Summer Weekend 2023: Field Trip to Ynys Môn, led by Rob Crossley

Club members enjoyed a weekend of fascinating geology on 10th – 11th September. We were led by Rob Crossley from Geo Môn geopark who had picked out some of the best areas of the island for us to visit.

On the Friday evening we assembled in the Heritage Centre in Cemaes. Dr Rob Crossley introduced himself as a solid rock geologist presently with Geomon but previously with Robertson Research.
Rob outlined the deep basement geology of Anglesey with reference to the now available mapping of the whole area including the North Sea and Irish Sea. He noted the deep seated west-east dislocations which underlie the predominantly northeast-southwest Caledonian grain and which are of importance to defining the structure of the exploited oil bearing basins. They do incidentally influence the position of the North Wales coastline.
He then introduced the early geologists who studied Anglesey, notably J.S.Henslow, Edward Greenly and his wife, Annie. Henslow was a geologist way ahead of his time, otherwise famous as a tutor of Darwin. He published a comprehensive "Geological Description of Anglesey" in 1822, the result of a brief exploration with accurate sketches and a map which is surprisingly similar to Greenly's which was produced a hundred years later and is still the basis of present mapping.
Rob distributed a comprehensive handout covering the proposed two day excursions and a third for participants to follow on Monday, on the way home.

One of the most important features of Môn geology is the Gwna Melange, described and published by Edward Greenly in 1919. This was the first melange described, and still attracts geology students from all over the world. (Wikipedia describes a melange as “a large-scale breccia, a mappable body of rock characterised by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically consists of a jumble of large blocks of varied lithologies. Both tectonic and sedimentary processes can form mélange.”)

text by Tony Thorp and Chris Simpson - photos by Chris Simpson

Day 1
The first location was just east of Wylfa power station, facing a cliff exposure in which the Gwna Group melange, containing large quartz clasts, was cut by a dyke exactly as shown in an accurate sketch by Henlow. We surmised that he probably stayed at the large house nearby which had been demolished when the power station was built.
As we walked back to Cemaes we could see further along the coast one of the famous enormous limestone clasts in the melange in which Margaret Wood had found fossil stromatolites, the oldest fossils in Wales.

Next location was Parys Mountain, the spectacular remains of what was once the largest copper mine in the world. It contains a vast open cast pit and a mass of shafts and galleries. It remains a source of highly acidic mine effluent which was still flowing out. The various complex iron oxides and hydroxides with their colours ranging from ochres to reds and purples give the residual tips their "out of this world" character as made use of in scenes from Dr Who. We could see the tight overturned folded structure of the bedding, within which the richest ore was extracted by open cast mining.
The last location of the day was the cliff section at Lleiniog, near Beaumaris. This is a section through Quaternary till and gravel deposits. The gravel initially looks like the deposits of outwash streams and rivers, but further examination shows that they are more likely sub-glacial meltwater channel deposits. These are under hydraulic pressure and can therefore change directions in three dimensions. Even so, it was difficult to interpret some of the changes in level. On the beach below the cliff were pebbles and boulders eroded out of the till, some showing striae and some coming from northern England with the Irish Sea Ice Stream. There were also large more local limestone ones from the limestone quarried nearby at Penmon

On Wylfa Head, just North of the nuclear power station. The vertical rock face behind Rob has four separate lithologies

A closer view showing how different the colours and textures of the various melange components are.

A little further North on Wylfa Head. A distance view with a power boat for scale. Just right of the centre, there are some lighter coloured rocks in the distance

A closer view shows several adjacent large pale blocks of quartzite with no relationship to the surrounding darker schists.

Looking across Cemaes Bay from Wylfa Head. There is a well-demarcated white area of rock at sea level in the centre of the picture, about 1.5km away.

At closer view this pale area of rock is in fact a single large block of limestone, several hundreds of metres across, making up part of the melange

Day 2
On day 2 we met at the small RSPB car park just south of the main visitor centre. We took a short walk to the clifftop where the South Stack formation cliffs are intersected by a major dyke. Looking along the line of the dyke to where it cut the opposite cliff we saw how it made a dog-leg inland. This led to an object lesson on how dykes interact with the grain of the bedrock. The line of a dyke is determined at depth and, if it lies at a low angle to the grain, it has to progress en echelon with a dog-leg at intervals.
We took the clifftop path towards the reserve and Ellin's Twr, a good viewpoint of South Stack lighthouse and the section in he cliff below before returning to the car park and motoring around the headland to park near to the Holyhead Harbour Breakwater. This was built in the 1850s with a strong substructure of large blocks of local Holyhead quartzite topped by Carboniferous limestone. Fossil brachiopods and and corals were exposed on the limestone capping walkway. Apparently there was a move to use limestone for the structure but it was more subject to erosion.

