Ice-sheet Britain
Series Title:- The Orkney Riddle
28/29
Blog Title:- Ice-sheet Britain
This is an attempt to contribute to a better understanding of how the topography of Britain was created by the presence of thick ice-sheets on its surface through multiple cold periods, over many millions of years.
Summary
We, in northern Europe, live in a post-glacial world.
Every thing that we see in the geology of the British Isles, all of it with rare exceptions, is the product of geological events that occurred at the end of the last ice age.
The glacial periods, as they occurred over the last 150,000 years were short bouts of heavy snow which was allowed to build on the mountains of Britain and Norway.
Year on year thick snow built up on the mountains.
The height to which these ice sheets formed is said to be 1500 metres. This is a theoretical assessment and may or may not be accurate, but the thickness of ice was certainly hundreds of metres.
This sheet of ice, as it perched on mountain Britain was static.
It did not move, but remained, perched on the mountains of Britain and Norway.
The entire period of the ice age from 150,000 years ago the gulf stream flowing up the Atlantic Ocean brought the weather systems that still batter northern Europe.
The airflow from the gulf stream maintained lowland Britain as free from thick ice. It was cold, but there was a snow line , an altitude below which snow laying was seasonal, not permanent.
This snow line was very low in a period between 120,000 and 70,000 years ago but at 70,000 years ago it rose, melting the ice sheets at the foothills of the mountains.
The snow line remained low from then until 30,000 years ago, or later, the mountain top ice sheets remained in place, building more height.
After 30,000 years ago the snow line fell, so that all of Britain was coated by snow of varying thickness, from almost nothing to hundreds of metres.
After 21,000 years ago the snow line rose quickly in at least two superheated episodes.
This caused the mountain ice sheets to collapse fast, and pound the underlying geology to dust, rubble, and slush.
This action caused the creation of valleys, some of which were blocked at both ends, forming ribbon lakes. Other valleys enabled huge ice streams, rivers of ice, sludge, and rocks to drain out to coastal areas. These ice streams rolled across wide open spaces, metres thick, leaving drumlins and moraines at waters edge. Where the ice stream became entrapped in northern valleys their melting was delayed into more recent millenia, when the water melted out of the "glacier" leaving hummocky moraines on the valley floor.
All these features represent the events that occurred as a result of the ice sheet over Britain melting and breaking up. Very little of the country was completely unaffected by the deglaciation. Huge amounts of water, sediments and rocks were distributed across lowland Britain, temporarily raising coastal sea levels.
Prehistoric people were roaming this cold landscape from at least 43,000 years ago. We call them "cavemen" simply because that is where we find their bones, in caves. The world in which these people lived has been completely lost, mostly washed away by the meltwater runoff from collapsing ice-sheets.
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The map above is a topographic rendering of the land surface of Britain. It demonstrates that the group of islands that make up the British Isles are adjacent to the Atlantic Ocean, at their northwestern approach.
Significant features in the topography are the mountains of Scotland, Northern England, Wales, Cornwall, and Ireland. These mountains are sliced by valleys, and some locations are defined by these features. For instance, the "The South Wales Valleys", "The Lake District", the "Glens" of Scotland and the Yorkshire Dales.
In contrast , in the south and east of southern England are the fens of east Anglia, the Wolds of Lincolnshire, the Downs of Kent and Sussex, the heaths of Dorset, the plains of Wiltshire, the rolling hills of England.
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The geological map of Britain, above, indicates that those same locations where major valleys prevail are the places where the older rocks surface, rising up to form the major mountain ranges.
Not to say there are no valleys in southern England, but those that are there do not define the landscape.
The paradox here is that while the geology of northern England and Scotland consists of older, harder rocks, the valleys cut into them are deeper and more impressive than any in in the south where the geology is by several degrees much softer.
Academia would suggest that the lack of deep valleys in the south was due to the absence of glaciers there, but interpretation of more recent research implies a broader picture.
The Encyclopedia Britannica defines the valley in the following way:-
"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."
While valleys are a characteristic feature of the British landscape, similar features are also present in coastal areas around the archipelago. These come with various titles such as tunnel valleys and palaeovalleys.
One definition of these features is given by N Aitkenhead et al, in "The Pennines and adjacent areas. British regional geology" as follows:- "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."
Beside this definition, there are others, and the scientific/academic community generally ascribe the formation of valleys to ice movement laterally along their extents, or to the effect of "ice-pushing" to deform the geology, but there are various good reasons to be sceptical about the truth of these theories.
Assigning tunnel valleys at sea an alternative , sub-glacial derivation is also problematic, so presented here is a different approach which demonstrates how both valleys on land, and tunnel valleys at sea were formed in a way not previously considered; by the vertical collapse of the edge of an ice sheet, the Impact Valley.
In this view the Impact Valley is the basic component of a post-glacial landscape in north-western Europe, and possibly elsewhere.
Unfortunately, the theory finds little agreement with current universally agreed ice age history, and I cannot draw upon the opinion, experience, or clout of any academic specialist in the field, just data!.
The subject is huge, drawing data from multiple disciplines, including geology, geography, dynamics, sea-level data, channel erosion, soil mechanics, water flow dynamics, meteorology, and archaeology; none of which am I an expert in!
A basic tenet of this theory is that laminated ice-sheets did not move, slide, or flow across any surfaces. Solid glacial ice formations in valleys did not move laterally along those valleys. The flowmarks and scratches which are common on northern English and Scottish higher grounds were caused by meltwater and sludge ice streams flowing away from upland ice-sheets. These would have contained sand, silt, gravel, broken rocks, and erratics. They may also have contained ice blocks, and they may have moved down slopes at speed, or very slowly.
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Impact Valleys 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.
