Geology Special Interest Group

Visiting the Raistrick Rock and Fossil Collection

Geology Group — Fri, 02 Feb 2024 11:21:42 +0000

UWFS Geology Group visit to Craven Museum Skipton 31/01/2024

The Craven Museum hosted a session attended by 15 members of the Geology Group. Just right for a very cold and windy morning!

Several trays of exhibits from the Raistrick Collection were made available in the Education Room. The group appreciated seeing samples of rocks and fossils with explanations from Josephine Drake, Steve Warren and Shirley Everett.

The rocks of particular note were Fluorite with large right-angled crystal formations indicating slow cooling of magma. Samples of Fluorite with Malachite (small samples seen on the ground on the Pikedaw trip) and Barite which Steve Warren told us is an important ingredient in drilling muds used in the petroleum extraction industry because of its high density.

Fluorite from JD collection.

Right-angled crystal of Fluorite from JD collection

Malachite and Azurite. JD collection

Josephine Drake passed around Black Marble which is actually a fossiliferous limestone and looks glossy black when polished, as seen in Dent Church; Tufa (deposits of calcium carbonate) which can be seen on rocks behind Janet’s Foss. There were many fossils to marvel at, Crinoids, Corals, and Brachiopods, a reminder that the Yorkshire Dales limestone was formed under a warm tropical sea near the equator.

We were shown Graptolites, fossils of primitive animals from an even earlier time which appear to resemble Viking runes, examples of which are rarely seen in the Dales but possible seen on a dry stone wall near Helwith Quarry in Ribblesdale during a previous Geology trip.

The group then enjoyed a walk round the Museum looking at rocks, fossils, photographs and interpretation boards of geological significance in the Craven area demonstrating how Geology has shaped the landscape, local human activity and history.

This meeting was a follow-on from a visit in November to Cliffe Castle Museum Keighley where there are several dioramas explaining Geological processes, each museum adding to our knowledge in different ways.

A coffee shop was the next stop for discussion, friendly chat and some humour.

Leaders Pragna Unia and Josephine Drake

NB photos of samples are from the JD Collection and cannot be seen in the Raistrick collection in the Craven Museum.

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Pikedaw, Malham Cove, the Mid Craven Fault and Glaciation

Geology Group — Tue, 31 Oct 2023 21:24:51 +0000

Geology Field Trip 25th October 2023

Craven Fault Zone Vegetation Change

We left the YDNP car park in Malham via Back Lane and Burns Barn with the view above from Straight Lane with typical Field Barns in the foreground.

The Craven Fault system formed as the Askrigg block of Granite underlying the Yorkshire Dales kept the Carboniferous rocks higher while the land on the other side of the fault fell forming the Craven Basin where the limestones are many hundreds of feet lower and covered by later rocks of the Bowland series, mainly gritstones.

The start of the walk was in sunshine and we could easily note the light coloured rocks of limestone and green grass on the righthand side of the fault zone. Overlying the gritstone of the Craven Basin with more acid rocks the land is wetter browner and more rushy, the visible bed-rock is also much darker, as seen on the slopes ahead.

Climbing up the hillside, at first on limestone, noting the climb from the car park over Burns, a reef knoll, we looked back towards Gordale and could see the fault zone carrying on up to Weets Top part of the Craven Basin.

Looking across to Weets Top over Cawden, a Reef Knoll

On the top left of the photo can be seen the limestone crags just beyond Gordale, with the peak of Weets top on the right skyline. Nearer is Cawden a typically shaped Reef Knoll formed of shelly debris deposited on the slope of the fault between the top of the Askrigg Block and the Craven Basin.

Entry to Lead Mine

Climbing higher we came to an adit from the earlier lead mining with a spoil heap in front, again showing a variation in vegetation.

Climbing higher still another adit, with a stone arch and date stone of 1872, was used to bring out the mined copper ores.

Adit for Copper Mine

To the left of here was very rough ground in front of the Bowland rocks of the Craven Basin, a landslip.

The humps of the landslip were formed post glaciation, perhaps due to the weathering of the gritstone causing collapse downslope.

By now the cloud and mist had descended but with enough visibility to beware of horses emerging!

