Namurian Succession; Upper Bowland Shales to Pendle Grits, Forest of Bowland

One of the great joys in  living in the Lord Howell’s barren north of England is to live close to the finest scenery in England, with the Dales and Lakes 30 miles away  and the Forest of Bowland on my doorrstep. The fells go up to 570 metres and are capped with Namurian Pendle Grits (i.e Millstone Grit). These lie on the Upper Bowland Shales, which in turn lie on the Lower Bowland shales of upper Dinantian, which then lie  on the Worston Shale Group of mudstone and limestone, also Dinantian. These are described in the BGS memoir of Sheet 67 The Geology of the Country around Garstang. As more of a historian of geology than a geologist it pleases me that the first to name these shales as Bowland Shales (or rather Bolland Shale) was John Phillips, nephew of William Smith, in 1836.

To show the geological context I  have an annotated ancient geological map of Lancashire   below.  The Bowland Shales outcrop between the Craven Faults and the “M6 Fault” aka the Bilsborough and Grimsargh faults. West of the fault the Carboniferous are downthrown by thousands of feet and lie under Permo-Trias capped by Glacial Drift. Hence they are inaccessible.

Bowland geol map

 

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Above is the entrance to the Roseacre prospect of Cuadrilla which is very flat terrain at 10-20 metres altitude.  The Bowland Shales lie a mere 8000ft below my bike. A mile from here is the Elswick gas well which has been pumping out gas from Permian Colleyhurst Sandstone since 1992 – and no one realised it was there. The second photo is taken near Eagland Hill looking towards the Forest of Bowland. The turbine in the middle distances is 2 km west of the Bilsborough fault. To the east of that the terrain rises gently on Bowland Shales and then leads up to Beacon Fell (264m) which is capped by Pendle Grit. All the higher hills of the Forest of Bowalnd and Pendle Hill are capped with Pendle Grit.

The terrain gets more rugged and the photo (bleow left) is looking down the Trough of Bowland (at 280m) towards the Sykes Anticline in the middle distance  and is on the obscured contact of Upper Bowland Shale and Pendle Grit. Just round the corner on the right is a good exposure of UBS. The right hand photo is up Bleasdale Water where the river has cut through a recent (20,000yr?) landslide of PG overlying UBS. These landslides are common in Bowland, but this is my favourite.

 

 

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The general aspect of the Forest of Bowland is soft valleys which then rise up to fells/hills capped by Pendle Grit. The left photo below is looking down the Hodder Valley (UBS & LBS) from Totteridge Fell looking towards Longdridge Fell capped by PG. The right photo is looking south from Longridge Fell at Pendle Hill, the type area of the Pendle Grit, which has its type section in Little Mearley Clough which is a gully on Pendle Hill directly above the farmhouse.

 

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The several memoirs of the British Geological Survey describe many sections in the Craven Basin and I have taken to including one on my walks (up to 15 miles) in the Forest of Bowland. The finest section is the 1000ft section at Little Mearley Clough (https://www.ribblevalley.gov.uk/download/downloads/id/8978/little_mearley_clough_sssi) which is now an SSSI, which I shall visit shortly.

However this focuses on one a few miles west of Garstang  which superbly demonstrates the transition from UBS to PG over several metres. It lies on the north of the public footpath going from Bleasdale over Fiendsdale Head north of Holme House Farm  and follows the line of a stone wall marked on the OS Map (SD 57484760) mentioned in the BGS memoir for Garstang Sheet 67 on p46, but no description is given beyond “the gradational character of the base of the formation [PG]……”

The exposures of PG overlying UBS lie some 60 to 120 metres up a 120 metre high gully. The photo below shows an alluvial fan in the lower part, which covers UBS. The craggier part above is where grits begin to appear and then predominate. The RH photo is part way up  immediately above where UBS begins to peek through the overlying alluvial fan and debris of PG. Some eroded-out UBS is seen in the lower left of the photo. It shows clearly thin sandstones beginning to replace shale and then getting thicker and more massive higher up.

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Both in this gully and elsewhere BS is apparent from disintegrated shale rather than outcrops as in this photos, in which a bullion has resisted the vagaries of nature. The strata are dipping shallowly NE.

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Slightly further up the shales are more competent and less “muddy” indicating the change to more fluviatile and shallow water conditions. Note the mudstone beds about 10cm thick and the very fissile nature of the more shaly material above and below. (It is this fissile nature of the shales which makes them suitable for Hydraulic Fracturing. )

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Above this sandstone bands became thicker

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but at times overlay thick layers of shale, which in close-up showed

 

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the transition from UBS to PG, with an increasing proportion of gritty beds, but there was something more interesting

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in the form of clastic dykes about 10-12 cm wide, in which gritty material intruded the shales, probably from below. The dykes stopped at the overlying grit bed and I suggest they were intruded form below.

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Dykes were found over a distance of several metres

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This photo is taken abot a metre from the walking pole in the previous photo and shows (not very clearly) two dykes running at about 30degrees to the other two. (I have not found any reference to clastic dykes in any of the BGS memoirs so I cannot say whether this is a unique occurence.) Any petroleum geologist will realise the significance of these dykes.

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Further up grit bands of increasing width began to predominate and shale bands gradually became thinner and ultimately

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the strata gave way to massive Pendle Grit , which can be beds several metres thick. This photo also gives a good view down the gully . The lowest UBS exposure lies just below and to the right of the dark green “shrub” halfway down. This photo also looks downto  the flat  farming and wooded area around Bleasdale Church, which is underlain by Lower Bowland Shales with Pendleside sandstones and mudstones.

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This continued up the last few metres of this local succession, when the slope eased off and it was difficult to identify beds due to a considerable covering of vegetation

 

 

 

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Finally here are some slides of a typical UBS sequence of shales and mudstones at an exposure on the confluence of fiendsdale Water with Langen Brook SD599501 about 3 km NE of the previous site. These are not recorded in the memoir of Geology around Garstang, which was published in 1992 and have been exposed in the intervening period of time. Unfortunately this is an isolated outcrop and lack of exposure means that I could not trace the gradual change into PG further up the beautiful Fiendsdale. However they show the alternating sequence of mudstone and shale with the occasional bullion. As I was short of time when I found them, they will need another visit. Incidentally to inspect them I had to cross the stream three times, but avoided wet feet.

 

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Neither of these outcrops are close to the road as both are  4 km from the nearest road.

My aim in this photographic cross-section is to give a visual idea of the Bowland Shales as exposed in the Craven Basin, thus enabling some visualisation of the shales lying 8000ft below the Fylde which Cuadrilla have applied to carry out exploratory drilling.

In the futue I shall do similar photologs of the Little Mearley Clough on Pendle Hill and other exposures in the Forest of Bowland, some of which are oozing oil.

I conclude with a scenic shot taken halfway between the two sets of exposures at Fiendsdale Head.

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3 thoughts on “Namurian Succession; Upper Bowland Shales to Pendle Grits, Forest of Bowland

  1. Rob

    Thats some nice work Michael, particularly spotting the sand dykes and sills. You’re well on your way to writing a guidebook ! As downhole imaging and seismic processing gets better in the oil and gas field, the advancing resolution is showing that those ‘injectites’ are often important in conventional wells. Theres several oil and gas fields such as Marathon’s Volund Field which ‘work’ because of the larger scale versions of the same process.

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  2. Pingback: Bowland Shales – type succession | Peddling and Scaling God and Darwin

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