As far as i understand it, the Atlantic Ocean is the only deep area of oceanic water that connects the southern oceans that circle the planet, and heated equatorial waters, with the northern hemisphere.
Such was the speed of melt in deglaciation periods that ice sheets crumbled at speed, falling like a cliff-edge collapse, on underlying geology, gouging deep linear grooves through pre-existing glacial deposits and natural bedrock alike.
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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.
In our time, the power of explosive force is usually stated as a comparison with the yield of a ton of a commonly used explosive, 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 916kg, which, falling 1000 metres, will land on a surface with an impact of:- mass x acceleration due to gravity x distance fallen = 916kg x 9.81m/s/s x 1000m = 8986 kilojoules
Therefore 8986 kilojoules is the kinetic energy of impact of 1 cubic metre of ice falling 1 kilometre, and the equivalent in terms of the explosive power of TNT is:- (8986 divided by 4184) which is 2.147 kilograms of TNT
The whole 1000 metre column of ice falling on substrate, at a total weight of 916 tonnes would cause an estimated impact energy of (2.147 x 1000 / 2), = 1,073, kilograms of TNT [kilogram TNT equivalent, times total height, divided by 2, for the average height of the falling ice]
That is just over 1 tonne of TNT impact for just a single one metre by one metre, thousand ton, vertical spike of ice.
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It is probably important to understand what happened in the "Last Ice Age", as a simplified timeline.
Scientists define this period as starting at around 120,000BP and finishing close to 12,000BP. Between those two dates there built up and remained an ice sheet on the mountains of Scotland, Northern England, Northern Ireland, and Wales. While there was a dramatic period of deglaciation between 70,000BP and 60,000BP, those icecaps remained in place, and from 60,000BP to after 30,000BP much of lowland Britain and Ireland was home to cold-loving species, but also Upper Paleolithic people. The land that these creatures occupied was dominated by the effects of meltwater streams that continued to surge throughout the period, from upland ice-sheets.
Sometime after 30,000BP, in a very brief freeze, the whole of the British landscape, including the North Sea was blanketed with an ice sheet of varying thickness. In the south-west of England, there was little or no ice, and elsewhere, away from the hills, the thickness was probably up to a couple of hundred metres. There was no sea-ice, and while the Norwegian Channel was free of ice , a lagoon lying along the east coast of Britain, was divided from it by a raised ridge of higher ice-covered ground that connected Dogger Bank/Doggerland to the Atlantic Ocean close to Shetland.
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Impact Valleys are likely to have been caused, in periods of deglaciation, as a 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, winter?, 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 it may have simply melted, or the impact of the collapse may have been minimal. As the edge retreated though, towards higher ground, 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.
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. This is, of course an oversimplification of the way that features were created, but it gives a rough idea of the processes involved.
SITE UNDER REVIEW
Case Study [1] - The English Channel
Reference 1:- Denudation of the continental shelf between Britain and France at the glacial–interglacial timescale by Claire L. Mellett, David M. Hodgson, Andrew J. Plater, Barbara Mauz, and Ian Selby.
"The continental shelf between Britain and France has received attention in the literature for many years because of its potential to preserve a record of the timing and mechanism that led to the isolation of Britain, as a geographical island, from the European continent during the mid-quaternary. Despite ongoing debates regarding the timing of erosion the most recent evidence supports a Marine Isotope Stage (MIS) 12 age, at least for the initial breach, with an English Channel–North Sea marine connection during highstand, at some point between MIS 12 to MIS 6. The resulting palaeogeographical configuration of Britain and Northwest Europe has implications for the migration of flora and fauna including hominids throughout the Pleistocene."
"Additionally, reorganisation of drainage basins and funnelling of freshwater discharge through the English Channel during cold stages as a consequence of breaching of the Straits of Dover, may have contributed to destabilisation of the Atlantic thermohaline circulation. A number of mechanisms have been proposed to explain breaching at the Straits of Dover including gradual erosion as a result of fluvial downcutting and catastrophic flooding . The bedrock morphology of the continental shelf in the eastern English Channel, particularly the erosional bedrock bedforms preserved in the Northern Palaeovalley, have been interpreted as the product of high magnitude flows linked to erosion at the Straits of Dover by megaflood events. However, this interpretation was based on bathymetric data of the sea bed only, and did not consider the sedimentary record preserved in the subsurface stratigraphy. Distinguishing catastrophic events from ‘normal’ sedimentary processes requires an understanding of how fluvial and marine processes, over multiple sea-level cycles, interact to create the morphological and sedimentary history of the continental shelf."
Figures above, "Seismic reflection profile and interpreted panels illustrating seismic facies character and association for deposits overlying surface T3 and ME. (a) (above) Bathymetry in the eastern English Channel showing the morphology of the SB Palaeovalley at its confluent with the Northern Palaeovalley (Bathymetry © British Crown and SeaZone Solutions Limited. All rights reserved. Product license No. 112010.009). ..... Locations of cross-profiles G–G' and H–H' are shown as white lines. (b) Schematic representation showing the stratigraphy and depositional context of sediments preserved within the SB Palaeovalley adapted from Wessex Archaeology (2008). Sedimentary facies are provided in italics. ......."
"One of the most striking features of the channel belt deposits is variations in colour within sedimentary facies Gm. The gravels are clearly heavily weathered and show evidence, based on their colour, of different degrees of secondary iron oxide development (Hurst, 1977). This would suggest that subsequent to deposition, channel belt sediments were sub-aerially exposed for a sufficient period to develop a red to dark brown coloured soil prior to erosion by wave ravinement."
The depth of the seabed at section G-G' is in the region of 50m to 60 metres, and the base of paleovalley SB is around 70 metres. Sea-level flooded this palaeovalley to the seabed surface either side of it, in around 10,000BP.
