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上次咱們在懷俄明州的盆地發(fā)現(xiàn)了什么呢？它能否告訴我們落基山下降的原因呢！咱們接著聽！
Rock and cobbles eroded down from the Bighorn Maintains and filled up the basin to at least a thousand feet, the height of the Pumpkin Buttes.
"Pumpkin Buttes are unique because this used to be the actual surface level of the basin itself. The rest has been eroded away a thousand feet of sediment to the basin that we see now."
But the rubble found here is nowhere near enough to have covered the Owl Creek Mountains. Mclaughlin traveled to Darton's Peak, one hundred miles west in the Bighorn Mountains. On a cliff, 9000 feet high, he finds granitic cobbles that are strikingly similar to the ones on the Pumpkin Buttes. They too, are from the core of the Rocky Mountains.
"The core of the Rocky Mountains are made extensively of granite. Much like what you see here, these are from the Bighorns that have been transported down, rolled and smoothed along their way to create these smaller boulders and cobbles."
This is strong evidence that cobbles eroding from the Rockies filled in the basins and valleys to at least 9,000 feet, slowly burying the Mountains under their own debris. Where once the mighty Rockies stood, there was now a grey barren plain with only the peaks of the old Mountains piercing the surface.
The same process has happened in other mountain chains too. There was evidence that the European Alps were also cut in half by erosion. At their base, scientists found hills formed out of millions of tons of rock that had cascaded down and reduced their height. But the story of the eroding Rockies wasn't over yet. After erosion turned the landscape into a great cobble field, another disruption happened. Evidence for this is a layer covering the top of the cobbles.
"It’s very light, it's very fine-grained. It's actually of volcanic ash. As you can see, it's made of very very fine-grained sediments compared to this boulder conglomerate, just made of big hunks of rock. It sits directly on top of this unit and it was laid horizontally from mostly ash-fall."
小編有約：這期終于給咱們解開了這個謎團，為什么洛基山脈會下降，你有了答案嗎？能告訴Daisy嗎？
