Academic Reading — Test 10

Reading passage

Along the western edges of Norway and the indented coastline of Scotland lie some of the most dramatic landscapes in Europe: long, narrow inlets of seawater flanked by steep rock walls that plunge directly into deep, still channels. These features are known as fjords, and although they appear timeless, they are in fact the product of a relatively recent and violent chapter in the planet's history. Fjords were not shaped by rivers or by the sea acting alone. They were carved by ice, during a series of cold periods collectively referred to as the Ice Ages, the most recent of which ended roughly eleven thousand years ago. The essential agent in the creation of a fjord is the glacier. During the coldest phases of the Ice Ages, snow that fell on the highlands of Scandinavia and northern Britain did not melt in summer. Instead it accumulated, layer upon layer, until its own weight compressed the lower snow into dense ice. When this mass of ice grew thick enough, gravity set it in slow motion, and it began to flow downhill towards the coast. A glacier does not behave like flowing water; it moves only a few metres in a year, yet the forces it exerts are enormous. The most important point to grasp is that a glacier erodes by two distinct processes. The first is abrasion, in which rock fragments frozen into the base of the ice act like the grit on sandpaper, grinding and polishing the bedrock beneath. The second is plucking, in which the ice freezes onto blocks of rock and, as it advances, tears them away entirely. Crucially, the glaciers did not carve their valleys at random. They tended to follow the courses of existing river valleys, which already provided a natural channel running from the high ground down to the sea. As the ice exploited these pre-existing routes, however, it transformed them completely. A typical river valley has a cross-section shaped like the letter V, narrow at the bottom where the river runs. A glacier, by contrast, is far wider and grinds away at the valley floor and both walls simultaneously. The result is a broad, steep-sided valley with a flat or gently rounded base, whose cross-section resembles the letter U. This characteristic U-shape is one of the surest signs that a landscape was once occupied by ice, and it is visible in countless valleys that no longer contain any glacier at all. Because ice is so much heavier and more powerful than water, a glacier could excavate its valley to a depth far below the level of the sea. This explains a feature of fjords that often surprises visitors: many are astonishingly deep, and some plunge to depths greater than the open sea immediately beyond their mouths. The Sognefjord in Norway, the longest and deepest in the country, reaches more than thirteen hundred metres below the surface of the water in places. Yet near its entrance the seabed rises sharply to form a shallow ridge called a threshold, or sill. This sill is composed of rock and debris that the glacier deposited at its furthest point, where its erosive power finally weakened. The sill is significant because it partly isolates the deep water trapped behind it, sometimes limiting the exchange of oxygen between the inner basin and the open ocean. The final act in the making of a fjord came not from ice but from the sea. When the climate warmed at the close of the last Ice Age, the great glaciers melted and retreated back into the mountains. The enormous quantity of water released raised global sea levels considerably. The sea advanced inland and flooded the deep, empty troughs that the ice had left behind, drowning their lower reaches and filling them with salt water. It is this combination of glacial excavation followed by marine flooding that gives a fjord its defining character: a glacial valley invaded by the sea. The Scottish examples, often called sea lochs, were formed by exactly the same sequence of events, although the ice sheet that covered Britain was generally thinner than the one over Norway, and the resulting inlets are correspondingly less deep. Today the fjords are valued for more than their beauty. Their sheltered, deep waters provide ideal conditions for shipping and for fish farming, and the steep rivers that tumble into them are widely harnessed to generate hydroelectric power. They also serve as natural archives, for the layers of sediment that settle on a fjord floor preserve a detailed record of past climates. In studying how these inlets came to exist, geologists are reading a history written by ice across many thousands of years.