Sediment layers in the section drawing are described and dated here:-
(Gm) Very poorly sorted matrix supported sandy gravel. Gravel typically comprises sub-angular to sub-rounded flint clasts. Matrix is medium to coarse sand. Fluvial. Date: 21,200BP
(Sfp) Fine to medium sand with frequent laminations of silty clay. Laminations are generally fine but can be up to 1 cm in thickness. Clay is occasionally organic rich. Alluvial or tidally influenced. Date: 15,100BP.
(Sh) Poorly sorted slightly gravelly fine to medium sand with frequent outsized gravel clasts. Gravel component is sub-angular to sub-rounded, fine to medium size and of various lithologies. Frequent shell fragments throughout. Shallow marine tocoastal (nearshore). Date: 14,200BP to 11,900BP
(Smw) Generally well sorted occasionally moderately sorted slightly gravelly medium to coarse calcareous sand. Shells, both whole and fragmented are frequent. Occasional organic mottles and inclusions of granule size coal. Coastal–shallow marine". Date: 9,000BP
In her conclusion Claire Mellett writes: "Evidence is preserved to support major erosion during MIS 6. However, sedimentary processes during this time alone cannot account for the morphology of the Northern Palaeovalley as it was at least partly re-sculpted by fluvial processes during MIS 2. Events of catastrophic magnitude are not necessarily responsible for formation of the palaeovalley network preserved in the eastern English Channel and it is much more likely that the landforms were created by processes of non-catastrophic magnitude operating over long (104 yr) periods of time."
Significantly, she recorded a date for layer in the floor of the gulley. The gulley, which I would interpret as an impact valley was dated to 21, 200BP. This feature was just one of a myriad of cuts and grooves in the floor of the English Channel, all of which are connected through the Dover Strait by a deep Channel called the Lobourg Channel.
This passage of deep water is, in turn connected to a group of subsea ridges and gullies called the Norfolk banks.
Reference 2: Geology of the southern North Sea. United Kingdom offshore regional report By T D J Cameron,
"Norfolk Banks
The Norfolk Banks rest on a relatively flat surface at 20 to 30 m depth (Figure 121). They consist of accumulations of sand on an erosional surface of Pleistocene deposits that in the interbank areas is either exposed or covered by a thin lag gravel. The largest of the Norfolk Banks, Well Bank, is over 50 km long, 1.7 km wide and rises to 38 m above the adjacent sea floor, although some banks are even higher at over 42 m (Gaston, 1972).
The Norfolk Banks can be subdivided into a group of more nearshore, parabolic banks connected by low cols to form a zig-zag pattern; and an outer group of more linear banks. The innermost banks generally have sand waves on their flanks, the outermost banks tend to have a smooth profile. The parabolic form exhibited by the nearshore banks is believed by Gaston (1972) to be a stage in bank development where a single bank may eventually split to form further banks. These banks are more active than the offshore linear banks, perhaps in part due to the greater tidal-current velocities closer to the coast. Many of the Norfolk Banks probably originated at around 7800 years BP (Jelgersma, 1979), although it is likely that the nearshore banks are more recent.
Net sand transport is in opposite directions on opposite sides of banks, as evidenced by the direction of asymmetry of sand waves on their flanks. Nevertheless, there is a dominant north-westerly transport direction (Gaston and Stride, 1970). The linear banks are generally asymmetrical towards the north-east, with the steeper slopes up to 7°, and more gentle south-westerly slopes (Figure 122). Internal seismic reflectors within some of the banks dip to the north-east (Houbolt, 1968) (Figure 122), indicating a migration in that direction which may, or may not, be continuing at present. These internal reflectors probably represent boundaries between cross-bedded sets of sediments (e.g. McCave and Langhorne, 1982).
By studying hydrographic charts dated between 1851 and 1967, Gaston (1972) found that many of the banks have elongated towards the north-west, the direction of net sand transport. Gaston (1972) also found that during the same period, some of the more nearshore banks, such as Haisborough Sand, the Hewett Ridges and Smith's Knoll, had correspondingly migrated hundreds of metres to the north-east. The central part of Ower Bank had apparently migrated over 700 m to the south-west. However, after a similar study of hydrographic charts published between 1886 and 1950, McCave and Langhorne (1982) could detect no movement of Haisborough Sand to the north-east. The evidence for modern movement of these banks perpendicular to their long axes must therefore be regarded as equivocal; the evidence for migration in the direction of their long axes is better.
The banks to the east of Great Yarmouth (Figure 123) have shown measurable displacement almost entirely parallel to the coastline in the direction of their long axes. Since 1866, South Cross Sand may have moved several hundred metres, and the Scroby Sands over 1 km, to the north (Craig-Smith, 1972). South Cross Sand shows dipping internal seismic reflectors which reflect this migration (Figure 124). On the other hand, Lorton Sand has moved over 1 km to the south during the same period. These nearshore banks are separated by channels which have been scoured to a level beneath the base of the bank sediments; Scroby Sands is effectively isolated on an elongate high. Similar highs are seen beneath some of the Thames Estuary banks, where D'Olier (1981) believed that such features predate bank formation. In the case of Scroby Sands, it is likely that at least some of the scouring is modern, and similar hollows interpreted as due to tidal scour occur adjacent to more linear Norfolk Banks (Donovan, 1973). These scoured deeps may serve to fix the banks on to the topographic highs, restricting lateral migration. Evidence from tidal-current data suggests that the Norfolk Banks formed at a greater angle to the prevailing tidal currents than they exhibit at present (Howarth and Huthnance, 1984), which may indicate that to some extent they are already moribund.
