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

Bolders Bank Formation

 Glacial Prehistory 

Bolders Bank Formation 

 

"The Bolders Bank Formation characteristically consists of reddish to greyish brown, stiff diamictons that are generally massive but in places possess distinct, commonly arenaceous layering, and deformational structures. The majority of its pebbles, of which chalk is the most conspicuous component, are derived from the sedimentary rocks of eastern England. The pebble content tends to diminish eastwards. In general, the formation is less than 5 m thick, and commonly less than 1 m is preserved in deep-water areas west of the Dogger Bank, although to the east of Lincolnshire it may be 15 to 20 m thick (BGS Spurn Quaternary Geology sheet). " 

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

 

The plan above demonstrates that the Bolders Bank Formation merges into the Wee Bankie Formation, which in turn merges with the Marr Bank Formation. 

 The plan at the top of the page demonstrates that the cut for the Outer Silver Pit, at the southern edge of Dogger Bank slices into and through the Bolders Bank Formation. 

"The pebble content tends to diminish eastwards" suggests that the material of the formation is bedload for meltwater overflow around and south of Dogger Bank. 

"Stiff diamictons" may suggest that an ice sheet has rested on the formation's surface.

Jeffery Nicholls 

South Ronaldsay 

Orkney 

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

  

Tattershall

 

Glacial Prehistory 

Tattershall 

An instance of how the climate was warmed by 43,000BP, is described by Peter Kent :-

“Fluvial sand and gravel exposed in quarries near Tattershall contain evidence of a short but marked mid-Devensian climatic amelioration (Girling, 1974). An organic silt bed within involuted gravel overlying the Ipswichian peat has yielded insects indicative of a high-latitude continental tundra environment, but a slightly higher bed contains a thermophilous insect fauna—some taxa of which are now confined to southern Europe—implying summer temperatures at least as warm as those of southern England today. Radiocarbon dates from these deposits and correlatives elsewhere in England show that this warm interval, the initial phase of the Upton Warren Interstadial Complex, occurred around 43 000 years ago. It was too brief for the climatic improvement to be reflected in the flora. The overlying sand and gravel in the Tattershall area are appreciably cryoturbated and contain numerous remains of mammoth (Mammuthus primigenius), woolly rhinoceros (Coelodonta antiquatis), bison, reindeer and other tundra-favouring mammals.”

Longtown Castle

 

Glacial Prehistory 

  Longtown Castle 

 

It was 1978 when I worked at Longtown Castle, a motte and bailey medieval castle on the Welsh border in Herefordshire.

The castle is on a ridge that carries an ancient track way from Abergavenny, north to Hay- on-Wye. 

A deep valley runs parallel to the Longtown ridge, and beyond that was the Black Mountain, a high ridge, also connecting Abergavenny to Hay- on-Wye. 

The scenery, topography, and geology are difficult to ignore here, and I would sit in the site hut, sipping tea and looking out and wondering how the valley and the ridges on either side of it were formed.

Anything I might have learned at school suggested that the valley must have been formed by either a glacier sliding along it, or by water flowing along it.

Neither option looked feasible here. The valley was roughly 10 kilometres long, 1 kilometre wide, and fell 300 metres along it's length.

Looking at the valley over the rim of a cup of tea, at tea break, I could never see that the amount of rainwater falling on the valley would ever be enough to erode the several million tons of rock that was missing from the valley, not even in many millions of years.

I began to cook up an idea that the valley was actually created, not by a scraping glacier flowing along it, but by an ice sheet edge collapsing.  

This would explain most British valleys, and ribbon lakes, and the valleys that cross small islands, from sea to shining sea.

It would also explain multiples of valleys that lie parallel to each other, and that are separated by ridges like the ridge upon which Longtown Castle sits.

Gradually the idea took shape a couple of years ago, as i started to research more deeply into the recent, post-glacial geology of Britain and Norway.

I found that academia is completely locked onto the idea that mountain valleys were formed by the flow of glaciers along them.

In fact though, what is actually happening, is that ice sheet edges are collapsing, gouging out valleys as they fall, and the resulting meltwater with ice blocks, dust, and broken rocks are sliding gently down any available slope until they reach water. 

Finally, this flow of meltwater, dust, ice and rocks reach water where the ice melts out leaving a moraine at the waters edge.

The passage of the ice, dust, and rocks mixture is marked by striations in the exposed geology, being currently understood to be formed by the movement of solid glaciers rather than this looser mixture of materials. 

If my calculations are correct this melt would have supplied meltwater at a volume of over half a cubic kilometre for every day of deglaciation in that valley. This would have (did) raised sea level significantly in the brief periods when extreme temperatures and extreme melts took place.

