Ice-sheet Britain

 

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. 

Understanding the geological processes of recent prehistory maps out where land may have existed; land which is now completely absent. This might suggest how people,  plants, trees, and animals may have been able to migrate across to Britain from continental Europe. 

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.

 

  

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.

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.

 

Case Study [1] -  Norway 

The impact valley, the fundamental earthmover, that ground away the mountains of northern Europe is defined by a set of characteristics.  These characteristics are not absolute markers, but they are a fair indication of a definition for the features. They are:-

1, They are linear features cut into sediments or bedrock. 

2. They tend to follow contours, and are often in the foothills of higher ground.

3. They occur in locations that are parallel to coastlines, both onshore and offshore. 

4. They also run in parallel sequences,  ridge to ridge and valley to valley.

5. They are commonly deeper in their longitudinal midpoint, or higher, falling away in two or more directions. 

6. As a result of their lack of fall geometry they often contain ribbon lakes.

Also , in general terms:-

7. The ice-sheet over Britain and Norway was static. Ice streams running away from collapsing ice sheets traveled at speeds that were directly proportional to the slope on which it travelled- no slope, no speed, no travel.

8. The melting of the ice sheet was dependant on proximity to warm water, and warm air.  Ice-sheets adjacent to the Atlantic Ocean would melt before inland ice, and lowland ice before hilltop ice. On a relatively level and large area of land, such as Sweden the dominant source of heat affecting ice sheets would be air flowing across it in gulf stream weather systems. 

The effect of the air flows across Scandinavia is to run up against the retreating front of the ice-sheet,  which collapses, in place, retreating towards the north-east against breezes coming from the south-west.  Consequently the orientation of the linear features on Sweden is commonly aligned from north-west to southeast. 

 

The following map of the Bergen area from "Configuration of the Scandinavian Ice Sheet in southwestern Norway during the Younger Dryas," by Jason P. Briner et al shows dates for exposed bedrock and perched boulders. 

 

The average date for all these measurements 14.2ka BP. On bedrock alone the average is 17,8kaand on perched boulders 12.4ka BP.

The range of dates on bedrock is from 12.7 to 23.1 ka BP , and from perched boulders 10.4  To 16.2 ka BP. 

Quoted from "Late Quaternary ice sheet history and dynamics in central and southern Scandinavia"  Timothy F. Johnsen, refence location above

"Paper IV: Åreskutan TCN

Johnsen, T.F., Fabel, D., Stroeven, A. High elevation cosmogenic nuclide dating of the last deglaciation in the central Swedish mountains: implications for the timing of tree establishment. Manuscript."

"TCN exposure ages of glacially transported boulders from the summit of Mt. Åreskutan (1420 m sl) in central Sweden are consistent (adjusted mean age = 10.6 ±0.6 ka, n = 3) and similar to lower elevation dates for deglaciation from the region. "

The distance from Bergen to Åreskutan is roughly 500 km. The earliest date on bedrock is 12.7ka BP,  500km from 12.7ka BP to 10.6ka BP .  Bergen is a coastal area, so an early date for the retreat of the icesheet is to be expected. 

  

Referring to research in the Kvikkyokk area (location above) of norther Sweden,  in :- "Tracing the last remnants of the Scandinavian Ice Sheet: Ice-dammed lakes and a catastrophic outburst flood in northern Sweden" by Carl Regnell et al

"Large deposits and erosional landforms from a large glacial lake outburst flood are mapped along the Pite River Valley. The glacial lake outburst flood is dated to 10.3 cal ka BP, based on crosscutting relations between flood landforms and raised shorelines in the Baltic Sea basin.  We consider this age interval to be representative also for the last ice sheet remnants in northern Sweden."

  

  

Kvikkyok is cloes to Sarek, in the map above. The valleys of the region are loosely parallel to each other, and commonly filled with ribbon lakes.

   

      

The Baltic Sea coast of Sweden,  above, shows strong parallelism for the features on sloping ground, across from the ridge of high mountains with ribbon lakes, down to sealevel. 

Between Åreskutan and Kvikkyokk relative dates are 10.6ka BP to 10.3ka BP on a distance of 500 kilometres,  roughly 300 years to retreat 500km.    

Inland from Bergen the the mountains are high enough that they remain topped by glaciers. In this region there is little evidence of parallel impact valleys. 

Case Study [2] - The North Minch 

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 coastal region here is adjacent to the north Atlantic Ocean,  and it is quite unique in that it is the only deep water passage that connects equatorial waters with the Arctic circle.  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 "ploughman's or MSGL", (Mega-scale-glacial-lineations).  These lineations were caused by ice blocks and rocks carried along in the stream scouring the channel floor.

A Parallel Valley Array 

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.

    

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.

Glaciers melt - in place

  

A glacier retreats slowly between 11,000BP and around 7000BP,  dropping its contents onto the valley floor on which it had rested.

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Coastal Impact Valleys and seafloor features 

 

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. 

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.

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.

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. 

    

 

Around the southern end of the Dogger Bank are the East Bank Ridges and the Outer Silver Pit.

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. 

     

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. 

And on Land 

The chalk hills adjacent to the Dover Strait are covered with shallow valleys that are parallel to it, and to each other. They represent the retreat of the ice sheet away from its southern extreme. The retreat of this ice sheet took 6000 years,  from 21,000BP to 15,000BP, to migrate from Dover to Wales. The progress was not necessarily linear, the thickness of ice being dependent on altitude, latitude, and probably undefined topographic circumstances. The melting of this ice-sheet retreat is likely to have occurred slowly, and  un-dramatically.

 

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. 

 

 The land surface over Caithness,  above is characterised as formed by ice movement,  and has been used elsewhere to suggest major ice-sheet flow across from the Moray Firth northwards to the Pentland Firth. 

The great height of the cliffs experienced daily by drivers on the A9 coastal road here should advise researchers that a flow of this nature is not possible. 

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. 

Elsewhere

  

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.

  

  

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 in various ways.

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"Ice-sheet Britain" is the first of a series of articles which provide accounts describing the development of British geography in the Quaternary period.

"Relative sea-level" provides additional evidence to support this discussion. 

"Archaeology of the North Sea " describes evidence in the written scientific literature which suggests a local context in the North sea that would have allowed a rodent (the Orkney Vole), and other creatures to travel from Europe to Orkney without setting foot on British soil. 

"Walkable Land in the North Sea" charts the probable existence of a land bridge from Holland to Norfolk which allowed flora, fauna, and people, to commute between Europe and Britain in the early Neolithic era.

Having provided some reasoning that would suggest that Neolithic people walked from Europe to Britain, it is more difficult to sustain the idea that they used boats to sail from Scotland to Orkney. "Neolithic Migration to Orkney" leads into a myriad of articles that describe what happened in early Neolithic Orkney, and again what happened when people could no longer walk from Caithness to South Ronaldsay. 

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:-

 

An untidy array of impact valleys (mostly) over northern England 

 

The English Lake District 

  

Vaguely organised impact valleys on northern Scotland 

 

All opinions my own, unless referenced otherwise.

Jeffery Nicholls 

South Ronaldsay 

Orkney 

Jiffynorm@yahoo.co.uk