Ice Streams

 

Glacial Prehistory 

Ice Streams 

An ice stream is a meltwater flow running away from the collapse of an ice sheet onto dry land geology.

The material of which the ice stream is composed, is water, ice blocks, stones, gravel, sand and silt in proportions that would allow the bedload, sand gravel etc, to travel with the water down available sloping terrain towards the sea, or anywhere that pooling of the water can be achieved. 

The ice stream, as it crossed terrain, caused the bedload that it carried to erode the underlying geology, in some places leaving evidence of its passage. 

The evidence that remains shows up as lineations, pot-holes, erratics, striations, drumlins and moraines. 

 

Above is a basic plan of flowsets from northern Scotland in "An introduction to the history of glaciation and sea-level around Nairn and south of the Inner Moray Firth" by Jon W. Merritt et al

Many of these Lineations are likely to have been formed in valleys that were cut by impacts, ie, they are impact valleys along which erosive ice streams have traversed scarring the geology. 

 

  The chart above offers an interpretation of existing landform information,  suggesting where known ice streams are thought to have traversed across Britain. 

It is simplified, but even so there are problems with this data, primarily that the ice stream is often required to flow uphill.

     

The diagram above is from:-

"Dynamic cycles, ice streams and their impact on the extent, chronology and deglaciation of the British–Irish ice sheet" by Alun Hubbard et al

It illustrates how computer modelling visualise the flow of ice sheets and ice streams across Britain without reference to geology behind the imagery.

The following article describes the current view of what these ice streams consist of and how they got to be where they are.

In, "Late Devensian deglaciation of the Tyne Gap Palaeo-Ice Stream, northern England", by Stephen J. Livingston et al," glacial Lineations flowing down from the high Pennine Hills, pictured above, are described in an introduction, as follows:-

The mountainous spine of the central sector of the BIIS was characterized by high-elevation cold-based plateau ice caps bisected by eastward-draining terrestrial ice streams such as the Forth, Tweed, Tyne and Stainmore (Fig. above) 

Upland ice-dispersal centres such as the Cheviots, Southern Uplands, Lake District and Pennines played an important role in seeding and modulating ice flow through these fast-flowing corridors, and thus in controlling the relative dominance of the Irish Sea Ice Stream (ISIS) and North Sea Ice Lobe (NSL), both of which have their onset zones in this region. This is reflected in the geological record, which indicates complex, multi-phase ice-flow behaviour and repeated marginal fluctuations.

The Tyne Gap Ice Stream (TGIS), flowed eastwards towards the mouth of the River Tyne through a predominantly bedrock-floored, 15–30-km-wide mountainous pass located between the Cheviot Hills and English Pennines. The TGIS was fed by ice from dispersal centres in the Lake District and Southern Uplands and was a major tributary of the NSL, which flowed southwards down the east coast of England. 

The Tyne Gap corridor is heavily streamlined below 400 m asl, with lineations typically <1.5 km long, although forms up to 4 km have been observed. Lineations are composed of both till and bedrock, including visually striking streamlined bedrock ridges controlled by the geological structure, particularly evident in the western Tyne Gap region. This has resulted in obliquely angled lineations, which may have formed during a single ice-flow phase. The ridges often display a distinctive asymmetry, with steep north and west faces and gently dipping southern and eastern flanks.

This is explained by the distinctive geology, which comprises Lower Carboniferous sedimentary rocks composed of rhythmic sequences of limestone, shale, sandstone, greywacke and coal (units 5–20 m thick) intruded in places by the resistant strata of the Whin Sill dolerite that dip 110–20˚ SSE towards the edge of the northern Pennines.

Further to the north-east the strike of the strata swings around to the NNE, with a dip of 55–10˚ to the east. This was transverse to ice flow, resulting in crag-and-tail features where till has been smeared across the ridges. Till is more extensive in this region, with more subdued lineations orientated east to ENE.

The illustration shows four flowsets by which glacial material in some form has traversed from Pennine England out towards the North Sea. The flowsets emerge from the Forth Valley, the Tweed Valley,  the Tyne Valley, and the Stainmore Valley. 

The Tweed valley feeds offshore into the North Sea at a location that is known as the North Sea Lobe which follows the English coast south and around Dogger Bank to the East. 