More importantly we met the shaft to the Calamine mine, Iron oxide, which was found during mining for Copper, as a sediment on the floor of the cavern system and was extracted though this shaft.

Calamine Mine: 23 Metre Fall

To the west of here was a large pool, the washing area, surrounded by debris and very small pieces of minerals of copper, Azurite – blue, and Malachite – green, great finds as shown here

Malachite and Azurite. JD collection

We finally emerged on to the Bridleway between Settle and Malham via Stockdale lane, the latter being part of the western end of the Mid CravenFault.

A little further on but now standing on the wall is an old cross base but modern shaft, Nappa Cross, leading us towards Langscar Gate.

After lunch we arrived at the top of Watlowes, a dry valley formed by a large meltwater flow under the glacier then altered by weathering. Note the east (left) side is more gently sloping, in the sun, than the west side in the shadow. The effect of the sun causing freeze thaw is offered as an explanation.

Though the valley is generally dry, excess rain can cause some water to collect (when all the underground passages are full of water) as in 2015 when Storm Desmond also caused a flow over Malham Cove, a spectacular sight for those who saw it.

We managed the difficult walk eventually and arrived at the top of Malham Cove a large area of Limestone Pavement, with its blocks of limestone Clints, and gaps Grykes formed by weathering, the weakly acidic surface water dissolving the weaker parts of the rock. Walking over this landscape is very difficult so some of us climbed the hill behind to access the steep steps down to the base.

Pavement at top of Malham Cove

Top of steps down side of Cove

View from steps showing Lynchets

Lynchets are very early Field Systems, seen as ridges in the fields caused by plowing.

Cove from direction of Malham

Looking back to the magnificent Malham Cove, believed to have been formed as a result of the fault in the valley above, water flow over causing weathering so that the lip retreats backwards. There continues to be debate about this as there is a mismatch between the size of the valley and the size of the Cove.

The black marks seen on the cove are caused by mosses and lichens growing in seeping water, not the soot as proffered by Charles Kingsley in The Water Babies!

An excellent days walk and Geology field trip, enjoyed by 10 members, all very supportive and patient.

Leader Josephine Drake

Photos Ann Shaw and an occasional one by Josephine Drake

Update: Further information on mines around Malham can be found in “British Mining No 55”, section “Some questions concerning the history of the Pikedaw Calamine Caverns, Malham Yorkshire” by Jack O Myers

Also “British Mining No 97” section “The Malham Mines” by ~Mike Gill and Mike Squirrel.

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Stump Cross Caverns Geology Trip 31st May 2023

Geology Group — Wed, 07 Jun 2023 19:03:00 +0000

A Geology Group Visit to Stump Cross Caverns

Dr John Helm is a UWFS member of long standing and a Geologist who now works part time at the above caverns, he was kind enough to lead a group of members round these caves.

His initial illustrated lecture helped us to understand their formation and history adding to our interest in the visit. The site became an early Tourist attraction helping its preservation. Over the years much has been learnt from cave systems about past climates, fauna and flora, and beliefs.

Water landing on the land surface percolates down through the underlying slate of the Yoredale series, north of the North Craven Fault, drains through the cracks and along the beds of limestone until it reaches the impermeable rocks below. The dissolving of the limestone is accelerated by mildly acidic water formed from the carbon dioxide in the air and the acidic vegetation. The water eventually drains out through a resurgence, at Stump Cross Caverns this is at Black Hill Resurgence to the south of the main Hebden Greenhow road. It has taken millions of years for this system to have formed. Though a space cannot be dated, the features formed in the cave can be using radioactive decay of Uranium and Thorium.

When the water table is high and water fills the growing gaps a rounded tunnel or Phreatic cave system is formed, the acidic water wearing away all the sides. When the water table falls and the water runs only in the base of the gap a Vadose system ensues. The former becoming the latter can make a Keyhole system.

Stalagmites growing upwards, Stalactites growing downwards

As calcite containing water slowly drips from the roof of a cave a calcite Stalactite forms and hangs in increasing length from the ceiling. When the drop falls more quickly and lands on the base a Stalagmite grows upwards. The combination of time and imagination can make all sorts of shapes of these features.

The Organ, flow stone and imagination.