Our final location was Llanddwyn Island.
This offers the most spectacular exposures of the Gwna group lavas which were erupted underwater forming pillows with infill and strands of jasper between them. Where the rocks had been washed clean, the Gwna melange showed up the different colours and textures of its varied clasts with pink and red chert and quartzite cobbles contrasting with green schists.
The sand dunes are constantly shifting and the exposures were more extensive than the last time we visited.

At the Western edge of Llanddwyn Island, this is another part of the melange with a much smaller scale mixture of components compared to Wylfa Head.

A closer view of the same area.

An area of boulder clay from the last glaciation sitting between two pillow lava outcrops. This had previously been covered over (and protected) by dune sand.

A closer view with pillow lavas on the right. The boulder clay also contains pillow fragments, consistent with its glacial origin. Now it has been exposed, it is likely that this remnant from the last ice age will have been completely eroded away in a few years.

Rob Crossley explain how these structures formed. Note the black structures in the cliff face immediately above his hands.

A closer view shows that these are coal fragments within the sandy/pebbly matrix. Their orientation gives clues about the direction of flow under the glacier.

This slide shows the range of clast size seen in the cliff face. But how did the vertical interface in the lower centre of the picture between the pebbly matrix area and pebble-free area develop? We did not have a definitive cause for this unusual appearance!

Day 3
Rob had given us an itinerary to follow on our way back via Menai Bridge and Caernarfon. First stop was at the blueschist exposure round the Marquis of Anglesey's column. This was a disappointment as the car park and access was full of plant. The only access was a small path up from the main road. At one side was a small collection of blueschist hand specimens (Thank you, if you left it there!) but the column itself was barricaded off. The large exposures were still exposed and pieces of blueschist were scattered around in the wood.
A mile or so further along the A5, Telford's 1826 Menai Bridge is worth visiting in its own right. The supporting towers in Carboniferous limestone show some good fossils with rather lovely corals. Rocky Carboniferous exposures below were not very clear from the roadway but could be better exposed at low water.

The final location was the along the low cliffs near Plas Menai Sports Centre just off the Caernarfon road. A major Tertiary dyke intersects the coast here with smaller offshoots cutting the cliffs between it and the sports centre. The pebbly to cobbley conglomerate varies from grey where it was cooked by the intrusion to red nearer the centre.

That rounded off a most rewarding visit to Anglesey where we enjoyed good weather and some interesting and spectacular geology.

Field trip to Aberystwyth and Clarach led by Dr Charlie Bendall August 23rd 2023

Field trip to Aberystwyth and Clarach with Dr Charlie Bendall to investigate sedimentary structures in the Aberystwyth Grits Sunday 20th August 2023

Some of the group on the beach with Charlie Bendall

Our August field trip, led by Dr. Charlie Bendall, followed the coast from Aberystwyth to Clarach. This coast is famous for its sedimentary structures, including "Turbidites". Charlie had provided copious background information on Gravity Flows of all sorts for prior study and, after members assembled at The Castle, he broadly outlined the full range of terrestrial and marine slides, slumps and other gravity movements, often described confusingly by a proliferation of ill or loosely defined terms.

A marked degree of folding at the base of Constitution Hill, Aberystwyth. Charlie explained that there was no evidence that this folding was tectonic in origin. He thought it was an extreme degree of soft sediment deformation before complete lithification.

We then walked down to the first location.
This was a 3m deep cutting below the castle alongside the promenade where a series of thin to medium bedded sandy and silty couplets was exposed. These were good examples of turbidites which could be classified in terms defined by Bouma. They clearly had an erosive base with a normally graded sandy Bouma unit A, grading into a finer unit B with some parallel lamination and a unit C with some cross lamination.

Walking to the north end of the promenade, the cliffs below Constitution Hill expose more Aberystwyth Grits folded in a conspicuous plunging anticline, with associated faulting. The bedding was somewhat thicker here with sandier beds alternating with darker shales.

