Interest in extremely long periods of time sets geology and astronomy apart from other sciences. Geologists think in terms of billions of years for the age of Earth and its oldest rocks ― numbers that, like the national debt, are not easily comprehended. Nevertheless, the _________________________
are important for environmental geologists because they provide a way to measure human impacts on the natural world. For example, we would like to know the rate of natural soil formation from solid rock to determine whether topsoil erosion from agriculture is too great. Likewise, understanding how climate has changed over millions of years is vital to properly assess current global warming trends. Clues to past environmental change are well preserved in many different kinds of rocks.
