【正确答案】 C

【答案解析】[解析] 1-5 Now that you know how sedimentary rocks are formed, the next step is to look at various shapes and learn to read them. On our next field trip, we'll see several of the formations called mesas. This landform gets its name from its flat top. "Mesa" means "table" in Spanish. The Spanish people who explored the area thought these flat-topped hills looked sort of like tables. A mesa is wider than it is high--kind of like a large table. We'll also see a variety of other formations, such as buttes, spires, and pillars. All of these spectacular forms are the result of the erosion of rocks of differing hardness. Water erodes rocks both mechanically and chemically. The fast- moving water of rivers carries silt, gravel, and rock debris, and this scours the rock underneath. Slow-moving standing water also erodes when it enters tiny rock pores and dissolves the cements holding the rock together. On a mesa, conditions are optimal for erosion. With enough time, even the durable top of a mesa will decrease in size. The sides of a mesa are often made of shale or softer sandstone. The slope of the sides will increase the water's speed and force as it runs down. Freezing and thawing loosen the surface rock. Debris carried by the running water cuts away the softer surface rock. As the softer base of the mesa recedes, the edge of the top is weakened, and it eventually cracks, splits, and falls. As a mesa is shrunk in size by water, it may be cut into smaller landforms. If these smaller remnants are at least as high as they are wide, they are called buttes. The great buttes we'll see were all created by water rather than wind erosion. Further erosion can change a butte into a tower or spire. This is because the shaft of the spire is usually harder than the base on which it stands, and like a mesa or butte, it's capped with a rim of even harder rock. The spires you'll see were left standing after the sandstone around them eroded away. You can see why they're also called chimneys. I mean, they sort of jut up from the sandstone floor. Further erosion of the softer rock may reduce the spire to some interesting and really weird forms. We'll see some hourglass-shaped rocks, mushroom-shaped rocks, and a sort of strangely eroded pillar. Over time, erosion finally topples these rocks to the ground. They might remain there as boulders, or they might undergo further erosion that completely demolishes them so they disintegrate into pebbles. Finally, these pebbles end up as the sand we walk on as we explore the surface of the plateau. 1. What is the main purpose of the talk? The main purpose of the talk is to explain how erosion shaped certain landforms. The professor says: All of these spectacular forms are the result of the erosion of rocks of differing hardness; On a mesa, conditions are optimal for erosion; Further erosion can change a butte into a tower or spire; Further erosion of the softer rock may reduce the spire to some interesting and really weird forms.

【正确答案】 D

【答案解析】[解析] Why does the professor say this: "A mesa is wider than it is high--kind of like a large table. We'll also see a variety of other formations, such as buttes, spires, and pillars." The professor's purpose is to preview what students will see on a field trip. Just before that the professor said: On our next field trip, we 71 see several of the formations called mesas. The students will also see a variety of other formations

【正确答案】 A、D

【答案解析】[解析] What reasons are given for the erosion of a mesa? One reason for the erosion of a mesa is that the rock on the sides is softer than that on the top: The sides of a mesa are often made of shale or softer sandstone. Another reason is that the force of water cuts away the softer rock: The slope of the sides will increase the water's speed and force as it runs down; Debris carried by the running water cuts away the softer surface rock.

【正确答案】 B

【答案解析】[解析] Listen again to part of the talk. Then answer the question. "The spires you'll see were left standing after the sandstone around them eroded away. You can see why they're also called chimneys. I mean, they sort of jut up from the sandstone floor." Why does the professor say this: "I mean, they sort of jut up from the sandstone floor." The professor's purpose is to describe the appearance of spires by comparing them to chimneys.

【正确答案】 B

【答案解析】[解析] Listen again to part of the talk. Then answer the question. "Further erosion of the softer rock may reduce the spire to some interesting and really weird forms. We'll see some hourglass-shaped rocks, mushroom-shaped rocks, and a sort of strangely eroded pillar. Over time, erosion finally topples these rocks to the ground. They might remain there as boulders, or they might undergo further erosion that completely demolishes them so they disintegrate into pebbles." Why does the professor say this: "They might remain there as boulders, or they might undergo further erosion that completely demolishes them so they disintegrate into pebbles." The professor's purpose is to show that erosion continually changes the shape of rock. She explains how erosion reduces spires to other forms and eventually to pebbles.