英语听力：自然百科 黄石公园的夏天 Yellowstone—4

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This plateau is the created bowl of a huge sleeping volcano. And melting rock below the surface still pushes the land up to the cold of high altitude.

 

But it has not been the only force to shape the land.

 

Millennia³ ago, the volcano was covered by an ice sheet thousands of feet deep.

 

The few glaciers⁴ that border Yellowstone are an echo of a time when ice scarred and smoothed the entire plateau.

 

When these great ice sheets retreated, they left pulverized⁵ rocks in their wake, the foundation for fertile soil and some of the richest natural grasslands⁶ in America.

 

But in May this year, in the heart of Yellowstone, that grass is still beneath thick snow - something this ground squirrel was clearly not expecting.

 

But as the sun's strength returns, spring bulbs start to push through the late snow - at last a sign that winter is losing its grip.

 

Bluebirds, too delicate to survive deep winter here, now return to take advantage of the fist hatch of stoneflies from a thawing⁷ river.

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