Where water flows very slowly over a slope and forms a small pool, here the calcite is deposited at the edge or rim forming a small dam, now known as Rimstone or a Gour.

Rimstone, Gour, forming pools

The use of a UV light torch can highlight the different minerals in the rocks and cause fascinating scenes.

Minerals in the rock fluorescing under UV light

It was good to see many children in the caves each age group following a different “version” be it Fairy Doorways, Dinosaurs Eggs or just the straight knowledge base but all learning something while having fun.

Lunch afterwards gave us all a chance to discuss what we had seen and reflect on how lucky we are in the Yorkshire Dales.
Many thanks to Dr John and all at Stump Cross Caverns, do check out their website for more excellent information.
Josephine Drake

Photos
Ann Shaw and Stella Hughes.

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A Field Trip to Haw Crag Quarry Bell Busk 28 September 22

Geology Group — Thu, 13 Oct 2022 19:06:46 +0000

Seven of us met for a shorter afternoon session to look at Carboniferous rocks in the northern part of the Craven Basin, visible in a small quarry near Bell Busk.

Much of the Yorkshire Dales lies on the Askrigg Block, a large block of granite that underlies the rocks and causes uplift of later rocks. The Craven Basin lies south of the Block and is separated from it by various faults, including the Mid Craven Fault. During the earlier Carboniferous time the Block was covered by a shallow warm tropical sea and Limestone formed. At the same time the Basin was filled by a deeper sea with rivers to the north draining into it and depositing mud. Between the Block and the Basin were shelf or ramp areas, slopes thus giving shallower areas of sea where muddy limestone could accrue. Sometimes tectonic activity would cause landslides on the shelves.

A track from the Aire bridge in Bell Busk leads up to Haw Crag quarry, the walls of the track are of limestone rocks often containing fossils of coral, found in the warm shallow seas.

From above the quarry there is an excellent view to the north of the Cracoe Reef Knolls, formed on shelves in the warm seas on the Block and containing lots of fossil of shelly creatures.

To the west Malham Cove could be made out and the change from green grassy vegetation to brown rougher vegetation signalling the approximate line of the Mid Craven Fault. Looking to the south in the distance was a high point in the Basin, Pendle Hill but nearer to were drumlin hills, mounds of glacial till left by a retreating glacier.

Within the quarry were two exposed faces of rock, lots of large to very large boulders jumbled up but in one area over-lying even larger blocks of vertically bedded (layered) rock which had clearly been upturned by incredible forces. The big boulder bed or conglomerate is known as the Haw Crag Boulder Bed and has been an SSI since 1954 as deemed significant geologically. The vertically bedded rocks are Hetton Beck Limestone.

The question remains how did these rocks get here in this disorder? One theory is as suggested above an avalanche, or turbidite, falling down the shelf or ramp.

Smaller horizontal beds of rock were seen below the level of the above, all looking sedate and in their correct orientation, we concluded these were beds of the lower Thornton Limestone laid down before some of the above activity.

A fascinating view of past environments, while also seeing the wider views of the local geological setting.

Leader Josephine Drake

Photos Ann Shaw J Drake

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Geology Field Trip to Combs Quarry Helwith Bridge 31 August 2022

Geology Group — Wed, 07 Sep 2022 10:08:31 +0000

Royanne Wilding, a member of UWFS until she moved to Giggleswick, and a retired Open University lecturer in Geology, led a group of 9 of us to Combs Quarry in good weather.

The rocks quarried until the late 1880s were the Horton Formation, known as Horton Flags and W R Mitchell of The Dalesman wrote an interesting article on their widespread use.

The Horton Flags are Silurian rocks formed from muddy deposits in deep seas when what was to become England was south of the Equator 450-400 million years ago. Intermittent volcanic eruptions occurred which left layers of fine ashes on the sea surface to then sink down and form different intermediate layers.

Rocks of the Horton Formation -layers of lightly metamorphosed mud deposits with volcanism ash between

Tectonic Plate collision and the closure of the Iapetus Ocean caused these deposits to significantly fold and later compression altered the nature of the rock to change or partially metamorphose into the harder rock present today. Local faulting caused smaller changes which can be seen in the rocks with displacement across the gap.