Jeffery Nicholls 

Orkney 

Jiffynorm@yahoo.co.uk 

Impact Valleys

  

Geological Prehistory 

Impact Valleys 

 

A view of the Olchon Valley and the Black Mountain beyond. The ridge of the Black Mountain marks the Welsh border from the county of Herefordshire. 

The valleys of Britain,  particularly those in upland counties, are generally presumed, by academics and geologists to have been cut by rivers of ice or water flowing along them.

This is in spite of every indication that both schemes of valley creation are actually extremely unlikely. 

The actual erosive force that carved out these landscape features is much more likely to be by a vertical impact.

Think of a block of ice, a one metre cube, falling onto a car  from 500 metres up, at the edge of aan ice cliff face. How much damage would the ice black do to the car? Or what about falling on the roof of a house?

How much damage would the fall of a complete ice wall trimmed off the ice cliff edge do at a width of a metres, a height of 500 metres, and a length of 100 metres, if it fell onto dry ground?

Definition of a valley 

"A valley is a long depression on the Earth's surface that commonly hosts a river and lies in a relatively flat plain or between hills or mountains. Valleys can be formed through various processes, including river erosion, glacial activity, or tectonic action.

Tectonic valleys, also known as rift valleys, are formed by the subsidence of the Earth’s crust between faults. River valleys are created by the incising action of rivers. Glacial valleys, often U-shaped, are created as glaciers erode the landscape.

Valleys come in different forms, such as canyons, gorges, and rift valleys. Canyons are deep, steep-walled valleys cut by rivers through resistant rock. Gorges are narrow, deep valleys. Rift valleys are elongated troughs formed by the sinking of a segment of the Earth's crust between faults."

( Encyclopedia Britannica )

The floor of the North Sea is also composed of an undersea landscape of linear valleys which are commonly called tunnel valleys or palaeovalleys. 

Valleys forming under existing polar glaciers are apparently formed in the way described below:-

"Tunnel valleys formed when subglacial or englacial meltwaters flowing under great hydrostatic pressure, loaded with ice and rock debris, incised channels into the underlying bedrock." From "The Pennines and adjacent areas. British regional geology" by N Aitkenhead et al"

However there is no evidence that the valleys of Britain and the North Sea were created in a subglacial environment. 

There is significant evidence that these valleys, particularly on rising or high ground, were formed by the vertical impact of falling ice as described above. 

*

The Impact Valley 

The Impact Valley is the basic component of a post-glacial landscape in north-western Europe, and the "Parallel Valley Array " is an organised arrangement of impact valleys. 

These features are not yet recognised or understood by the academic community that studies glaciers and ice sheets.

They are the basic building blocks of a fundamentally novel theory for the creation of the British landscape as a result of the events in the last ice age.

 

*

Impact Valleys and Parallel Valley Arrays were created as a result of the sudden rise in temperatures that took place in periods of deglaciation at the ends of cold periods. ("Ice Ages")

Their prevalence across western Europe is due to the warm Gulf Stream waters that flowed north through the Atlantic Ocean from equatorial America. This climatic system that caused the melting, continues to maintain a "unique " climate in the northeastern Atlantic Ocean. 

Such was the speed of melt during those periods that ice sheets crumbled at speed, falling like a cliff-edge collapse, on underlying geology, gouging deep linear grooves through glacial deposits and natural bedrock alike.

 

The huge thickness (and altitude) of the ice sheet would have tended to conserve icy conditions, both in its bulk, and in its capacity to reflect the heat of sunlight. On the ground, adjacent to the ice sheet though, the suns energy would have been absorbed, creating the conditions in which the edge of the ice sheet could be undercut. The undercut edge of the ice-sheet would then fall, causing damage to the underlying substrate.

The thickness of ice over Britain and the North Sea is believed to have varied to a maximum of 1500 metres, and in some places may have been almost nothing. 

The force of impact that falling ice would have had on any ground upon which it landed, is calculated as a quantity of joules, or kilojoules, but that unit of measure is almost impossible to relate to reality.

The Snowline 

The thickness of ice over Britain during the last ice age is believed to have varied from a maximum of 1500 metres down to nothing.

Ben Nevis, in Scotland, is the highest mountain in Britain, at 1,340 metres. 

Under present day Alpine conditions:- "The snow line appears when there is a cut between the surface and the orography. Temperature sensitivity of snow cover duration, analytically derived from the state function, is extreme at the median snow line (snow probability 0.50). Alpine-wide mean altitude of the median snow line is 793(+ / −36)m in winter and 3.083(+ / −1.121)m in summer. - The snowline climate of the Alps 1961–2010" by M Hantel et al"

An ice sheet of a kilometre thickness would make the elevation of the ice above sea level over much of Scotland and northern England, over 2 kilometres. The temperature at that elevation would be seriously low, even while warm sunny weather was keeping the ground level rivers flowing.

