听力题 M: It''s good to see you back. How was your trip to Arizona? Did you see the Grand Canyon? W: yes, it was fantastic. Now I know why it is listed together with the Nile River as one of the great natural wonders of the world. Here, want to see the picture I took? M: Oh, what a view! It is even bigger than I had imagined. I remember reading about the Grand Canyon and I think it said that it was formed suddenly when the earth''s crust split open during an earthquake. W: Well, not that quickly. Look, here is the picture of the canyon, that river and its tributaries have been wearing the canyon floor away for ten million years. M: Then the canyon is the result of swale erosion. I''ll bet geologists have made some interesting discoveries here. W: Not only the geologists, archaeologists have found the bones of extinct animals in caves in the canyon walls. In a cave one hundred and forty feet above the river, they found drift wood that dates back as far as thirty seven thousand years. M: That means the river must have been a hundred and forty feet higher up when it carried the driftwood into the cave. W: It''s very possible, but of course who knows. The Grand Canyon is full of mysteries. Wouldn''t you like to go to Arizona some day? M: You bet I would.