Above is an illustration from:- "The mixed‐bed glacial landform imprint of the North Sea Lobe in the western North Sea" By Dave Roberts et al

It is:- "A bathymetric overview of the North Sea showing the position of BIIS (British and Irish Ice-Sheet) and FIS (Fennoscandian Ice-Sheet) ice margins, flowlines and ice divides during MIS 2 glaciation. The NSL (North Sea Lobe) was a distinctive lobe of ice that flowed southeast from the Firth of Forth region towards Norfolk in eastern England towards the end of MIS 2 glaciation." 

  

The illustration above is from the same report. "Streamlined bedforms in drift across the seafloor north of core 132VC. Some, but not all, bedforms appear to be seeded from perturbations on the seabed. It is also possible to see that some ridges are more ovate in planform. Yellow areas are bedrock highs with very thin glacial sediment cover. "

Current thinking around ice streams is further discussed in:-

"Streamlined hard beds formed by palaeo-ice streams: A review"  by Maarten Krabbendam et al

"Ice streams have flow velocities many times greater than surrounding areas of sluggish or intermediate-velocity ice; they form the arteries of ice sheets and are crucial for regulating the flow dynamics of ice masses (e.g., Bentley, 1987; Joughin et al., 2001; Bennett, 2003; Rignot et al., 2011; Stokes et al., 2015). In modern ice sheets, ice streams can be mapped using direct satellite observations of surface ice velocities (e.g., Joughin et al., 2010; Rignot et al., 2011), whereas palaeo-ice streams in Pleistocene ice sheets can be reconstructed by mapping out their geomorphological footprints, consisting primarily of discrete flow sets of elongate bedforms such as mega-scale glacial lineations (MSGLs) (e.g., Clark, 1994; Patterson, 1997; Stokes and Clark, 2001, 2002, 2003; Everest et al., 2005; Golledge and Stoker, 2006; Hughes et al., 2014; Spagnolo et al., 2014).Within the former Laurentide Ice Sheet, for example, at least 100 such palaeo-ice streams have now been recognised (Margold et al., 2015).

Generally, MSGLs are implicitly or explicitly assumed to comprise unlithified, soft sediment (principally till) that has been modified subglacially into very long drumlins and megaridges, although the exact mechanism by which this happens (erosion, accretion, deformation or a combination of these) is still uncertain (e.g., Boyce and Eyles,1991; Clark et al., 2003; Stokes et al., 2013; Spagnolo et al., 2014). However, increasing attention is being directed to elongate subglacial forms on bedrock surfaces, known as ‘hard beds’ (e.g., Bradwell et al., 2008a; Graham et al., 2009; Eyles and Putkinen, 2014). The geomorphological analysis of ‘hard beds’ is important given that large parts of the Northern Hemisphere Pleistocene ice sheets rested upon, and flowed across, hard beds composed largely of Precambrian shield rocks, flanked by Palaeozoic sedimentary strata. The same broad pattern is seen in Greenland and probably Antarctica (e.g., Goodwin, 1991; Livingstone et al., 2012). Analysis of hard-bed streamlined forms can aid in reconstructing palaeo-ice streams, assist in understanding the primary controls on ice streaming itself (see also discussions in Winsborrow et al., 2010; Livingstone et al., 2012) and has potential implications for the origin of MSGLs in general (Eyles et al., 2015)."

Videos appear on social media websites of disastrous flooding events where a mountain based melt, or breaching of a dam, has caused huge volumes of water with sludgy sediments loaded into it to descend on a lowland village. 

The damage that these floods cause to the village the landscape around them are quite significant,  not least the slow movement of cars and houses down river. 

It is likely that this is the sort of scenario in which most of the ice streams of Britain and the North Sea were formed. 

In many locations the amount of meltwater water being added to the landscape and thence to the coastal waters would have been in the order of cubic kilometres a day.

What percentage of that volome would have been rocks, gravel, and sludge?

What speed would the water flow at?

What speed would the bedload move at?

Jeffery Nicholls 

Orkney UK 

Jiffynorm@yahoo.co.uk 

#BIIS #Quaternary #geology #glacial 

Striations

   

Glacial Prehistory 

Striations 

  

Striations are features present on some stone surfaces that have grroved surfaces that may have been eroded by glacial ice streams. They are present under materials that may be glacial, and if they have not been scarred by this material its difficult to see how they were deposited. 