The height of the ice sheets during an ice age would have been maintained, and increasing, until a critical temperature environment was reached forcing the undercut and collapse of leading edges of the ice sheet.

While prehistoric creatures and environments were coexisting with early humans , in the middle of the last ice age, these ice sheets would have been both gaining height, and melting on low ground simultaneously, the high ice looming over the lowland landscapes.

The Impact 

The force of impact that falling ice would have had on the ground upon which it landed, from this height, is calculated as a quantity of joules, or kilojoules.

However, to understand the potential power of impact that this event would have had, it has to be compared to one of the largest measured man-made explosions in history, that of the atomic bomb that exploded over Hiroshima.

On August 6, 1945, Hiroshima became the first city in the world to be struck by an atomic bomb. The bomb, called Little Boy, was deployed by a B-29 bomber named the Enola Gay. The bomb was airburst at 580 meters (1,900 feet) above the city, and the explosive yield was estimated to be the equivalent of 15,000 tons of TNT.

The ton of TNT is a unit of energy defined by convention to be 4.184 gigajoules, (4,184,000,000 joules) which is the approximate amount of energy released in the detonation of a metric ton (1,000, 000 grams) of TNT. In other words, for each gram of TNT exploded, 4.184 kilojoules (or 4184 joules) of energy are released.

Assuming that the ice sheet depth was commonly 1000 metres thick over northern Britain, the impact of the collapsing ice can be calculated as follows :-

The mass of a single cubic metre of ice is 1000kg, which, falling 1000 metres, will land on a surface with an impact of 9810 kilojoules

Therefore 9810 kilojoules is the kinetic energy of impact of 1 tonne falling 1 kilometre, and the equivalent is (9810 divided by 4.184) which is 2.335 kilograms of TNT

The whole 1000 metre column of ice falling on substrate, at a total weight of 1000 tonnes would cause an estimated impact energy of (2.335 x 1000 / 2), = 1,170, kilograms of TNT

That is just over 1 tonne of TNT, which is very little compared to the atomic bomb over Hiroshima, at 15,000 tons TNT, but the column of ice that falls to create that energy is just a single one metre by one metre, thousand ton, vertical spike.

A single square kilometre of collapsing ice would cause an accumulative explosion equivalent to 66 Hiroshima sized atomic bombs.

If my science here is debatable, can we agree that the damage caused by the fall of just one cubic metre of ice , one tonne weight, falling from a height of 500 metres or 1 kilometre, would cause significant damage for the surface it landed on?

The individual palaeovalleys of Parallel Valley Arrays, are likely to be the result of high summer temperatures melting an edge of the ice sheet, and causing that edge to collapse, creating deep eroded gullies where it landed.

Over cooler periods the rate of collapse slowed, and the impact of the erosion reduced, leaving raised ridges between deep valleys. 

When the planet warmed after 21,000BP, the low level ice sheets of southern England and the North Sea collapsed, retreating north. 

Where ice sheet thickness was low, the impact of the collapse was minimal, but as the edge retreated, thickness increased and valleys were cut into ground beneath it.

Where the ground consisted of glacial sedimentary dump, from a previous ice age, the cut of the valley was deeper than it would be if it landed on harder, or older, bedrock materials. 

Where the underlying substrate was rock, that rock was pulverised and became mixed with the shards of ice that had fallen on it, at the foot of the ice sheet wall. 

The Ice Stream

This ice and rock mixture, in a soup of meltwater sludge would form a huge heap where it had fallen.

There, lubricated by meltwater, this heap of ice, rocks, and silt, would slide slowly down any sloping surface that was available to it. 

When this mixture arrived at seaside or lakeside water the ice would melt out , leaving a moraine of rocks and soils.

*

The Black Mountain Parallel Valley Array 

 

The Impact Valleys (Parallel Valley Array) for the Black Mountains (above) are on the Welsh border of Herefordshire. These are a group of valleys that run more or less parallel to each other. Some, to the west, at the Brecon Beacons, run downhill from a ridge at the North of the area. At the east though the floors of the valleys are mainly flat, horizontal. 

Current thinking would probably suggest that these features were caused by the movement of glaciers along tnem, but even if the valleys were to be filled with snow there would be no impetus for the mass of ice in its frozen state to move along them. 