Alternating sandstone and mudstone beds at the foot of Constitution Hill. Some of the sandstone beds are very thin, hardly disturbing what would otherwise have been a single thick mudstone bed.
Also visible just above the thick basal sandstone unit is a distinct layer of concretions within the overlying mudstone bed

A closer view of the concretions

The second part of the trip was to explore the cliffs north of Clarach. Most of the group took the cliff walk but Bill and I took the easy option of driving and avoiding the long walk back to the car.

An example of soft sediment deformation within a sandstone bed

A small detour took us to the south of the bay where we (probably) located a thin bentonite which had been noted from earlier research. Such beds are the result of volcanic eruptions which distribute ash into the sea over a wide area so they are good marker units for dating.
This bed was not too obvious as it was obscured by a black algal deposit. It also seemed quite hard, but this can happen during diagenesis.

A thick sandstone bed at the Southern end of Clarach Beach showing Bouma A and Bouma B units.

Crossing the bridge over the river, we continued along the beach.

The cliff face underneath the coast path at the Northern end of Clarach.
At the base are beds of the Aberystwyth Grits. Above that is a grey layer, and above that a brown layer. These are both of glacial origin. The grey layer is material arising from Welsh glaciers, while the brown layer is material arising from an Irish glacier. The Irish glaciers eroded rocks with a high iron content, so the brown colouration is a useful marker for glacial material arising from an Irish glacier

As the tide was falling we could clamber round to another inlet and a cave where the undersides of beds could be observed. There flute casts and burrows had been beautifully preserved. In higher beds nearby concretions had developed in a thicker shaley mudstone. The way these develop is not fully understood. Sometimes they start from a visible nucleus, possibly a bit of fossil, but often there is nothing visible. They could start with a crystalline particle which could be at a molecular scale and grow into the surrounding sediment, cementing it and, given space and stable conditions, develop a spherical structure. Interesting forms like "cone-in-cone" can be produced by the complex interaction of crystal growth with sediment particles.

A cone in cone concretion within a sandstone bed at the Northern end of Clarach Beach.

A higher power view of the concretion. (We later discovered a pebble on the beach which was an intact concretion, eroded out of the beach rock by wave action)

A cave at the Northern end of Clarach Beach. The base of the sandstone bed forming the roof of the cave shows prominent flute casts.

An enjoyable trip exploring Aberystwyth Grits, turbidites and Bouma interpretations with an accomplished leader wound up with a cliff walk back to Aberystwyth.

Photos by Chris Simpson and text by Chris Simpson and Tony Thorp

Summer Weekend Field Trip to Gower 9th - 11th September 2022 led by Prof. Kokelaar

Summer Weekend Field Trip to Gower 9th - 11th September 2022 led by Prof. Kokelaar

The weekend commenced with a talk by our leader Prof Peter Kokelaar ( retired George Herdman Professor of Geology University of Liverpool) which covered the general geology of the area and also his own findings from the work he had carried out on Gower.

Gower is composed almost entirely of rocks of Devonian and Carboniferous age with only remnants of the once extensive Mesozoic cover. The Devonian rocks comprise the Old Red sandstone (ORS) which were deposited in arid to semi-arid environments.These rocks are divided into red marls and brownstones ( lower ORS) and quartz conglomerate (upper ORS) which are separated by an unconformity. The ORS form the highest ground on Gower and occur in the cores of anticlines. The high ground of Cefyn Bryn, Llanmadoc Hill and Rhossili Down are anticlines consisting of ORS.
The main bulk of the rocks of Gower are comprised of Carboniferous Limestones with deposits greater than 1000m. These rocks were laid down after marine transgression over the ORS with further regression/transgression sequences over time culminating in the typical facies of Carboniferous rocks i.e. Carboniferous limestone, Millstone Grits and the Coal Measures.
Towards the end of the Carboniferous period the Variscan Orogeny propagated from the south in a northerly direction causing extensive folding and faulting of the Devonian and Carboniferous rocks.
The whole of Gower was affected by ice on numerous occasions. The last glaciation, the Devensian, reached a maximum about 18,000 years ago and destroyed many of the features created by previous glaciations and is responsible for most of the glacial landforms preserved in Gower today.

Port-Enyon Bay

On a gloriously sunny Saturday we parked in Oxwich and then walked, with various stops, to Oxwich Point.