A break in the layers, a Fault

The two ends of the walking stick show the amount of displacement caused by the fault

Further Tectonic plate activity meant the crust that was to become Yorkshire had moved to just north of the Equator in Carboniferous time and Limestone was formed from shell debris in warm shallow seas, the horizontal layers of rock now seen above the steeply dipping Horton Formation. The Devonian Rocks, formed when the crust was above the sea level between Silurian and Carboniferous times, in hot windy dry desert conditions, have since been removed by massive erosion thus leaving a large time gap between the Horton Formation and the Limestone known as an Unconformity. Because the horizontal rocks are lain on a dipping layer of rock this is known as an Angular Unconformity.

The horizontal layers of Carboniferous Limestone resting on top of steeply dipping Silurian Rocks

After a well earned rest over lunch we headed back towards Arcow Tarmac active quarry which shows much more of the bedding of the lower rocks, again with local folding forming “hills” or anticlines and “hollows” synclines.

Red arrow at the top of Anticline
Blue arrow at the bottom of Syncline

In deep seas with shelves and slopes avalanches of mud occur, known as Turbidity Flows. These were discovered when the first Telegraph lines laid in the Atlantic were broken, that is the force of Turbidity Flow. Outcrops of rock above the Austwick to Helwith Bridge road were examined to see examples of rock formed by Turbidity Currents. The Bouma Sequence is the name given to the layers of deposition forming the rocks.

Layers of rock forming more than one Bouma Sequence

A Base of Bouma and high energy deposits
B Layering as energy drops
C cross stratification and ripple marks

When the force of flow begins to lessen it first drops the larger grains which also gouge the underlying surface creating eddies and hollows so the lowest layer of Bouma is uneven. As the forces lessen finer particles are lain in layers, these too might be affected by flow of water forming ripples, later things quieten to allow layering and finally the finest of debris in water settles over time.

Here in the rock outcrops we were able to see magnificent Flute Marks, the casts of the hollows created by the first powerful flow. Then the graded layering of particles as the flow lessens followed by some ripples. Nowhere was the full sequence seen but it was possible to recognise the different layering.

The base of the Bouma showing Flute Marks

Royanne had done a splendid job in educating and entertaining us on what one can learn from careful observation of rocks about past environments. A very big thank you.

Josephine Drake

Photos mostly Ian Hughes occasional Jo Drake

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Field Trip to Askrigg Mill Gill and Ballowfield 27 July 2022

Geology Group — Tue, 09 Aug 2022 08:38:31 +0000

Having read Jim Hutchinson’s excellent report on a similar trip, Mill Gill Force and Ballowfield’s Nature Reserve, a small group of us met in Askrigg village centre opposite Skelldale House used in the earlier television series of All Creatures Great and Small.

Lesley Collins was our excellent and very knowledgable leader taking us past the Old Mill, first a Corn Mill, used to make hay-rakes and lastly used to produce Hydro Electric Power for Askrigg but sadly no longer in use, just various remnants of leat, pond and buildings.

In the gill below, the bedrock was the Gayle Limestone, one of the lower bands in the Yoredale series of rocks known as Cyclothems, layers of limestone shale and sandstone which then repeat as shallow seas encroach the land followed by mud deposits from river deltas and finally sandstone, larger grains deposited on the top.

As we walked up hill through the woods we could gradually hear the waterfall, not a torrent but quite a spectacle after the rain following the very hot spell. Here could be seen the sandstone layer gradually giving way to another limestone this time the Hardraw.

Bedrock Gayle limestone with higher Yoredale rocks above of shales and mudstones.

Mill Gill Force

Churt filled fossil

Top of Disher Force

After lunch at Ballowfields Nature Reserve, once the site of a holiday camp for people from the East Coast, we looked at the Wet Groove spoil heaps from earlier lead mining in the area. Climbing up the side of Eller Beck and Disher Force we were able to find fossils of coral, brachiopods and some chert in the Middle Limestone, now above Hardraw and Simonstone Limestones.

Climbing higher we looked at the Eller Beck Hushes, water had been collected above the highest local limestone, the Undersett, then flushed/hushed down over the rocks to clear the surface and expose the mineral veins in the bedrock. We were also able to make out a geological feature known as the Carbeby Basin.