The views above are taken from Longtown Castle in western Herefordshire. They both look along the Olchon Valley, and across it to the high ridge of the Black Mountain, the Welsh border. Longtown castle , from which the photo is taken stands on a ridge that runs parallel to the Black Mountains. 

The Olchon river is a small stream in the bottom of the valley. The limits of the catchment area for the stream is the horizon, in the distance of the lower view.

I suggest that the depth of ice over in this area could have been 500 metres, but it may have been more.

For the purpose of this example I suggest that the area of an ice sheet over the olchon valley is 12 kilometres long and 1 kilometre wide, 12 square kilometres. 

Because of the reduced thickness (500 metres) of ice, the impact in the Olchon valley, of the collapsing ice sheet, would be half a tonne of TNT, or 33 atomic bombs per square kilometre. 

As it seems likely that the warm period that melted the ice lasted over a summer season; 12 square kilometres, 33 bombs per square kilometre--- the valley experienced an impact equivalent to an atomic bomb, along its length roughly on every day of that deglaciation year.

This melt caused to be released 16.5 x 10^9 litres of water to find its way to the sea every single day, 16,500 tonnes.

Those 16,500 tonnes of water would have raised the level of the sea around the coast of Britain, locally, by several metres. Evidence for this is in the charts produced for the British coastline by Ian Shennan. 

  

The chart above demonstrates the change in relative sea-level for coastal locations around Britain and Ireland. 

The graphs show that at around, or just after 15,000BP the level of the sea around our coasts rises dramatically by 10 metres or more. 

This rise was caused as meltwater from fast melting and collapsing ice sheets reached coastal locations and was unable to escape due to coastal topography. 

The water that arrived at the coast from the mountains of Britain carried with it ice, rocks, gravels, sands, and silts. These were Ice Streams that traversed the landscape scouring the geology as it passed.

Jeffery Nicholls 

Orkney 

#BIIS #LGM #Quaternary #Geology #glaciation #IceAge #ice-sheet 

Dorset Barrow

 

Glacial Prehistory 

Dorset Barrow 

  

"My" Dorset Barrow was on Canford Heath, close to Poole. It was on the edge of a gravel cliff that extends from Wimborne, along the south coast to St Catherines Hill, north of Christchurch.

When I, and the diggers I was working with, were excavating the barrow, I camped in my tent alongside it to make sure that the boys that hollered in the valley below during the day, did'nt trample my beatifully cut section across the barrow during the moonlit nights.

Aa I think I may have said, the barrow was a round mound right at the edge of a low cliff edge. At a distance from the mound was a gravel bank around it, and beyond that a ditch with a sharply cut base, "v" shaped.

The mound was made up of cut turves from the surrounding area that may have been stacked against a post that slotted into a post hole that was found at the centre of the barrow.

There was enough carbon in the posthole to date the barrow to 1000BC.

Someone had spent a long time in prehistory sitting on the mound and knapping flints. There was a huge spread of struck fkakes on top of the barrow, but the quality of the flint and the knapping make it seem unlikely that any useful tools were created by the effort.

Underneath the barrow though was another story.

Here, a layer, not very deep, of white sand contained a small group of blades and cores from an early period of prehistory, the upper palaeolithic. 

I had dug at the mesolithic site on Hengistbury Head. There the blades knapped from flint were tiny, and the cores from which they were struck, equally small.

Here the long skillfully struck blades were beautiful, and i later found that they date to between 40,000 and 20,000 years ago when the weather was a bit chilly.

To find a trapped ground surface underneath the barrow like this was quite extraordinary. These things are usually found in caves.

At the end of the excavation every last drop of soil had been removed from the surface at the edge of the cliff. All that was left was the circular ditch and the post hole in the centre of it.

When I left the site for the last time I had a tremendous feeling of loss, that I had destroyed something that had meant something to somebody thousands of years ago. 

It had been excavated to prevent it from being used by bikers as a launch place for off road dirt track riding.

It changed something inside me for ever.

Jeffery Nicholls

Orkney 

#BIIS #LGM #Quaternary #Geology #glaciation #IceAge #ice-sheet 

Erratic Boulder

Glacial Prehistory 

 Erratic Boulder

In Dorset Barrow I explained that Bournemouth, Christchurch, and Poole were all on or near a plateau of gravel.

This plateau that must be dated to some time in the last million years has been cut through in many places, and as a result the area is dotted with steep roads. These hilly roads are notorious. Richmond Hill in central Bournemouth, Sea Road in Boscombe, Gravel Hill in Poole. As a cyclist in the area, these hills were a bugger to cycle up, and a quick thrill to ride down.