Port-Enyon bay like Oxwich bay lies in the core of a syncline with the softer Millstone Grits ( Marros) being easily eroded to form the bay. From our vantage point, at the first stop, we could view the solifluction terrace alongside Port - Enyon bay. This is almost a flat terrace that ends in a steep bank at the beach. This was produced during the last glaciation ( Devensian) when glaciers reached northern Gower and the area would have had an Arctic Tundra type climate with permafrost. Freeze -thaw action on the rocks lead to rock fracturing producing rock fragments. During summer thaw water could not drain through the frozen ground so that it became unstable and slid down the slopes leading to a thick layer on the rocks below and hence the terrace.

Solifluction terrace Port-Enyon Bay

Prof. Kokelaar then went on to explain why when looking at the high surfaces one can see that they appear to be level. In fact they are sloping platforms at approximately 60m,120m and 180m. These are thought to be marine abrasion platforms that have been uplifted in pulses. The breakdown of the limestone in a marine environment is due to a number of processes of weathering and erosion which include wave action, sea water depositing salt crystals in cracks in rock which expand on heating fracturing the rock ( known as haloclasty), wetting and drying, splash and spray impact, mixing corrosion, dissolution, and biological action,
He then explained that the most probable cause of the pulsed uplift was related to a mantle plume which was originally centred in the area of western Scotland, north-eastern Ireland and east Greenland. He suggested “that continuously flowing fluids like in the hot mantle in a plume do not move steadily though the cooler mantle but form a series of waves or blobby pulses. Therefore uplift would have been pulsed leading to the stepped nature of the landscape.” The landscape would then be weathered and eroded as seen.

At the second stop we examined minerals that occurred as fault fracture fill. The site was usually covered but storm waves had led to the site being temporarily visible. The mineralisation was superb. The layers consisted of mm scale calcite, and haematite layering with some paler yellow presumably limonite present. It was explained that the colour banding reflects compositional and/ or temperature variations of the fluids forming the minerals through time. These fissures in which the mineral fill occurs form along or around faults in the upper few kilometres of the crust. These fissures can form in one of two ways either by displacement along a fault leading to the production of a void or by solution along faults or joints can create fissures in for example limestones, although the two are not mutually exclusive.

Fault fracture fill

Along our way to the last stop we viewed and discussed the origin of the slades. Slades are the short dry valleys that cut into the flat - topped cliffs. There have been various explanations for their formation but Prof Kokelaar suggested that the simplest solution is that they are remnants of minor karst valleys that existed before the late Pliocene marine planation. Therefore the upper reaches of these valleys were originally greater and then were shaved off by by the marine planation.

Discussing Slade formation

Our final stop was at a part of the beach which led to quite a discussion. This is one of the sites where what is known as Patella beach deposits occur and these deposits are also found at various other sites from Mumbles Head to Worm’s Head. The deposit is composed of various pebble, cobble and breccia conglomerates with a matrix of coarse sand and gravel. It is highly fossiliferous and the name Patella derives from it’s large content of limpet shells ( Patella Vulgata). Periwinkle and dog whelk are also present. These organisms live in the littoral zone.
The Patella beach is Late Pleistocene in age and interpreted as the result of a single exceptional storm with associated storm surge that caused debris to be catastrophically dumped beyond the reach of normal storm waves.

Prof. Kokelaar explaing the formation of the Patella Beach Deposit

The Sunday turned out to be misty and cool a complete turnabout from the previous day.
Our first stop was to the top of Cefyn Bryn at which point fleeces and windproofs were required. From the carpark we followed the path which runs north and stopped at the Bronze Age Round Cairn which comprises a massive collection of erratics which are representative of the area. The erratics consist of large numbers of quartzites along with UORS quartz-pebble conglomerates and LORS Brownstone Fm sandstones.

Quartz conglomerate

We then followed the path to the Neolithic burial chamber known as Maen ceti or Arthur’s seat. Prof Kokelaar explained that the chamber consists of a huge capstone and is supported by plinth stones. The capstone is a glacial erratic composed of quartz -pebble conglomerate from the north crop. It has an “exposure age” of approximately 23000 years and as it is an erratic some 300m from the southern limit of the Devensian ice it records the time of the Last Glacial Maximum.

Maen Ceti (Arthur's Tomb)

Lunch was taken atop Cefyn Bryn followed by the final excursion to Caswell Bay. We parked in the carpark and then walked onto the beach. An added bonus was the presence of Mediterranean gulls amongst the Black-headed and Lesser Black-backed gulls on the shore.