If the offshore migration of the banks is continuing at present, a continual supply of sand from the nearshore zone is required if the nearshore banks are not to decrease in size. This supply could come from rapid erosion of the East Anglian coast. It is unlikely that coastal erosion during the Holocene could have provided sufficient sediment to form the whole Norfolk Banks group. Clayton (1989) estimated coastal recession on the Norfolk coast to have been about 4.5 km during the last 5000 years, yielding about 2 x 109 m3 of sand. The volume of Well Bank alone has been estimated to be 2.4 x 109 m3 (Houbolt, 1968).
The banks consist of fine- to medium-grained sands which show a high degree of sorting. On Haisborough Sand, McCave and Langhorne (1982) found the sand to become finer grained across the bank from south-west to north-east, with the best sorting towards the bank crest. Shell content in the Norfolk Banks is low, less than 5 per cent on Well Bank (Houbolt, 1968). Very little is known of the internal composition of the Norfolk Banks, or indeed of any tidal sand ridge. Houbolt (1968) found no vertical gradation of grain size in a borehole through Ower Bank. A borehole through a moribund sandbank north-west of the Dogger Bank, just north of the report area, revealed only subtle vertical trends of grain size (Davis and Balson, 1992). Short cores taken on the steep flank of Well Bank indicate extensive bioturbation to depths of 55 to 60 cm from the surface, probably due to the burrowing echinoid Echinocardium cordatum (Pennant), although large populations of sand eels, which also burrow, are found on the same bank (Wilson, 1982)."
Reference 3: "Global sea-level rise in the early Holocene revealed from North Sea peats" by Marc P. Hijma, Sarah L. Bradley, Kim M. Cohen, et al
Hijma's sea level curve, for the southern north sea give a date at which sea level would have been 30 metres below present sea level of 9,000BP. As the base of the corridor that joins England to Holland is 30 metres below present sea level it follows that the separation of the two land masses occurred at that date 9000BP, ot 7000BC.
This series of measurements may be more accurate than those sources around Britain's coast as it may be less influenced by local meltwater dumping which would have maintained a higher measure of sea-level around Britain, than those on the European coast.
Reference 4: Geomorphology of the Axial Channel (Southern Bight, North Sea) by Morgan Vervoort, etc al
"The Holocene sea-level rise
As the Weichselian glaciers began their retreat, sea level began to rise from a eustatic low of around 120 m below present-day levels during the glacial maximum (Fairbanks, 1989). Regional sea levels were approximately 65 m below present at the beginning of the Holocene, 10 000 years ago (Figure 114), when the southern coast of the North Sea had probably only just encroached (Figure 115) into the report area (Jelgersma, 1979). Much of the area north of a line drawn approximately from The Wash to the southern edge of the Dogger Bank had been covered by lodgement tills or glacial outwash sands and gravels left behind during the retreat of the Weichselian ice sheets (Balson and Jeffrey, 1991). To the south, periglacial aeolian sands had been deposited. In late Weichselian to early Holocene times, fluviatile sands may have been deposited by rivers flowing across this land surface from eastern England or the European mainland.
Reconstructions of early Holocene sea levels in the southern North Sea are based mainly on radiocarbon dating of former coastal peat beds. Almost all of the data has come from peats now on land or in estuaries. In eastern England, peats have been dated from the Humber Estuary (Gaunt and Tooley, 1974), the fens surrounding The Wash (Shennan, 1986), the north Norfolk Coast (Funnell and Pearson, 1989), the Norfolk Broads (Shennan, 1987) and the Thames Estuary (Devoy, 1979; Greensmith and Tucker, 1971; 1973). Only a few peat samples from the offshore area have been dated (Godwin, 1960; Kirby and Oele, 1975), and because the North Sea was still some distance from its present coastline at the beginning of the Holocene, the earliest Holocene record is relatively poorly known. Jelgersma (1961; 1979) used data from Holocene deposits in The Netherlands and offshore to construct a Holocene sea-level curve for the North Sea (Figure 114). She used this curve, with known present-day bathymetry, to construct palaeocoastlines for various times throughout the Holocene (Figure 115).
Shells of molluscs typical of tidal flats are found at sites throughout the southern North Sea (Veenstra, 1965). Radiocarbon and stable-isotope analyses of such shells from sea-bed samples have been used to reconstruct the early Holocene palaeoenvironments (Eisma et al., 1981). The oldest radiocarbon ages found by Eisma et al. (1981) were between 9370 and 8260 BP in the Deep Water Channel area (Figure 115), which was initially flooded by the sea transgressing through the Dover Strait. Salinities at this time were brackish, around 13 parts per thousand. Younger samples indicate that the remainder of the southern North Sea became brackish between about 8000 and 7000 years BP, followed by fully marine conditions everywhere except in the Thames Estuary. Early Holocene mean water temperatures were around 10°C compared with 13.5° to 14°C at present. The flooding of the southern North Sea coincided with a sharp increase in average air temperatures in central England between 10 000 and 7000 years BP (Eisma et al., 1981).
In the early stages of the transgression, the Southern Bight contained a quiet, shallow sea of low tidal range. Once linkage between the English Channel and the North Sea was established (around 8300 years BP, Figure 115), there was probably a rapid transition to strong tidal currents and a large tidal range. Mathematical modelling suggests that this transition occurred when sea levels were approximately 10 to 15 m below present (Austin, 1991). This increase in tidal currents may have led to significant erosion of early Holocene tidal-flat sediments (Stride, 1989).
