Part of a lecture about glaciers and how they shape the landscape

Section 4. You will hear part of a lecture about glaciers and how they shape the landscape. First, you have some time to look at questions 31 to 35. Now listen carefully and answer questions 31 to 35. MAN: Good afternoon. Today we're going to look at glaciers, enormous masses of moving ice, and in particular at how they carve and shape the land beneath them. Let's begin with how a glacier actually forms, because it does not happen overnight; it takes centuries. In places where more snow falls each winter than melts away the following summer, the snow builds up, layer upon layer, year after year. As fresh snow piles on top, the snow buried underneath is squeezed under enormous pressure, the air is driven out, and it slowly changes, first into a grainy substance the French call névé, and at last into dense, solid ice. Once enough of this ice has gathered, and the mass becomes heavy enough, gravity takes over, and the whole glacier begins to creep downhill. MAN: Broadly, there are two types, and it is worth not confusing them. The rivers of ice that flow down existing mountain valleys are called valley glaciers. The vast domes of ice that bury entire regions, like Greenland or Antarctica, are something else altogether; these are known as ice sheets, and they can be kilometres thick. Before you hear the rest of the lecture, you have some time to look at questions 36 to 40. MAN: Now, how does moving ice wear away solid rock? There are two main processes, and they work together. In the first, called plucking, melt-water seeps into cracks in the bedrock and freezes onto the rock; then, as the glacier moves on, it tears those loosened lumps away and carries them off. Those torn-off rocks now become tools. Dragged along, frozen into the base of the glacier, they scrape and grind against the bedrock beneath, and this second process is called abrasion, rather like rubbing the ground with a giant sheet of sandpaper. Abrasion often leaves long, parallel scratches gouged into the rock surface, and these scratches have a special name: they are called striations. Geologists prize them, because they reveal the exact direction in which the vanished ice once flowed. MAN: Let's turn to the landforms glaciers leave behind, because these are what let us map ice ages long gone. Perhaps the most recognisable is the valley itself. An ordinary river cuts a narrow, V-shaped valley. But a glacier is vastly heavier and wider, and it grinds out a broad valley with steep sides and a flat floor, a U-shaped valley, quite unmistakable once you know to look for it. High up on the mountainside, at the very place where a glacier is born, it can scoop out a deep, rounded, armchair-shaped hollow, and this is called a corrie, though you may also hear it called a cirque. Then there is all the shattered rock and debris the ice has been carrying. When the glacier finally melts and retreats, it simply dumps this load, and any such deposit of dropped rocky material is called a moraine. One last, dramatic landform: along some coastlines glaciers carved their valleys right down below today's sea level; when the ice melted, the sea flooded in, drowning the valley and producing the deep, steep-sided coastal inlet we call a fjord. The coast of Norway is famous for them. MAN: Finally, why does any of this matter today? Glaciers still lock up most of the planet's fresh water. But as the climate warms, many are now shrinking at an alarming rate, and the landscapes they left behind, valleys, lakes and all, are a priceless record of the Earth's colder past. The lakes of the English Lake District, for instance, sit in basins that ice gouged out during the last ice age. We'll examine that evidence in more detail next week. That is the end of Section 4. You now have some time to check your answers.