Separating the Bournemouth plateau from St Catherine's Hill, north of Christchurch, is a wide valley joining Ringwood in the North with the south coast at Bournemouth Bay, close to Hengistbury Head. 

The River Avon wandered through this wide valley, and in the 1960s the powers-that-be decided to build a dual carriageway along it, linking Bournemouth to Ringwood. 

The whole route was farmland, and when the machines started to strip the soil to lay the footings of the new road, I and other members of the Archaeology group I belonged to were there, watching the ground they were cutting into, hoping to find stuff, anything, but no, it was not to be.

I had rather thought that the valley had been cut through by the River Avon meandering across it, but I was wrong. Had that been the case the earth would have been disturbed, perhaps brown or muddy, or so I thought. 

In fact the whole floor of the valley, flat as it was, consisted of bright yellow sand.

Watching the huge earth moving mechanical beasts rolling along valley floor rather dashed our hopes of finding anything at all.

There was just the one thing though. That one thing was a huge limestone boulder just buried in the yellow stuff. When the Archaeology group found this there was a lot of tooth sucking and it was generally agreed that this must have been a glacial erratic. 

It was a big lump of rock, and must have been a metre diameter, white stone, knobbly, probably limestone. 

I didn't think much about the ins and outs of the valley of yellow sand and the erratic in it at the time. I just stored it up to revive at a later date, if needed.

As far I understand it the nearest likely source for the erratic would be at or near the Cotswold hills which is considerable distance for a rock to be rolled. Fifty, a hundred miles? While the Bournemouth plateau was made up of gravel, this yellow stuff was not therefore local, so both sand and boulder both had to have come come from elsewhere. I couldn't see the boulder and sand arriving in a glacier. It had to come by melt water, and there had to be a lot of it!

As for the boulder, that was lifted by a charismatic member of the Archaeology group. He hired a crane and a flatbed truck, and removed it to become a feature in his own garden.

I have no doubt that it's still there!

Jeffery Nicholls 

Orkney 

#BIIS #LGM #Quaternary #Geology #glaciation #IceAge #ice-sheet 

Levallois Flake

Glacial Prehistory 

 Levallois Flake 

 

Drawing by E Cuypers

After my second season at Grimes Graves, in 1974, the director G.de G Sieveking asked for volunteers to work a small excavation he had planned in North London.

The place was in Acton, at a school, the Haberdashers Aske School. The property was changing hands and would become the Japanese School in the new term. 

Mr Sieveking had been given permission and support to excavate part of the grounds of the school as it was known that a knapping floor was present in the area yielding an unusual style of knapping technique, the Levallois technique. 

The flints that we were hoping to find were large and sort of triangular.

The site of our excavation was just outside the window into the head teacher's office, and as we had not been told about the circumstances of the change of hands of the property it felt slightly sacriligeous to be cutting into turf that we as children, not that long ago, would have been forbidden to walk on.

The turf was tidily cut and carefully stacked for re-laying. When that was done we were told that the finds that we could expect to find would be in the material immediately beneath the turves, 

That material was called "loess", which, we were told was Dutch for loose.

The material that we were digging through was not loose, it was the hardest soil that any of us had had the misfortune to be required to excavate. It was as hard as solid chalk, but even then chalk would have been much easier to work than this. There were a few mattocks on site, but not many, so for the four weeks of the dig we stabbed the ground with our trowels, accepting the punishment that the work did to our hands, wrists, and arms.

One of the only people equipped with a mattock was Phil Harding, and he used it to good effect, finding the only levallois flake of the entire excavation. 

Unfortunately his find received a direct hit from the matlock, breaking into a lot of shard-like pieces.

At the time it was not lost on me that while we were digging a material called "loose", what we were actually working was anything but loose.

It was not though until recently that, in researching geology I discovered the term "overconsolidated" which is material that has been subjected to a huge mass laid over it. Indeed the mass was so great that all the molecules of air and water that had been in the material when it was laid down were compressed out of it.

The actual material here was chalky white, and was probably of the same material as that I found under the Dorset Barrow. 

While the material under the Dorset Barrow was fluffy and loose, and indeed may have been wind blown, the stuff in Acton was nearly as hard as concrete, which I take to mean that at some stage in the last million years the ice sheets from at least one ice age were laid over parts of London. 

The weight of hundreds of metres of ice may well have been enough to overconsolidate the wind blown stuff.

Piece by piece, I begin to map out where the ice sheets over Britain were, and where they were not, or where they weren't very thick anyway. 

This in an effort to better understand what happened in the ice ages, and how they shaped Britain. 

Jeffery Nicholls 

Orkney 

#BIIS #LGM #Quaternary #Geology #glaciation #IceAge #ice-sheet