A simple picture of sea-level rise after the melting of Weichselian ice sheets is complicated by the effects of regional uplift and subsidence. In Scotland and northern England, isostatic uplift was responsible for the preservation of Holocene raised beaches, whereas in the southern North Sea region there has been regional subsidence of up to 2 mm/year during the last 4000 years (Shennan, 1989). Furthermore, detailed studies of sea-level change over the last 5000 years show a number of small oscillations (Devoy, 1979) rather than the smooth progressive rise shown in (Figure 114); these may be linked to climatic fluctuations. For instance, palaeoecological and sedimentary changes at many coastal sites show the influence of a possibly worsening climate, and greater freshwater throughput in estuaries, after 3000 years BP (Devoy, 1982).
Sediment input from rivers may have varied through the Holocene, reducing as vegetation cover increased. More recently, man's activities have had a significant effect on Holocene sedimentation through tidal-flat and saltmarsh reclamation, coastal engineering projects and flood defences, farming practices, and offshore aggregate exploitation."
Reference 5: Loess and coversands of northern France and southern England by PIERRE ANTOINE, JOHN CATT, JEAN-PIERRE LAUTRIDOU´ and JEAN SOMME
The map above indicates that regions on both sides of the southern north Sea had either loess, or coversands laid upon them , probably before the LGM, 21000BP. It is likely that these materials will have laid upon the substrate that now links Holland with England. How thick these materials were would tend to define when they were washed away.
ABSTRACT: Loess and coversands are widespread on either side of the English Channel. In southern England, loess is generally thin and discontinuous, but locally reaches thickness of about 4 m in east Kent. Coversands occur mainly in areas well north of the Channel, such as parts of East Anglia and the southern side of the Bristol Channel. Most of the loess and coversand deposits in southern England date from the Late Devensian cold stage (marine oxygen isotope stage (MOIS) 2), but there are also a few localised patches of older (mainly MOIS 6 and 12) loess, which are the dissected remnants of originally more extensive covers. In France loess is widespread and locally very thick (up to 12 m), especially in northern and northeastern France. In Normandy and in the Somme basin some long records indicate that accumulation began at the end of the Lower Pleistocene, at about 900 ka. Nevertheless the main accumulations of typical calcareous loess are related to the Upper Weichselian and Upper Saalian stages (MOIS 2 and end of MOIS 3 and MOIS 6). Indeed, before about 160–170 ka, loess deposition was generally restricted to special sediment traps such as fluvial and marine terraces exposed to the east or northeast. These older deposits are characterised by more sandy facies derived from local fluvial sources (mainly by northwest to north-northwest winds). Coversands occur locally along the coast of the Mont Saint-Michel bay, and over the northern part of the Seine estuary near Le Havre. The main differences in thickness and extent of the loess deposits between southern England and northern France are linked to their location in relation to the source areas (Channel and North Sea), to ice sheets, and to the main wind directions (northwest to north-northwest in France and western England but northeast in eastern England). Copyright 2003 John Wiley & Sons, Ltd.
Reference 6: What is a Bed Parting? by P. Harris.
Case Study [2] - The North Minch
Evidence of the deglaciation of Britain can be found in at least two forms. The first of these is the formation of the valleys described above from which huge volumes of rock, sand and soil were excavated.
The second land and seascape feature that was created in the process of deglaciation was the moraine. These features are currently attributed to ice-sheet movement, but in fact these are the deposits of ice-streams that have been broken out of mountain-top ice-sheets.
After the ice-sheet edge collapsed, the resulting stack of melting ice and broken rocks would drain off downhill making its way to the sea. As it crossed bare rocks the stones within it would leave scratch marks on surfaces, and when the whole stack reached seawater the ice would melt, leaving sands, silts, and rocks at a linear tide line.
On land , where an ice stream was unable to reach seawater it remained at the lowest point it could find, melting slowly and leaving hummocky moraine.
The Minches Channel between the Outer Hebrides and Scotland was a coastal region to which ice-streams drained. The ice-streams melted at the seashore , leaving successive moraines as sea-levels rose.
The channel that separates the Outer Hebrides from Scotland is the Minches. This is an impact valley that has been formed over millions of years. Its' proximity to the Atlantic shelf edge caused continuous snow cover in cool periods, and dramatic edge collapse of ice-sheets when climate warmed. The channel is parallel to the shelf edge, and there is a shallower channel between the islands and the shelf edge.
Skye is a volcanic formation, and it is therefore made of tougher stuff that resisted impacts.
Here we have rising sea levels sorting debris from ice streams, where they meet in the Minch channel. As sea-level continues to rise, water reaches the base of the actual solid ice sheet where the ice-sheet edge collapses.
A very thorough survey has been made of the seabed between Scotland, the Outer Hebrides and the continental shelf edge by Tom Bradwell, and his team, in "Pattern, style and timing of British–Irish Ice Sheet advance and retreat over the last 45 000 years: evidence from NW Scotland and the adjacent continental shelf"
"Figure 1. Location map showing study area in NW Scotland and the adjacent continental shelf. Places referred to in the text are labelled; terrestrial locations in roman font; hydrographic features in italic font. Inset box shows location in NW Europe. Red dashed line defines study area, referred to as Britice‐Chrono Transect 8 (or T8). Isobaths at 50, 100, 130 (=MIS 2 eustatic sea‐level minimum; not GIA corrected) and 200 m vertical intervals on shelf; with 100‐m isobaths beyond shelf. Bathymetry data from British Geological Survey and GEBCO 2014 sources. Topographic data from NERC Earth Observation Data Centre. Key pre‐existing age assessments (onshore and offshore) also shown, taken from Britice database (Hughes et al., 2011); colour coding relates to quality assurance (green = robust; amber = acceptable; red = unreliable) after Small et al. (2017). All published TCN surface‐exposure ages are re‐calculated using Lm scaling (CRONUS‐Earth calculator; Balco et al., 2008) and Loch Lomond Production rate (LLPR; Fabel et al., 2012). An erosion rate of 1 mm ka−1 is assumed. All ages are presented in calendar kiloyears (ka) before present. [Color figure can be viewed at wileyonlinelibrary.com]."
