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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. 辉纹；光条纹）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.
小编有约：Daisy在这一期忽然觉得在这一期冰使得这里的山谷变得很宽阔，从V形到U形，看来咱们的古话“水滴石穿”起到作用啦，所以有些道理还是相通的。
课后题目：在前面的几期中我们一起经历了落基山脉70多万年的经历，那么它都经历了哪些地理运动呢？
