Geologists have known for long that formation of mountains affects the Earth's ecosystems. But due to the uncertainty in establishing the age of the highest mountains, the relationship between formation of topography and tectonic processes is unclear. One way to ascertain the variation of physical features is the effect that the oxygen and hydrogen isotope ratios of rainfall decrease with elevation. Some geologists have used this technique to find out that the Sierra Nevada in USA was formed 16 million years ago ( Science , Vol 287, No 5456).