This is a record of some striations,  just one at the moment, it is on South Ronaldsay in Orkney. 

  

The illustrated location is in a bay in South Ronaldsay on of the islands of Orkney. The orientation and context of this sample feature is not a common occurrence in this location, but is an outcrop of a hard fine-grained rock which responded to abrasion by allowing its surface to be eroded with detailed lineations. 

Elsewhere softer or flakier materials simply broke up as material flowed over it.

   

  

 Above is the location of the the Orkney Islands north of mainland Scotland with location of St Margaret's Hope where the feature is to be found.

The arrow indicates the orientation of the striations.  The angle of fall of the striated rock surface is about 5 or 10 degrees, dropping towards the sea.

 

Above is the outcrop from the beach. 

Closeup of the outcrop striation group.

   

Above is the striated stone.

     A view, in the direction to which the striations are pointed. It shows the port of St Margaret's Hope, with the Pentland Ferries,  roll-on roll-off ferry Pentalina, in port.

  

The sketched chart shows island of South Ronaldsay with the location and orientation of the striated stone.

Also shown are the ridges of a series of parallel valleys crossing the island.

From "Bedrock, Ice age and Holocene  Environment in Orkney" by John Brown.

Chart shows striations across Orkney generally following a southeasterly to north-westerly direction. 

The flow direction of the striations on the bedrock stones are in complete contradiction to the general topography over the archipelago. Specifically, the flow of ice over the South Ronaldsay ridges and valleys is not tenable. 

Jeffery Nicholls 

Orkney 

Jiffynorm@yahoo.co.uk 

#geology #Quaternary #Orkney #Scotland 

    

Ice Age Story

Glacial Prehistory 

 Ice Age Story

 

  Introduction 

This is a story of Britain, and north-west Europe, as it passed through several periods of extreme cold, Ice Ages, the formative events that created the landscape that we live with.

It is not authoritative, and may be taken with a pinch of salt, but it is my best attempt, using current evidence, to understand the ice age of Britain. 

The general territory that the story covers is very roughly the Atlantic coast of Europe, but includes the Alpine mountains of southern France, and the northern mass of Scandinavia. 

Specific areas that are covered are Britain, Norway, and the North Sea. These constitute an area that academia deems to have been covered by thick ice sheets, the British and Irish Ice-Sheet (BIIS) and the Fenno-scandinavian Ice-Sheet (FIS). These ice sheets are thought by some to have been joined across the North Sea for part of the last ice age.

The theory that is proposed, studied, taught and learned, in universities, is that the ice sheets over northern Europe, and elsewhere flowed as solid ice Formations along glacial valleys, shaping them as they progressed.

Beginning 

The evidence tells a more nuanced story that has to start several million years ago with the movement of Britain away from Europe.

As the Atlantic Ocean opened up a great chasm between Europe/Africa and North/South America developed, so that Britain pulled away from Norway. The result was that a portion of the earth's crust between Britain and Norway remained open, a vast crack in the earth's surface, to be filled by sediments from surrounding lands. 

Ultimately, as the oceans filled up with sediments, and the ice-sheets melted, the seas invaded the space between Norway and Britain, forming the North Sea. 

In the last million years, and possibly before, the planet was cold, reflecting the sun's rays off from polar ice caps. There were brief periods of warmth in which ice sheets collapsed crashing into underlying geology, cutting impact valleys and dumping huge volumes of sediments across landscapes, and eventually into offshore waters. (Impact Valleys)

This process would have raised average sealevel on a continuous basis from a million years ago right up to the present day. 

It is difficult to assess what effect this carving out of geology had on landscapes of today, but evidence of sediments in the Trent Valley confirm that some of them go back a long way into prehistory, with sand and gravel beds dated up to a million years, and more.

Nascent North Sea

The period formally assigned to the last ice age period began at 120,000BP, and lasted until about 10,000 years ago. 

Before 120,000BP, there occurred the earliest significant event that is recorded in the geology of the North Sea. An extended warm period melted polar ice sheets causing Sea levels to rise allowing an insurgence of sea water to flood the area now the North Sea. 

