath a bedding plane, leaving a characterisitc 'T' or 'Y' shaped entrance.  It's quite easy to enter for tall people as nature has kindly provided a step in the waterfall.  Some people avoid a soaking by crawling along the ledge at top right, actually easier than it looks on the photograph.  About 800 metres of superb walking size cave passage can be explored, with cascades and formations, before a crawl in water leads out onto the moor.  I'll bring you back for a full exploration before very long.

The base of the crater containing Great Douk is choked with rubble.  It is highly likely that this was once a huge cave chamber that has collapsed, and that the waterfall we see today once cascaded into pitch darkness.

A view from the northern slope of the ravine shows the ledge up on the right.  Great care should be taken at the entrance where algae makes the rock very slippery - and the cave is very active, so should only be entered in calm weather.  In times of heavy rain it can be impossible to get near! In many ways this is a miniature version of Weathercote Cave on the opposite side of the dale - an underground stream crashing into daylight through a 'window' in the limestone, before once again disappearing into darkness.

This is the boulder-strewn floor of the crater of Great Douk - with the 'dig' of Great Douk Pot visible in the far right corner, where cavers have been tracing the course of the water as it heads through the limestone to emerge at the basement rocks in the valley bottom.

The Great Scar Limestone pavements of Scar Close, beyond Great Douk, have been protected from grazing sheep for years.  They are therefore thought to resemble such a limestone environment as it might have looked well before farming was thought of!  There wasn't a great deal of light today - but the beauty was undeniable.

This smooth plateau of Great Scar limestone was once covered by the 'sandwich' layers of the Yoredale series, as can be seen by the lower slopes of Whernside in the background - remnants of that once massive layer.  Glaciers in successive ice ages have effectively 'bulldozed' these layers away and scraped clean these magnificent pavements of white limestone.  The ice moved from right to left across the picture, and at its height would have covered even the slopes seen in the background.

The classic Yoredale slopes of Whernside with the pavements of Scar Close contrasting superbly.  This picture gives some indication of the height and width of the glacier that flowed down Chapel-le-Dale.

As these pavements are quite isolated from the Craven Faults, they tend to have massive irregular clints of unbroken limestone - unlike the latticed 'bar of chocolate' clints and grykes of Malham Cove, for example, which lie very close to the Mid Craven Fault.  Nevertheless, the grykes here are spectacular and are home to a variety of rare plants.  

A sandstone 'erratic' which was plucked from the valley sides higher up the dale and dumped here when the ice melted around 12,000 years ago.  

This weathered limestone boulder has been dropped by the glacier at an angle on a smaller piece of limestone resting on a clint.  The weather, wind and driving rain over thousands of years have reduced this supporting stone to a tiny pedestal.  This is a superb feature of the Scar Close pavements.  Beyond can be seen the Yoredale slopes of Southerscales Fell, the middle section of the Ingleborough massif.

Like a natural altar, the boulder has stood here for at least 12,000 years.  Notice how many of the surrounding grykes are filled in with glacial till - allowing grasses to grow - at least without our woolly friends around.

Classic grooving caused by water running off the elongated clint in the direction of the camera.

Looking south-west across the width of Chapel-le-Dale from the edge of Scar Close.  The Yoredale foothills of Whernside are on the extreme right, with Gragareth peeping behind.  Just above centre are the massive pavements of Scales Moor (see earlier post), scraped clean by the glacier, whose route can be seen from right to left across the picture.  The field in the foregound shows pavements peeping through a very thin layer of till, (clay, pebbles and boulders) deposited by the glacier.

The weathered clints on the extreme edge of the Scar Close Nature reserve - looking north towards Ribblehead.  A thin layer of till covers the pavements to the left.

Ingleborough broods over the lush landscape of Scar Close Nature Reserve.

Another wonderful stretch of Great Scar Limestone punctuated by rich vegetation.

A perfect combination:  Globeflowers, white limestone, and Yorkshire's greatest mountain.

Look closely at this isolated hole in the limestone and a very fine crack can be seen either side- the original weakness in the rock which rainwater and run-off has exploited to full advantage in hollowing a way through.

The Scar Close Pineapple?  Not many boulders dropped by the glaciers are free from sheep attack, so when they are - they tend to produce something quite bizarre.  

Looking the other way - showing the rich variety of wild flowers, grasses and mosses around the 'pineapple'.

Carpets of beautiful white cotton grass dominate the lowest Yoredale slopes before they meet the Great Scar Limestone at Scar Close.  That's Whernside rising beyond.

At Keld Bank, near Scar Close, minor faults reveal the pinkish-grey limestone of the lower beds of the Yoredale series; a very different limestone to the Great Scar both in looks and composition.  A stream runs down off Southerscales Fell and has an interesting journey from this point onwards ...

The lowest limestone beds of the Yoredale series were, if you recall, formed in very shallow muddy waters when sea levels fell, and doesn't it show here at Keld Bank - looking at this thinly-bedded, muddy-looking rock?  The stream has managed to cut through it to a shale bed beneath ...

But not for long!  It has soon encountered the limestone again ... and managed to vanish in its own bed, here at Keld Bank Sink. It quite simply disappears through a slot.

The stream used to sink further down before it found that 'new' route.  Here is its old course, still active in times of flood.

Exposed clay and boulders here at Keld Bank show the layer of glacial till that has been plastered over the underlying rocks.

The old route of the stream curved its way(top of photograph) to this larger slot into the Great Scar Limestone - and once a stream hits the Great Scar - the inevitable cave begins to form.

A 'manhole' into the streamway follows Keld Bank Sink through a wonderful cave passage: a hands and knees crawl to eventual walking and two lovely underground waterfalls - formed as the stream makes its way through the Great Scar Limestone.  

I didn't fancy crawling in without caving gear - but here's the passage from the manhole to the alternative entrance - with scalloped walls.  I'll be back before long.

Park in the second lay-by on the left just above the Hill Inn on the B6255. Scar Close is a National Nature Reserve requiring a permit to enter.  Write to the senior reserve manager, Colin Newlands, at Colt Park Barn or google it and telephone for permission.  Great Douk is the tree-filled crater looking like a small island on the image just off the south west tip of the reserve  (just below left centre).  A wall runs north east along the south eastern boundary of the reserve - clearly visible on the image.  Where it meets the first wall coming down off Southerscales Fell, just above the centre of the image - a careful search will reveal Keld Bank Sink.  Take your time and savour this unique place.

Stephen x

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