Caswell Thrust

Walking along the shore we passed where the Caswell thrust intersects the cliffs. Here the Langland Dolomite has been pushed up and over the Caswell Bay Mudstone. We carried on around the corner to a section of low limestone cliffs which we then scrambled up. The limestone was fine-grained ( micrite) and were covered in micro-rills. The micro-rills were narrow up to 2mm and were either straight or sinuous in shape. It was explained that they are an erosional structure caused by water flowing along the limestone via surface tension effects underneath sand. Prof. Kokelaar went on to explain that if the rills do in fact form under sand and as sand was now absent from these cliffs then at some point in the past the sand-dunes must have been banked up along the cliffs. In other words the sand has been washed away over time most probably due to storms.

Micro-rills on Micrite

This was our last venue and the end of a very informative weekend.

Summer Weekend Field Trip to Suffolk 2021

As covid gradually recedes, the club has restarted its annual weekend away field trips. This year we were in Suffolk being guided by Tim Holt-Wilson. We stayed around Saxmundham, and journeyed north on Saturday and south on Sunday.
Tim gave us an introduction to Suffolk geology on the Friday evening. The oldest beds we saw were the London Clay formation – Eocene marine mudstone at 52Ma. We also saw the Coralline Crag Formation (early Pliocene sandy, fossiliferous marine limestone around 4Ma) and the Corton Formation (glacial till and outwash deposits around 0.5Ma).

Bawdsey East Lane beach. The grey at the base of the cliff is the London Clay formation. The higher brown layer is the Coralline Crag formation. Extensive erosion is evident. The pipe dangling down is a field drainage pipe left exposed by rapid erosion.
There is 48Ma missing between these two beds; but the unconformity is not evident in the cliff face.

The “rock” forming the wave-cut platform of the beach is actually London Clay – very slippery and treacherous at times.

Many of the pebbles on the beach are actually rounded pieces of London Clay (scale is an A6 field notebook).

The Coralline Crag formation in-situ with pieces of shell protruding from the eroding cliff face (dangling drainage pipe is the scale)

A piece of eroded coralline crag material on the beach showing numerous fossil fragments. Nearly all of the fossils are fragmented, so identification is difficult (scale is the camera lens cap)

Cross bedding in the Corton Formation at Pakefield Cliffs

Suffolk was at the edge of the ice sheet in recent glaciations. During glacial times, there were three large rivers draining from higher ground further west across present-day East Anglia into the North Sea: one in the north arising from the south Pennines, a central one arising in the West Midlands and a southern one arising in the London area. These conveyed stones from these areas which are now found on East Anglian beaches.

A wide variety of pebbles at Pakefield beach – these reflecting the draining from a wide area of the west Midlands, possibly as far as Wales.

London flints on the beach at Benacre Broad.

Coastal erosion is occurring rapidly at many locations, although sand does move up and down the coast, and some areas show increased deposition. Dunwich was a major population centre in the Middle Ages, but erosion resulted in loss of >90% of the town, so only a small village remains today. We saw a fascinating model of Dunwich in the 12th century in the village museum with a yellow line half a kilometre inland representing the current coastline.

The Martello Tower at Bawdsey East Lane beach. These towers were built around the coast during the Napoleonic Wars to protect Britain. Now we have to protect them by installing rock armour to stop the sea eroding them away!

A tree recently lost to coastal erosion at Shingle Street

A sand bar at Shingle Street showing the deposition that occurs side-by-side with the coastal erosion.

One other feature of trade in the Middle Ages was that boats would come from many countries in Europe with local stones as ballast, then leave the stone as they loaded up their cargoes. Because there is virtually no building stone in Suffolk, these ballast stones would be incorporated in the walls of the churches and houses built at this time.

Flints incorporated into the wall of Orford Church. Look particularly at the second and third row up from the bottom

The second row up is formed of Swedish flints brought here as ship’s ballast. Note the “milky way” background

English flints in the third row up. Note the smooth glassy texture.

The Church of St Andrew at Covehithe. Originally a very large church, in 1672 the parishioners were given permission to knock most of it down and rebuild a much smaller church within it. The old church incorporates interesting building stones, many of which were re-used in the walls of the small church.

The old Franciscan monastery of Greyfriars at Dunwich. Tim Holt-Wilson (sitting far left) is going to show details of the building stones to members

Field trip to Waun Marteg July 25th 2021

The field trip on 24th July took us to the Waun Marteg forest, near Bwylch y- Sarnau. led by Tony Thorp

The four quarries visited took us through a sequence from the late Llandovery ( Telychian stage) to early Wenlock ( Sheinwoodian stage) The exposures form part of a syncline and the route took us from west to east from oldest to youngest rocks.

