例如，随着覆盖北美的大冰原在最后一个冰河时代逐渐融化，大量的水从冰原流出。

For example, as the great ice sheets that covered North America during the last ice age steadily melted away, huge volumes of water flowed from them.

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但是，一个大冰原的融化就不同了。

It would take the melting of an ice sheet to do that.

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上新世大约结束于三百万年前发生在北半球大冰原之间的碰撞。

The Pliocene ended around three million years ago with the onset of large ice sheets in the Northern Hemisphere.

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