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And geoscientists, they've known that the extent of the mountain glaciers in what's now the western U.S., it shrank and grew over the last 11,700 years. And this is after a period of time known as the Holocene. In the warm early Holocene, the Sierra glaciers got smaller, and then they expanded in the late Holocene. They shrunk in the last century, contracting from huge ice walls hanging above sea slopes in the small snow fields, barely surviving. And what's unclear is whether these glaciers ever fully disappeared.