SHAPING THE LANDSCAPE
Our landscape has been shaped by both internal and external forces constantly in motion across the planet, and, in certain periods of our geological history, ice has played a major part.
At the onset of an ice age, lowered temperatures and huge winter snowfalls mean some snow remains through the chillier summers. Due to compression and a freeze-thaw cycle, the snow crystals become granular and are known as névé. If this survives another season or two, it changes to firn, which is more densely packed, with the air squeezed out, locking the crystals tightly together. As more and more snow is added, the increasing pressure converts the firn to glacier ice. When it reaches a depth of approximately 164ft (50m), there begins an inexorable flow of a glacier, a river of ice, debouching under gravity from the deep recesses down onto lower ground.
These processes, seen today in glaciers on high mountains around the world, were helped by the action of freeze-thaw: ice that melts, refreezes and expands, breaking down rock structures. Rocks continually crash down onto the sides of the glacier and are carried along. The ice is often crevassed, with huge fissures that penetrate far below if the underlying rock is uneven. Deep within the glacier, percolated water remains, aided by extreme pressures, which prevent refreezing. This water provides lubrication of its base, helping it to flow.
Glaciers carve into mountains, excavating and reshaping them, transporting away the debris, known as till, and depositing it as long mounds of moraine, which can block the outflow of the glacier and form a lake that remains after the ice has gone. From the mountains of Scotland, north-west England, Wales and Ireland, glaciers scraped and gouged out the landscape, transforming the hollows into grand amphitheatres, known variously as corries, cirques or cwms. When two glaciers work back on each other, a sharp ridge, called an arête, can be formed. One of the UK’s most famous is the vertiginous Striding Edge on the eastern flank of Helvellyn in the Lake District. Beneath it lies the moraine-dammed lake, Red Tarn. During the ice age, glaciers flowed downhill, widening, entrenching and smoothing a path; transforming many valleys into ‘U’ shapes, including the Nant Ffrancon in North Wales.
Some 450,000 years ago, much of the British Isles was covered in an ice sheet hundreds of metres thick. Beneath an ice sheet, a vast amount of debris is deposited, and, as it melts, it reveals a landscape of low hills and ridges, called eskers, mounds or drumlins and hollows. The latter are formed by slowly-thawing lenses of ice remaining under the surface after the main mass has melted. This causes subsidence; these depressions then filling with water to form lakes. The area around Ellesmere in Shropshire is a good example.