Eller Beck Hushes, water was collected above then suddenly released to wash off the surface debris and reveal the mineral veins.

Below Ivy Scar further east along Ox Close, a grand bridleway from Bolton Castle to Askrigg and beyond, is the Knott, now a honeycomb of old lead workings. Here we were able to look among the spoil heaps for examples of Barytes, Fluorite, Calcite, and Galena with varying levels of success.

Ivy Scar, Undersett Limestone, the highest local limestone layer, and The Knott below – a Landslip?

The old Mine workings in the Knott

We returned to the cars by the same route with an excellent appreciation of our local landscape and the people who had worked and understood it in the past, with many thanks to Lesley for her guidance and explanations.

Report Josephine Drake

Photos Ian Hughes.

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Geology Trip to Brimham Rocks on 29 June 22

Geology Group — Tue, 05 Jul 2022 19:17:21 +0000

A group of 11 made up the Brimham Rocks Geology Trip on Wednesday, June 29^th. It was led by Stephen Lewis a very knowledgeable geological guide, author, and volunteer at this National Trust site.

Brimham Rocks is famous for its spectacularly exposed rock pinnacles that allow a three-dimensional insight into a complex ancient fluvio-braiding system. Whilst its 18 century visitors attributed the bizarre formations to the work of Druids, the 21^st century geologists are still researching into the scientific formation of these features.

The Millstone Grit of Brimham Rocks were formed about 320 million years ago. Research in the last five years has now led to the theory that the sediments laid down at Brimham and subsequent formation of the millstone made from sand, grit and rounded pebbles of quartz and some feldspar was the result of a braided river system carrying sediment from the newly formed Caledonian mountains.

The layers of sediments were washed down by storms and due to the velocity of the layering there was no time for fauna or flora to settle, hence there are no fossils at Brimham, with the exception, of a salamander footprint.

As the river braiding changed the direction of the flow of the river, the layers of direction of sediment would also change direction and these are really evident in the rock formation on site.

Brimham is also famous for the unique flame structures that are so rare to see in Britain as illustrated below on the bottom strata.

The weight of an overlying bed forces an underlying layer to push up through its overlying bed. The resulting pattern resembles flames in a rock cross section and is the direct evidence of earthquake movement.

Around 18,000 years ago, the northern Pennines were covered by an ice cap and the stone sculptures at Brimham are a direct result of this. Rocks that stood above the ice sheet were exposed to strong glacial winds blowing off the ice caps. These winds carried fine sand particles that effectively sandblasted the exposed rocks, creating the features that are so characteristic of this National Trust site.

The sunshine and dry weather added to the pleasure of the day. It was a fabulous and informative tour and well worth a visit.

Report Stella Hughes

Photos Ian Hughes

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Geology Field Trip to Crummack Dale 4th May 2022

Geology Group — Fri, 13 May 2022 15:54:41 +0000

Crummack Dale lies behind and above the village of Austwick and is an area where we can see the rocks, known as the Craven Inliers, Silurian rocks, that lie below the Carboniferous limestone that we are used to seeing in the Yorkshire Dales.

We climbed the hill out of Austwick noting many large rocks lying strewn about the upper fields, often covered in a beautiful yellow green lichen in map like shapes, Rhizocarpon geographicum, which is known to grow on sandstone and not on limestone.

Silurian sandstone rock with Rhizocarpon Lichen

Finally we reached the Norber Erratic Field, where more of the lichen covered rocks were often standing on pedestals of limestone suggesting transport of some power having moved these older rocks from elsewhere on to the top of younger limestone. Different rates of weathering have helped form the pedestals.

A Norber Erratic pedestal, Silurian sandstone on a limestone pedestal

We climbed down through limestone outcrops to the Sub-Carboniferous Unconformity at Nappa Scars, a cliff face with an undercut area of very different appearance, pebbly in a limestone matrix lying on a siltstone base. The sequence of events forming this cliff face is deep sea deposition of sands and mud in Silurian times, followed by a time gap, the Unconformity, representing the removal by erosion of all the Devonian rocks formed on land, then a rising sea level washed in pebbles from surrounding areas across a beach. As the sea deepened in a warm tropical climate the Carboniferous limestone was formed above.