We parked at the first quarry (location one SO 009 770) which exposes the junction between the Dolgau Mudstones and the early Wenlock Nant-ysgollen Mudstones.
The Dolgau Mudstones (Llandovery, Telechyian stage) are grey green, turbidite mudstones with burrowed hemipelagites. An example of the burrow-mottling was found in the scree and can be seen below. During the late Telychian tectonic events had become quiet. This allowed the re-establishment of oxic slope-apron sedimentation within the Welsh Basin which is characterised by the Dolgau mudstones.

Burrow Mottling

The Nant-ysgollen formation is the earliest strata in the Wenlock series and overlies the Dolgau mudstones. It marks the return of oxygen-poor bottom conditions and deeper water. It consists of thinly interbedded turbidite mudstones and laminated hemipelagites. The formation is in the Cyrtograptus centrifugus biozone and the base of the Wenlock series and the Sheinwoodian stage is equated with the base of the centrifugus biozone (although Mullins and Aldridge (2004) suggest that the actual level is at the top of that biozone or at the base of the C. murchinsonii graptolite biozone). The formation showed well developed cleavage which had a high angle dip and pencil cleavage was apparent.

First Quarry,SO 009770, with the junction between the Dolgau Mudstones and the Nant-ysgollen Mudstones

At the second location ( SO 013 771) the Nant-ysgollen mudstones were the predominant lithology. Bedding of shallow dip was apparent as were concretions and well developed pencil cleavage. Whereas at the third location ( SO 016 773) although the hemipelagites and pencil cleavage of the Nant-ysgollen mudstones were still present, towards the top of the exposure a fine sandstone probably of the Penstrowed Grits Fm was apparent. Within the scree pale calcareous mudstones were numerous.

Second location,SO 013771, shallow dip in the Nant-ysgollen mudstones

Horizontal bedding in the Penstrowed Grits Fm.

Location four (SO 012 776) was a large quarry within the base of the Penstrowed Grits Fm. with good example of sandstone mudstone turbidites. The Penstrowed Grits is an extensive sandstone lobe facies that was supplied from the south west end of the basin. The medium- to thick-bedded, high-matrix turbiditic sandstones are interbedded with thin siltstone turbidites and laminated hemipelagites. The muddy turbidity currents were probably supplied from the adjacent platform and slope rather than from much further south. The bedding was almost parallel at the SW corner of the quarry but the sandstone beds at the NE end of the quarry became jointed and the mudstone had developed pencil cleavage.

Pencil Cleavage

At the end of the visit a few of the members decided to carry on to a fifth location close to Bwlch-y-Sarnau (SO 035 750). A NW/SE fault crosses the quarry. The junction between the Nant-ysgollen Mudstones and the Penstrowed Grits Fm was exposed. The bedding in the Penstrowed grits was at a very steep angle and calcareous mudstones were again present. Good examples of current aligned graptolites were found.

Quarry Bwylch-y-Sarnau with member along the line of the fault

Current aligned graptolites found at the quarry

NOTES

Hemipelagite is formed by the slow accumulation of biogenic and fine terriginous material onto the sea floor. There are two types of hemipelagite laminated and burrowed. laminated hemipelagites occur under anoxic bottom conditions are dark grey and have a bedding parallel lamination, whereas burrowed hemipelagite occur under oxic bottom conditions is pale grey/green with darker burrow mottles and lamination is prevented by burrowing and scavenging benthos that constantly rework the sediment.

Pencil Cleavage
“Pencil cleavage is a low-temperature tectonic fabric formed due to shortening of clay-rich sediments like mudstones”
When mudstone or clay-rich sediments are deposited they will tend to break along bedding planes as the platy clay minerals give a natural bedding parallel fissility. If at a later time the rock is subject to another deformation then a second cleavage may develop with a different orientation to the first. This is due to pressure solution and reorientation of clay minerals. At some point this secondary cleavage will be just as pronounced as the first cleavage and the shale will fracture along both planes and form pencil like structures. Pencil cleavage can also develop where two tectonic cleavages develop in the same rock.

Mullins, G. L. & Aldridge, R. J. 2004. Chitinozoan biostratigraphy of the basal Wenlock Series (Silurian) Global Stratotype Section and Point. Palaeontology 47, 745–73