Glacial Prehistory

 

Orkney Riddle 

Glacial Prehistory 

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

It is not authoritative, and may be taken with a pinch of salt, but I hope that those who have learned, as I have, the geological history of Britain, will withhold any kneejerk response that may lurk unbidden in their bosoms.

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

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

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

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

As the Atlantic Ocean opened up a great chasm between Europe/Africa and North/South America, so Britain pulled away from Norway. The result was that a portion of the earth's crust between Britain and Norway collapsed into the crevice that had opened up, creating a groove in which the North Sea would ultimately develop. 

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

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

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

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

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

This condition seems not to have happened before 140,000BP. 

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

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

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

Also at this time, the ice sheet on Shetland was collapsing, and some of the meltwater from that event flowed south along the Scottish coast into a lagoon that had been formed by the after-wash from glacial meltwater. 

Ice age snowfall returned, possibly as late as 23,000BP laying a variable thickness of snow/ice across the British Isles and the North Sea. Very little snow built up on the south coast of England, though the South Downs in Kent may have had an estimated hundred metres thickness on it.

Additional thickness over the pre-existing mountain snow sheets raised them further, though it is likely that snow had been adding to these sheets throughout the previous, less cold periods.

The mainland ice sheets on England and Scotland extended off present coastlines a short distance and a high ridge of land joining Dogger Bank to the Atlantic Ocean at a point between Shetland and the Norwegian Channel, supported an ice sheet perhaps 250 metres deep. This would be thicker on the British side, and less so on the Norwegian side, where some heat from the Norwegian Channel reduced snow thickness. 

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

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

Between 21,000BP and 15,000BP the ice sheets retreated from the English Channel in a north-westerly direction, crossing the South Downs of southern England and arriving at the foothills of western England and Wales.

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

At about 15,000BP a very severe input of heat to the environment caused huge amounts of meltwater to be released to coastal waters. This effect may have caused the excavation of annual parallel valleys like those on the Welsh border with Herefordshire. Here the melt would have caused the release of meltwater, possibly in the region of half a cubic kilometre of water every day, with peak melt in summer and least melt in winter.

This release caused the sea level at the local coastline to oscillate, up and down annually, as seasons changed.

The meltwater released carried with it considerable amounts of sand and silt, with rocks and blocks of ice. This stream of ice and mud crossed the British landscape creating features in its path, and leaving sediments where the flow of water slowed.

Where the stream of materials reached water the solids either settled out of the stream, or backed up, forming moraines. 

At sea coasts these moraines were formed as parallel linear features aligned with the rise in line with local and global sea levels. 

At around 11,000BP the last of the mountain ice sheets had collapsed, and water from that event was mostly draining away with little effect on local sea levels, but in the valleys that had been gouged out by the collapse of ice sheets there remained, where the sun was less able to reach, rivers of solid ice. The ice carried the stones and sands that had been gouged out of the mountains and it wasn't until about 9000BP that these began to melt.

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

The lagoon, off the Aberdeenshire coast remained liquid during the last cold spell, and as the meltwater water fell off the mountains of England and Scotland it overfilled and overflowed around the southern coast of the north sea, between Belgium, Holland, Denmark, and Dogger Bank. 

The spine of land that divided the North Sea, north to south was available to mesolithic wildlife and people, and as sea levels rose it was gradually whittled away until in a series of collapses in the 4th millennium BC, more recent, softer sediments in the northern North Sea were easily flushed aside, opening up the whole North Sea. 

Mesolithic people roamed Britain, Ireland, and land now under the North Sea. The area that they would roam was across to the Norwegian Channel, a wide deep water Channel out of the Skaggerak that followed the south and west coasts of Norway. The northern extent of the mesolithic territory was at the Atlantic ocean between Shetland and Norway. There was a coastal inlet to the territory east of Shetland that had a southern shoreline just east of Orkney, and when a tidal wave was triggered north of Norway in around 8200BP it was funneled along this inlet to hit the Aberdeenshire coastline. 

In the middle of this mesolithic territory, now just off the coast of Aberdeenshire, was large lagoon left over from the ice age.

The people who found this landscape followed the post-glacial migration of trees, grasses, herbs, birds, reptiles, insects, and mammals.

As sea levels rose two routes were the last to be closed off, separating Britain from Europe. First to go was a crossing between France and Sussex, and the final division came with the demolition of land east to west between Norfolk and Holland, at some time in the 4th millennium BC. 

Jeffery Nicholls 

South Ronaldsay 

Orkney 

The Orkney Riddle (index here) is a series of blogs promoting the notion that Neolithic people walked from Scotland to Orkney to build the Ring of Brodgar. 

Email jiffynorm@yahoo.co.uk 

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