Figure 3. Mapped seabed moraines and grounding‐zone features within the study area (modified from Bradwell et al., 2019). Red boxes denote locations of other Figures. [Color figure can be viewed at wileyonlinelibrary.com]
The figure shows the location of a section line (Figure 18) that cuts through the Grounding zone wedges, 6, 7, 8, and 9.
Figure 18. North Minch (west trough) Quaternary geology stratigraphic interpretation based on SBP data (upper panel), and simplified lithologs ofcores 031‐034PC with facies interpretations based on X‐ray radiographs (lower panel). Figure modified from Bradwell et al. (2019).
Figure 18, (above) shows the section across the grounding zone wedges, 6, 7, 8, and 9.
Grounding zone wedges are generally understood to be leading edges of glaciers flowing off from ice sheets. The linear deposits shown above, as moraines and GZW’s, are thought to represent the locations where the glacier meets the local sea level, depositing its contents as the sea melts the leading edge.
In reality though the offshore water here is, and was, warm-ish and stormy. Storms are violent, and would have inhibited the maintenence of coastal ice-sheets.
Figure 26. Summary palaeoglaciological reconstruction of ice sheet and ice cap deglaciation in the NW sector of BIIS, centred on 58°30′W, 59°N. Palaeo ice margins (brown lines) based on all available geomorphological/geological evidence and Bayesian‐age‐modelled chronology. Solid lines where glacio‐geomorphological evidence is strong; dashed where connections are uncertain. Grey lines indicate pre‐MIS 2–3 palaeo ice margins on outer continental shelf, north of North Rona. Numbered ice‐sheet margins denote ages in calendar ka BP. Bold font, firmly dated; roman font, less firmly dated but ‘in sequence’. Map area to the NE is extended beyond study area to show optimal connectivity with adjacent Britice‐Chrono Transect (i.e. Shetland sector, T1; Bradwell et al., this issue). Base map is EMODnet 2018 data (present‐day bathymetry, not GIA corrected). DTM lit from NE/045. MTBH = mid‐trough bedrock high. Note: linear features in NW of base‐map image are survey artefacts. [Color figure can be viewed at wileyonlinelibrary.com].
In this plan-view the contours suggest the locations of sea-level shorelines through MIS 3 to MIS 2.
The moraines, or GZWs, that have been defined above, in "Figure 3", are the shoreline positions where meltwater ice-streams from collapsing ice sheets met with sea-level as it rose.
The meltwater travelled at speeds dictated by local topography, sometimes pooling, and sometimes fast. In quantity it carried several million cubic metres of material away from the static mountain ice sheets per day.
The meltwater ice-streams contained an estimated 10% of sands, silts, and stones of varied sizes, and 90% water and ice blocks. Where it arrived at coastal waters, the ice melted out, leaving the moraine material.
A better example of the dumping of moraines on the Atlantic coast is demonstrated by this rendition of seabed topography out of Donegal Bay.
Figure 19. Bathymetry of North Minch, east of the mid‐trough bedrock high. Besides the GZWs in the main trough, a number of moraine sequences are mapped 5–15 km offshore NW mainland Scotland relating to subsequent ice‐sheet oscillations. Core sites 012VC‐19VC are labelled. Thin grey line is geophysical data acquisition track (ship's track). Thick bue line is SBP line shown in Figure 20. Locations of Figures 21 and 22 also shown. TCN ages (squares) and OSL age (circle) from terrestrial sites are labelled (in white): roman font (this study); italic font (previously published ages; recalculated from Bradwell et al., 2008; Ballantyne et al., 2009). Abbreviations: ERM = Eddrachillis Ridge Moraine; GPCM = Greenstone Point Moraine Complex; RCM = Rubha Coigeach Moraines; AGFM = outer limit of Assynt Glacigenic Formation Moraines; WRR = Wester Ross Readvance; LE = Loch Ewe; GB = Gruinard Bay; LLB = Little Loch Broom; GZW= Grounding zone wedge. [Color figure can be viewed at wileyonlinelibrary.com].
Local sea level reached 50 metres below present day levels in around 17,000BP, and started to encroach upon the ice-sheet over Assynt, causing the edge of the ice sheet there to collapse.
The collapse of the ice-sheet edge caused linear impact valleys to be cut parallel to the 50 metre contour, (Figure 19, above) and between that line and the existing coast.
Figure 20. Seismo‐stratigraphy of Eddrachillis Ridge (part of East Minch Readvance complex) and surrounding Quaternary Formations in the eastern North Minch (interpreted from BGS sparker lines), with locations of JC123 core sites (modified from Bradwell and Stoker, 2015a). [Color figure can be viewed at wileyonlinelibrary.com].
In Figure 19 the section line for Figure 20 is marked, crossing the ridge in the seabed, north of Assynt. The ridges between the Eddrachillis Ridge, and land are described here as recessional moraines, but these would be impact valleys receding towards the mainland.
West of the Eddrachillis Ridge, where the meltwater sludge continued to flow out of the heart of Scotland until 20,000BP, is marked by "ploughmarks or MSGL", (Mega-scale-glacial-lineations). These lineations were caused by ice blocks and rocks carried along in the stream scouring the channel floor.
Case Study 3:- West Herefordshire
Below is a topographic map of a terrestrial group of valleys, showing their clear likely formation as a repeated annual event. This is a group of parallel valleys in Herefordshire, on the English border with Wales.
Taking just one of these valleys , as an example I estimate that the depth of ice over one of them 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 valley 12 kilometres long and 1 kilometre wide, 12 square kilometres, before it all collapsed and cut the valley.
