Impact Valleys

 

Orkney Riddle 

Impact Valleys 

Departing from archaeology to examine the geology of Britain as it has affected the topography of both Britain and the North Sea, the impact valley is a necessary component for an understanding of our prehistoric landscape. 

 

Olchon valley and Black Mountain from Longtown Castle 

 Definition of a valley 

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

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

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

( Encyclopedia Britannica )

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

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

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

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

There is significant evidence that these valleys, particularly on rising or high ground, were formed by vertical impact  - falling ice.

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The Impact Valley 

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

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

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

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

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

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

The Snowline 

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

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

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

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

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

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

The Impact 

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

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

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

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

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

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

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

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

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

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

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

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

Over cooler periods, 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, the impact of the collapse was minimal, but as the edge retreated, thickness increased and valleys were cut into ground beneath it.

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

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

The Ice Stream

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

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

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

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The Black Mountain Parallel Valley Array 

 

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

Current thinking would probably suggest that these features were caused by the movement of glaciers along tnem, but even if the valleys were to be filled with snow there would be no impetus for the mass 8of ice in its frozen state to move along them. 

  

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

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

Using detailed analysis, and my own interpretation, of Ian Shennans sea-level assessments (Relative sea-level changes and crustal movements in Britain and Ireland since the Last Glacial Maximum, by Ian Shennan) I suggest that the depth of ice over in this area could have been 500 metres, but it may have been more.

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

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

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

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

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

 

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

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

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

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

Jeffery Nicholls 

Orkney 

Jiffynorm@yahoo.co.uk