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Twenty thousand years ago,
两万年前
this part of America was at the edge of a vast ice sheet.
美国的这部分地区正处于广阔的冰原。
It covered much of the Northern Hemisphere.
它覆盖了北半球大部分地区
In some places, the ice would have towered over any modern skyscraper.
在一些地方，冰层比摩天大楼还高
When it retreated, just as in Yosemite,
当它退去，就像约塞米蒂
the ice left behind these boulders and rock smoothed and polished by ice.
冰带走这些岩石和留下一片平滑光亮的土地
The ice exposed a hard bedrock called schist
冰使一种叫片岩的硬岩床暴露出来
solid enough for sinking deep foundations to support tall buildings.
能挖出足够坚固的地基 来支撑高楼大厦
Without this bedrock, New York could not have been built so high.
没有这些岩床，纽约不可能建这么高的楼
The one part of the skyline that dips
大冰原堆积起散沙和沙砾
is the one place where the ice sheet deposited loose sand and gravel,
使一部分地平面倾斜
no good for building towering skyscrapers.
对于建造摩天大楼很不利
