The Present Is the Key to the Past
Part of Hall of Planet Earth.
This is a statement of the uniformitarian principle. Deceptively simple but enormously powerful, it means that the processes occurring today have operated throughout most of the Earth’s history. For example, an ancient sandstone formed exactly as a beach forms today—by the gradual buildup, over many years, of water-transported sands. The principle is one of the guiding rules for understanding rocks and landforms, reconstructing their histories, and estimating the time it took for them to form.
First Insights From Layered Rocks
Two of the most fundamental geological principles were outlined by a Danish physician, Nicolaus Steno (1638-1686). Hiking the mountains of Tuscany, in Italy, Steno realized that sedimentary rocks must have initially been deposited in horizontal layers, and that younger beds lie atop older ones. A century later, James Hutton came to another important realization: veins and dikes, both bodies of one rock inside another, must be younger than the rocks they invaded.