As it seems likely that the warm period that melted the ice in this particular valley lasted just one summer season, the amount of water trapped in this glacier would have been 12,000 metres by 1000 metres by 500 metres.
The melting of this ice caused to be released 12,000x1000x500= 6 x 10^9 cubic metres of water to find their way to the sea in that year.
The daily rate would be 6x10^9÷365: 16.44 x 10^6 cubic metres every day.
Then, if the average thickness of rock that was removed in the year of the valleys cutting was, say 50 metres (probably an underestimate). The total volume of rock removed would be 50x1,000x12,000 cubic metres, 6x10^8.
The average volume of rock being cut from the geology per day, during this event, is 6x10^8÷365=1,643,835 cubic metres.
1.6 million cubic metres of rock are flushed away by 16 million cubic metres of meltwater, (every day).
Those huge volumes of water and sludge would have raised the level of the sea around the coast of Britain, locally, by several metres.
Case Study 4:- The Trent Valley
Like many palaeovalleys, the Trent valley is deeper at its midpoint than it is at either end. This phenomenon, elsewhere, often results in the formation of ribbon lakes.
Also, like the Minches between Scotland and the Outer Hebrides, it was created by repeated deglaciating collapse episodes occurring over several million years.
Here though, evidence has been recorded for the sequence of events that cut the valley.
The chart above depicts, "Generalised profiles across the Trent valley near Newark (section a) and Nottingham (section b), showing the relationships of the principal Quaternary deposits."
These sections across the valley indicate, perhaps four periods of cutting, filling and recutting of the valley floor.
The Eagle Moor Sand and Gravel, the Balderton Sand and Gravel, and the Basingfield Sand and Gravel may have been outwash from impact collapses higher up on the Pennine Hills.
The Eagle Moor Sand and Gravel appears in this and other contexts to have been retained at high levels on hilltop locations. This suggests that this Gravel deposit is the result of an early deglaciation, and that later deglaciations have cut through it, and the surface it rested upon, and deposited more recent Gravel terraces on the floor of the Trent Valley.
The exact periods, when the valley profile was cut is not known. An exception is the latest filling, the Holme Pierrepont Sand and Gravel which is carbon dated to 11,300BP. This material is likely to have been washed from high hill glaciers to this location.
The plan of the Trent Valley, above indicates the positions of the York and Escrick moraines. These are commonly thought to have been the southern limit of the last glacial period. In fact though these are the locations in the valley where a mobile heap of ice blocks, and broken rocks, in a soup of meltwater sludge arrived at an entrapped lagoon of water in the Trent Valley.
At the water's edge the mobile mass melted, leaving the moraines.
Other glacial features
Pot-holes
At Linmere, in south Bedfordshire a series of circular pits have been excavated, in which were found Mesolithic flint artefacts and animal bones.
A total of 12 large pits are believed to date to the late Mesolithic period. The pits were 2.1–5m in diameter and 1.0–1.7m deep. All had steep sides but with a mix of concave and flat bases. Six pits produced animal bone and two produced struck flint. The latter was small (15 pieces) and only comprised blades; none is closely dated. The majority of the animal bone assemblage (8kg, c.400 fragments) derived from just two pits and was dominated by aurochs. Other identified species comprised red deer, roe deer and pig, represented in all cases by just one bone or tooth. Four pits yielded five fragments of animal bone that were suitable for radiocarbon dating and all returned late Mesolithic determinations: four in the mid-7th millennium and one in the later 6th millennium.
The pits.... were clustered around the three palaeochannels. The precise nature, origins and dating of the palaeochannels are uncertain. However, they appeared to lead down to the Ouzel Brook, suggesting that they may have been created by seasonal springs or run-off from the higher ground to the south. An association between the palaeochannels and the large pits seem highly likely as, despite extensive investigations in the area, no such pits have been found away from this part of the Ouzel Brook. (MOLA and Joshua Pollard/University of Southampton)
The palaeovalley here clearly formed, at least in part by glacial runoff, caused the formation of these features in the violence of its flow. When the Mesolithic peoples wandered across the developing post-glacial environment they came across the pits, and camping nearby, some of their detritus , bones and flints, were kicked into the pits, forming an early fill.
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Hummocky Moraine
Ice-streams running away from highland ice-sheets collapse, becoming trapped in cold valleys where reduced sunshine maintained solid ice streams in the Highlands of Scotland into the post-glacial period. Where the glacial ice stream was rendered stationary by local topography, it melted in place , depositing its contents as hummocky moraine.
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Coastal Impact Valleys and seafloor features
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The North coast of Aberdeenshire has an impact valley adjacent to it, the Southern Trench.
Further out, in the northern North Sea, in the Witch Ground region are the impact valleys of the Fladen Deeps and the Bressay Bank.
While the Southern Trench was cut by ice and rock sludge rolling off the Aberdeenshire coast, the Witch Ground and beyond, result from an ice sheet breaking up and receding to the north-east towards the Norwegian Channel.
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On the east coast of Aberdeenshire are a series of impact valleys, and farther off shore are the Devil's Hole group of valleys.
Here again, the channels close to the Aberdeenshire coast are from the impact of falling ice there, and the Devil's Hole group of impact valleys are from an ice sheet receding east towards the Norwegian Channel.
Between these two sets of impact valleys is an area of comparatively soft sediments that are likely to have been the floor of a shallow lagoon.
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On the east coast of northern England a series of impact valleys were formed by an ice sheet retreating onto the English coast.
The resulting ice stream flowed down slope, into the Farn Deeps, leaving striations and a sediment wedge.
Significantly, the presence of only one grounding zone wedge here confirms that this feature was laid in a non-tidal environment, water that was not subject to rising sea-level.