After this warm interglacial period snow began to fall from 120,000BP, continuing until around 70,000BP when there was a sharp thaw. At this time ice sheets on the Pennine Hill of Northern England and the mountains of Scotland collapsed, cuttimg impact valleys, and sending tidal waves across what was probably dry land in the North Sea area, to deposit huge volumes of sediments as Dogger Bank. (Dogger Bank)

Cold, but not that cold!

There followed a relatively mild period from around 50,000BP to around 23,000BP (Tattershall).

In this period the ice sheets remained at high altitude on the mountains of England Scotland, Wales and Ireland. But in the lowland regions of southern Britain and Ireland meltwater rivers flowed through a cold, and probably wooded landscape with marshy areas near the coasts. These places were home to cold tolerant creatures like the woolly rhinocerous, and to people using Upper Palaeolithic flint tools making technologies. (Dorset Barrow)

Also at this time, the ice sheet on Shetland was collapsing, and some of the meltwater from that event (Capeshore Formation) flowed south along the Scottish coast into a lagoon that had been formed by the after-wash from glacial meltwater, the Marr Bank lagoon. (Marr Bank Formation). There two additional deposits were laid with the Marr Bank Formation, the Wee Bankie Formation  and the Bolders Bank Formation. (Wee Bankie Formation) (Bolders Bank Formation)

Snowing again!

Cold weather and heavy snowfall returned to lowland Britain, possibly as late as 23,000BP,  laying a variable thickness of snow/ice across the British Isles and the North Sea. Very little snow built up on the south coast of England, though the South Downs in Kent, the east coast of England,  including London, (Levallois Flake) may have had considerable thicknesses of snow on them.

Additional thickness of snow over the pre-existing mountain ice-sheets of, Scotland, Wales, northern Ireland and northern England  raised those ice-sheets further.

The mainland ice sheets on England and Scotland extended off present coastlines a short distance and a high ridge of land joining Dogger Bank in the southern North Sea to the Atlantic Ocean at a peninsula east of, and separate from Shetland, but separated from Norway by the Norwegian Channel.

The Marr Bank lagoon lying off the Scottish coast, near Aberdeenshire remained unfrozen throughout this return to freezing conditions, supporting vegetation and wildlife. 

The ice-sheet spread across from Norfolk, linking up with Dogger Bank, to enclose the Marr Bank lagoon from the south. 

It is likely,  but cannot be proved, that the ice sheet also lay thickly around the North edge of the Marr Bank lagoon, where the Swatchway Formation is to be found.

Everything, everywhere, melts!

From about 21,000BP the very brief cold period at the end of the ice age ended, and a 10,000 year long deglaciation began.

Between 21,000BP and 15,000BP the ice sheets retreated rapidly across all low-lying areas, causing seasonal collapse differences to leave parallel valleys across many areas.(Parallel Valleys) 

Ice sheets on the French Alps collapsed sending meltwater North to the English Channel and the southern North Sea, where the speedy retreat of ice-sheets progressed northwards at a rate of roughly a kilometre per year.

At these intial stages of the melt the Lobourg Channel of the Dover Strait was cut, the Outer Silver Pit, at Dogger Bank also, and the Norfolk Banks. The South Wales valleys were cut around this time, and the Black Mountains of Brecon and Herefordshire. 

At about 15,000BP those ice-sheets on the high mountains began to fall, the meltwater causing Ice-streams to cut wide rivers and palaeovalleys across southern England. (Ice Streams)

These ice streams, containing varying proportions of stones, sludge, ice, and meltwater flowed across England and coastal waters leaving geological features where they passed.

These include glacial lineations,  moraines, and potholes. (Striations) (Moraines) (Pot-Holes) (Erratic Boulder) (Drumlins)

An ice sheet on Dogger Bank and the peninsula that bordered the west coast of the Norwegian channel in the North Sea also retreated, leaving huge gullies in the north of the area, as the Witch Ground, and the Devil's Hole, off the Scottish coast. (Swatchway Formation)

These edge collapses would have caused the release of meltwater, possibly in the region of half a cubic kilometre of water every day, with peak melt in summer and least melt in winter. This release, repeated on several coasts around the region caused the sea levels at those local coastlines to oscillate, up and down annually, as seasons changed.().

All Melted

At around 11,000BP the last of the mountain ice sheets had collapsed, and water from that event was mostly draining away with little effect on local sea levels.

