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For the last few million years, perhaps three million years, glaciers have come and gone from the Rocky Mountains. And every time they come across the landscape, they are capable of eroding that landscape at rates that are perhaps fractions of an inch per year, meaning that over the course of one glacial cycle, you perhaps erode ten, twenty feet of rock.
Ice also created the broad canyons. With every ice age, new glaciers ground their way down V-shaped river valleys, and turn them into broad U-shaped canyons.
For the glacier, the whole valley is its channel, so any place where the glacier touches, the wall is capable of eroding it. And therefore the walls will be made more vertical on the edges and be flattened on the base until it gets to a now, a U-shape, which then propagates downward.
Ice also explains the presence of these boulders. They hitchhiked at the bottom of the glacier down the frozen valley, when the last Ice Age came to an end and the glaciers melted about 10,000 years ago, the boulders were left behind. Scientists had found two pieces of evidence that were responsible for the jagged looks of the Rockies today. A solitary boulder falling to the area could have only been transported here by ice. Striations （ n. 輝紋；光條紋）showed scientists that a glacier at least a thousand feet thick covered the Rockies. Ice was responsible for the dramatic shape of the Rockies today. But the mountains keep evolving. Recently, scientists discovered alarming evidence that they may collapse into a deep rift.
For the last 70 million years, compression, erosion and ice have sculpted the Rocky Mountains to their present formation, but the geology that created this impressive mountain range has also the potential to destroy it. Over the last 25 million years, a gigantic rift has been opening up at the southern end of the Rocky Mountains.
小編有約：Daisy在這一期忽然覺得在這一期冰使得這里的山谷變得很寬闊，從V形到U形，看來咱們的古話“水滴石穿”起到作用啦，所以有些道理還是相通的。
課后題目：在前面的幾期中我們一起經歷了落基山脈70多萬年的經歷，那么它都經歷了哪些地理運動呢？