The basal Silurian Sandstone covered by pebbly limestone with the base of the Great Scar Limestone above

We walked north on Crummack Lane noting the field from which the erratics had been plucked by a grinding Glacier then transported south but uphill over a brow and onto the Norber Erratic Field.

The source of the Norber Erratics

Walking northeast on a field footpath we walked over hills of rock formed by Anticlines and a Syncline, folds of rock into hills and dips respectively, which could also be seen against each hillside.

We reached our lunch spot and looked in the stream to find some fascinating looking red and green striped rocks at Moughton Whetstone Hole. The rings are Liesegang Rings, formed by oxidation of the iron in the rocks during the Devonian and early Carboniferous when the rocks were part of a continent. They were later worked to produce Whetstone or sharpening stones.

Moughton Whetstone with Liesegang Rings

As we returned down the Green lane we were able to look up to the left hand cliff face and once again see upper horizontal limestone lying on almost vertical rocks above the screes, another Unconformity. The screes of loose rock were formed by periglacial weathering, the effects of freezing and thawing of the rocks.

The Studrigg Unconformity

The flowers throughout the day were an added pleasure, particularly the early Townhall Clock and the later Bird’s Eye Primrose.

By the time the five of us returned to Austwick the earlier rain and dullness had disappeared and been replaced by lovely sunshine, and reflected our mood of satisfaction at a good days mental and physical exercise.

Leader Josephine Drake

Photos Ann Shaw

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Terrawatch, and the height of mountains.

Geology Group — Sat, 09 May 2020 20:56:45 +0000

In a short article in The Guardian of the 6th May entitled ‘Terrawatch’ Kate Ravilious poses the question: would Alaska’s Mount Denali be as tall if it were situated on the Equator? A recent study has shown that the erosion of high mountains in Polar regions ‘lighten the load’ and hence create higher mountains than might be expected.

The study, by Jorg Robl at the University of Salzburg and colleagues, studied 16,000 of the world’s highest mountains comparing height, steepness and thickness of the underlying crust which supports them.

They found that some of “… the steepest mountains are found at the highest latitudes, where the effects of glacial sculpting predominates”. They showed that the underlying crust does not need to be as thick as it would for a non-glacial peak.

I have heard something similar before, that erosion reduces the mass of material and thus the mountain ‘floats’ higher, i.e. gains height – the Principle of Archimedes?

Text and Image: Jim Hutchinson.

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Stratigraphy

Geology Group — Sat, 09 May 2020 11:17:50 +0000

I wonder how many of you are familiar with Geo Supplies Ltd of Chapeltown, Sheffield? “The first choice of the professional geologist for 36 years” is their claim. They sell geological supplies, courses, literature and equipment and publish the quarterly magazine “Down to Earth” which a friend of mine subscribes to and sometimes passes on.

I think it was in one of these that I read a review of Paul Lyle’s book “Introducing Stratigraphy” published by Dunedin Academic Press Ltd of Edinburgh in 2019 and which I purchased from Geo Supplies earlier this year. Price £16 and no P & P. It is a book of 133 pages divided up into seven chapters plus an Epilogue, a Glossary, Further Reading (and Image Credits). And the images are very good, as are the diagrams and illustrations. Paper and print quality are excellent. The book is 22cm square so it won’t go in your pocket if you are out on a walk so you must read it before you go.

In the Preface Paul writes that “It is aimed at undergraduate students learning stratigraphy for the first time, amateur geologists wishing to understand the formalities of stratigraphic nomenclature,and the general reader who wants an explanation for the distribution of different layers of rock across the landscape”. That aim is achieved though as a ‘general reader’ by the time I got to Chapter Seven “The application of stratigraphy” a measure of concentration is required. But this is a book that I enjoyed enormously.

The author writes that his hope is “that the reader will use the book to explore and understand the concept of geological time and see how the science of stratigraphy can be used to measure and interpret the changes that have taken place throughout Earth history”.

The diagram of the geological succession at the Grand Canyon and that of geological time from the origin of the Solar System to the present alone are worth the price of the book. I’m about to begin reading it again the better to absorb it all.

Image is of Hunstanton Cliffs.

Text and Image: JimHutchinson.

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