In the highland valleys that had been gouged out by the collapse of ice sheets there remained rivers of ice, valley glaciers. These, like the ice-sheets, were static, and where the sun was less able to reach in the valley floors they remained as glacial rivers of solid ice. 

The glaciers carried the stones and sands that had been gouged out of the mountains and it wasn't until about 9000BP that these began to melt.

Again local sea levels rose as the ice melted in the valleys, and the debris that had been stored within the glacier simply gravitated to the floor of the valley, remainng now as hummocky moraine. (hummocky moraine)

North Sea develops

The spine of land that divided the North Sea, north to south was available to mesolithic wildlife and people.

As sea levels rose the spine of land was gradually whittled away until in a series of collapses in the 4th millennium BC, more recent, softer sediments in the northern North Sea were easily flushed aside, opening up the whole North Sea. (Archaeology in the North Sea)

There was a coastal inlet east of Shetland that had a southern shoreline just east of Orkney.

Two tidal waves were triggered by submarine landslides in the North Sea north of Norway. The first was the Nyegga Slide in 18,000BP, the second the Storegga Slide  in around 8200BP. These may both have been funneled along this inlet, between Shetland and the extension of Doggerland from the south, to hit the Aberdeenshire coastline.

Migrations 

During the mesolithic,  and early neolithic eras, Ireland was connected to Scotland by land; England was connected to Holland; and Scotland was connected to both Orkney and Shetland. 

Forest, heaths, and marshes formed with the inward migration of animals, plants, birds, and insects from Europe. 

During the 4th millennium BC the strands of land that connected these locations were stripped away by rising sea levels. The geological anomaly at the north West coast of Europe became the British and Irish archipelago.  (Walkable Land in the North Sea)

Between Caithness in Scotland, and south Ronaldsay in Orkney, land in the North Sea was removed.

The beach that connected these two places remains, vestigially, as the island of the Pentland Skerries, and there is a submarine raised landform attached to it, called the Sandy Riddle. 

The story of how the people who watched sea levels separate Orkney from Scotland reacted to this catastrophic event is told in "Orkney Riddle"

Jeffery Nicholls 

South Ronaldsay 

Orkney 

 #BIIS #Britice-Chrono #Quaternary #glaciation #LGM #neolithic #prehistory #archaeology #Orkney #Scotland 

Swatchway Formation

 Glacial Prehistory 

Swatchway Formation 

 

In “Late Pleistocene and Holocene deposits in the Witch and Fladen Ground area, northern North Sea” by Fred Jansen, He divides each of these layers, Swatchway and Witch Ground Formations, into Upper and Lower strata. The earliest deposits are the Swatchway Formation, of which the earliest strata is the Lower SwatchWay Beds.

Jansen writes, “The Lower Swatchway Beds are heavily overconsolidated, shallow Marine sediments which form pronounced ridges, interpreted as ice Pushed ridges." (The pronounced ridges would be the peaks of ridges between valleys caused by the collapse of ice sheets as they retreated, away from warm water in the Norwegian Channel, towards Scotland.)

“A core in the Swatchway Beds nearby Old Devil’s Hole (74-J62-Z, Fig. I) contains still unidentified peaty material as an admixture in Fine sands, which was 14C dated. The section between 130 and 166 cm Below the seafloor yielded a conventional radiocarbon age of 33 800 (+2600, --2000) y BP and the sample from 248 to 251 cm of 37 500 (+ 1800, --1500) y BP.”

The Upper Swatchway Beds, filling in the hollows between the Ridges, are composed of usually normally consolidated glaciomarine Deposits, with locally some probably subglacially folded structures. The Swatchway Beds Are roughly 50 m thick.”

The Upper Swatchway Beds would be the residue of materials left behind after impact erosion formed the ridges.

