The evidence helps put to rest the idea that the western USA once hosted a high, Tibet-like plateau that eroded to form the mountain ranges seen today.

The researchers analysed the ratios of hydrogen and oxygen isotopes in 2,800 rock samples collected from mountains in western North America in order to calculate the composition of ancient rain that fell on the rocks. Water containing heavier isotopes tends to fall first as a cloud rises, so by measuring the ratio of heavy to light isotopes, it’s possible to infer the elevation of the land on which the rain fell. The elevation trends revealed in the data could then be used to create a history of the entire range.

‘Where we got a huge jump in isotopic ratios, we interpret that as a big uplift,’ said Hari Mix of Stanford University in California.

‘We saw a major isotopic shift at around 49 million years ago, in southwest Montana,’ Mix said. ‘And another one at 39 million years ago, in northern Nevada,’ as the uplift moved southward. It’s generally believed that the uplift was caused when the Farallon plate peeled away from the underside of the continent as it was forced under the North American plate.

February 2011