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In the southern North Sea (above) the Norfolk Banks are an array of impact valleys which are parallel to the Norfolk coast. These features connected with a series of impact valleys through the Dover Strait, and into the eastern end of the English Channel.
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Around the southern end of the Dogger Bank are the East Bank Ridges and the Outer Silver Pit.
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Close detail of the surface of the Dogger Bank, above, demonstrates a ridge and valley system on its surface, gradually receding North. These are likely to be impact valleys, and the ridges between them, cut into the surface of the banks as a thin ice sheet edge retreated northwards.
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A deep Channel in the floor of the Dover Strait, the Lobourg Channel, connects from the Norfolk Banks in the North Sea to multiple valley forms in the floor of the eastern end of the English Channel. (See Figure 16). One of these valleys has been dated to 21,200BP.
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On Anglesey impact valleys run downhill towards the sea , both on the island, and on the mainland adjacent to it. They traverse the island from the south-west to the north-east, their progress retreating in a south easterly direction towards the higher hills of Snowdonia.
Although the Menai Strait could be an impact valley it is more likely to have formed several million years ago when this continental shelf region was forming. The island of Anglesey would at that time have simply split off from mainland Wales, drifting off into the Irish Sea.
Elsewhere
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The topographic map of western Europe shows the lowlands of northern France and the Netherlands, as they rise up against the mountains of the Mediterranean coast in the south.
As in the Trent Valley, of England, impact valleys are cut in the foothills of the mountains, roughly along contours, as the altitude of the land rises.
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The Scandinavian landscape, of Norway and Sweden, is a relatively simple geological structure, a huge lump of crystalline geology. The weather system that melted its ice sheet was also relatively simple. Warm air and water flowing up from the equator brushed the surface of the ice sheet, melting it inland from the coast , and up the spine of the landmass. Successive years, summers, or warm periods, caused edge collapse to retreat north-east along the ridge. Parallel Valley systems were left in the retreat, a very different landscape pattern to the, mostly chaotic, ice sheet retreat on Britain.
Post-glacial landscapes
Huge volumes of meltwater were released by the fast retreat of the icesheet. These melt periods occurred over short time intervals in any location, tens or hundreds of years, rather than thousands. As the globe warmed the locations of melting ice-sheets migrated northwards.
The result of the meltwater deluge was to send huge quantities of sludge across lowland Britain, and into coastal waters.
As the meltwater and its contents crossed land and coastal waters it carved striations on surfaces, eroded deep potholes, and formed drumlins and moraines in various ways.
Sometime between 12,000BP and 10,000BP the pace of melting slowed and stopped. Rivers began to be drain rainwater away from high ground, out to the Atlantic, (instead of sludge).
Most of the North Sea area was land, but Britain was connected to Europe, Ireland, and Orkney/Shetland. All of the glacial material on the surface of the North Sea had arrived in place in the previous 60,000 years, carried by cascading meltwater streams.
As these sediments were carried by water they were only loosely compacted, and as sea-levels began to rise they were easily eroded away.
The process of erosion was slow, but with the continuous barrage of high seas from Atlantic gulf stream weather systems any soft sediments blocking access between the Atlantic Ocean and the North Sea would inevitably be broken through.
The date of that breakthrough would be after 10,000BP, which is roughly when Doggerland in the North Sea was a territory roamed across by people and animals.
Seaworthy boats were present in the North Sea at 4,000BP, and it is reasonable to suggest that the breakthrough that separated Britain from Europe occurred between the dates, 10,000BP and 4000BP.
The clear linkages of large populations of Neolithic people moving around between Europe, England, Scotland, Orkney, and Shetland demonstrate that land joined those places into the 5th and 4th millennia BC.
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Also, see:- Walkable Land in the North Sea
Next:- Quaternary Addendum
Back to the beginning of the Orkney Riddle
Image sources
The geology of the Malin–Hebrides sea area United Kingdom Offshore Regional Report By J A Fyfe,
Pattern, style and timing of British–Irish Ice Sheet advance and retreat over the last 45 000 years: evidence from NW Scotland and the adjacent continental shelf. TOM BRADWELL et al
The geology of the northern North Sea. United Kingdom Offshore Regional Report By H Johnson
The geology of the central North Sea. United Kingdom offshore regional report R W Gatliff et al)
The mixed‐bed glacial landform imprint of the North Sea Lobe in the western North Sea by Dave Roberts et al.
The geology of the southern North Sea. United Kingdom offshore regional report By T D J Cameron et al
Large-scale glacitectonic deformation in response to active ice sheet retreat across Dogger Bank (southern central North Sea) during the Last Glacial Maximum by Emrys Phillips et al
The geology of the English Channel: United Kingdom Offshore Regional Report By R J O Hamblin et al
Geology of the country around Ramsgate and Dover. Memoir for 1:50 000 geological sheets 274 and 290 (England and Wales) By E R Shephard-Thorn
Quaternary of Scotland Edited by J. E. Gordon Scottish Natural Heritage, Edinburgh, Scotland. and D. G. Sutherland Edinburgh, Scotland.
Geology of the Nottingham district: Memoir for 1:50 000 geological sheet 126 (England and Wales) By A S Howard,
Eastern England from the Tees to The Wash. British regional geology, Sir Peter Kent
More images:-
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| An untidy array of impact valleys (mostly) over northern England |
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| The English Lake District |
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| Vaguely organised impact valleys on northern Scotland. The Great Glen, containing Loch Ness will also be a several million years old seismic split, pulling the mountains of of northwestern Scotland away from the mountains of southeastern Scotland. |
All opinions my own, unless referenced otherwise.
Jeffery Nicholls
South Ronaldsay
Orkney
Jiffynorm@yahoo.co.uk
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