"The Swatchway Formation has an amorphous, or structure-less, seismic signature, with sporadic subhorizontal reflectors. It has a lithofacies of mud and sand with scattered lithic and shell fragments, The sparse, benthonic foraminiferal assemblage is dominated by the cold-water species E excavatum var. clavatum, which suggests deposition in boreal to arctic seas (Stoker et al., 1985)." (Johnson)

Elphidium excavatum var clavatum (A most abundant calcareous foraminifer in cold-water areas (temperatures below 2°C) on the Arctic shelves; inhabits all sediment types; has a complex pattern of distribution; often dominates foraminiferal assemblages in extreme environments, such as low salinities, turbid waters, and low food supply, including sites proximal to tidewater glaciers.) ( Foraminiferal Research at Byrd Polar Research Center)

Perhaps more significantly, the foraminifera are accompanied by moderately abundant populations of dinoflagellate cysts; a limited spectrum is co-dominated by O. centrocarpum and Bitectatodinium tepikiense Wilson, indicating northern cool-temperate conditions without significant sea-ice cover (Harland, 1988a). " (Johnson)

Lower Swatchway Beds laid down in boreal to arctic seas without significant sea-ice cover after 37500BP. They are heavily overconsolidated, meaning that it was compressed under a thick ice-sheet. The Upper Swatchway Beds are the fill of erosive valleys cut into the Lower Swatchway Beds and they are generally accepted to be dated to between 18,000BP and 15,000BP. 

"The principal 14C samples used by HOLMES were recovered from two borings, a commercial boring at Josephine Field and the IGS boring 74/7 at roughly 60 km off Aberdeen (56058 ' N, 1°07 ' W, INSTITtrrE OF GEOLOOICAL SCIENCES, 1975). From the upper sample of the Josephine Field boring a.o. large fragments of Taxus were dated which is a thermophilic tree. An early glacial age of 23 170 y BP, as was measured, is therefore impossible for biological reasons, and the material most probably was contaminated with younger carbon. If this is the case, the older dates of the boring are also suspect." (Jansen)

The finding of Ewe Tree wood at a location near Aberdeen and off the Scottish coast requires some explanation. Initially, if the radiocarbon date is ignored the existence of the tree growing there at any date from 60,000BP to say 12,000BP, changes completely how we view the climate of northern Europe, and specifically the North Sea. 

The tree may have grown in that location , or it may have floated there, but "The lower 15 m of the Shell-Fugro boring are correlated with the Upper Swatchway Beds. The pollen spectra of this section has a considerable contribution of tree pollen." So this makes it likely that other trees were growing in the area of the Swatchway Beds both after the LGM, perhaps 18,000BP, and/or before the LGM, before 23,170BP.

The implication here, is that the period in which the Last Glacial Maximum gripped Britain was perhaps not more than 5,000 years long, enough time to add hundreds of metres of ice to the mountain ice sheets of Britain and Norway, and to dump a varying thickness of snow on southern England, southern Ireland, and much of the North Sea land area.

The cores 72-J1-Z and 72-Jg-Z, located further south (Fig. 1), were taken from the Lower Channel Fill which, at least partly, developed simultaneously with the Upper Swatchway Beds. The pollen spectrum in the lower section of 72-J1-Z also belongs to group III, a high Hystrichosphaeridae percentage pointing to a marine environment (DE JONG, 1976). The mollusc fauna lacks arctic species, suggesting that the sea was even warmer than today (SPAINK, 1973). The mollusc assemblage of core 72-J9-Z resembles the present fauna, but indicates a somewhat shallower sea level (SP.~aNK, 1976a). (Jansen)

Sources  

(Johnson) The geology of the northern North Sea. United Kingdom Offshore Regional Report By H Johnson, 

Jeffery Nicholls 

South Ronaldsay 

Orkney 

#BIIS ##Britice-Chrono #EarthSciences #LastGlacialMaximum #IceSheets #RelativeSeaLevel #Glaciers

Wee Bankie Formation

 Glacial Prehistory 

Wee Bankie Formation 

"Overconsolidated sandy and gravelly till with interbeds of fluvial sand and pebbly sand, and, less commonly, accumulations of coarse sand and gravel."

The Wee Bankie Formation rests along the east coast of northern England and southern Scotland. It also follows the southern edge of the Marr Bank Formation, and as an overconsolidated material is likely to have been overlaid by ice-sheets until deglaciation, in spite of the Marr Bank being essentially ice- free.

(Cameron) The geology of the southern North Sea. United Kingdom offshore regional report By T D J Cameron, 

 

The chart demonstrates that the Wee Bankie Formation links between the Marr Bank Formation in the north, and the Bolders Bank Formation in the south. 

Jeffery Nicholls 

South Ronaldsay 

Orkney 

#BIIS ##Britice-Chrono #EarthSciences #LastGlacialMaximum #IceSheets #RelativeSeaLevel #Glaciers

Marr Bank Formation

Glacial Prehistory 

Marr Bank Formation

The Marr Bank Formation is described as being deposited in a shallow estuarine environment in "The geology of the central North Sea. United Kingdom offshore regional report by R W Gatliff".

"Sand, fine-grained, poor- to well-sorted, soft to firm, grey to red brown with abundant lithic granules and pebbles. Locally silty."

"The Marr Bank Formation includes "muddy sediments, especially towards the north-west but it is generally formed of sands of varying grain sizes and degrees of sorting (Stoker et al., 1985). Gravelly layers contain a variety of lithic clasts of Scottish provenance, and wood fragments and clay balls occur sporadically. No dinoflagellate cysts have been recovered from the formation."

"A poor ostracod fauna and a moderately abundant population of foraminifera (Thomson, 1978) indicate that the shallow waters in which the formation was deposited were high-boreal to arctic in temperature, and inner shelf to estuarine in character." (Gatliff)

 

The chart above indicates the bathymetry of the North Sea. Generalised contours are marked on the plan. A 40 metre contour is marked around Dogger Bank in the south. Further north the 80 metre contour extends up the middle of the North Sea separating the Marr Bank Formation from the Norwegian Channel and coast. 

The base floor of the Marr Bank Formation is 100 metres below sealevel while the seabed at 80 metres below sea level is made up of outcropping formations of the MIS 6, Coal Pit Formation, and older Formations. 

This would indicate that sediments have been lost here and that Marr Bank lagoon was eroded out, initially in MIS 4 and that there was at least a 20 metre height difference between Marr Bank and the Norwegian Channel. 

North of Marr Bank is the Witch Ground, a very deep area of the northern North Sea which is likely to have been carved out by impact valleys cutting down into the Swatchway Formation, and being filled by the Witch Ground Formation.

  

 The section drawing above describes the relationship between the Wee Bankie and Marr Bank Formations. They merge together,  the Wee Bankie Formation being harder than the Marr Bank Formation. It is likely that the Marr Bank Formation was once as thick as the Wee Bankie Formation,  but, as a soft material, was winnowed away during deglaciation. 

During the Last Glacial Maximum, the Marr Bank was a shallow lagoon of unfrozen water along the British coast. It was separated from the Norwegian channel by a ridge of land that reached from dogger bank to the Atlantic ocean north of Norway

The chart demonstrates how the Marr Bank Formation links through the Wee Bankie Formation to the Bolders Bank Formation. 
 

Jeffery Nicholls 

South Ronaldsay 

Orkney 

 #BIIS ##Britice-Chrono #EarthSciences #LastGlacialMaximum #IceSheets #RelativeSeaLevel #Glaciers

Capeshore Formation

 Glacial Prehistory 

Capeshore Formation 

  

 

  

Around Shetland a deposit called the Capeshore Formation was laid approximately from MIS 5 to MIS 3.  The Capeshore Formation material is described as "Soft to firm, highly to moderately silty clay with sporadic shell fragments and pebbles." (BGS)

It overlies Ferder Formation and older layers that have been cropped by the rise of sealevel into the North Sea region. If the drawing is correct this may have been the first significant incursion of the Atlantic Ocean into the area.

Diatoms in the sediments are identified as:- "The foraminiferal assemblage is dominated by E. clavatum, Cassidulina reniforme Norvang and Protelphidium orbiculare . " (Johnson) , and the the conditions in which this organism developed are:- "The Cassidulina reniforme–Elphidium clavatum, Textularia spp. assemblage dominates the foraminifera fauna near the glacier proximal that indicates increased sedimentation and meltwater runoff." (Chatterjee)

The presence of this species suggests that meltwater was running away from Shetland probably through MIS 5, 4, and 3.

The small size of the Shetland Isles, and their closeness to the Atlantic Ocean caused an ice sheet sitting on them in MIS 5 to collapse, effectively rendering the area ice-free through MIS 3, 2, and 1.

Sources  

(Johnson) The geology of the northern North Sea. United Kingdom Offshore Regional Report By H Johnson, 

Jeffery Nicholls 

South Ronaldsay 

Orkney 

 #BIIS ##Britice-Chrono #EarthSciences #LastGlacialMaximum #IceSheets #RelativeSeaLevel #Glaciers