Geologists have divided mountains into four classifications, according to how they were formed. All mountains are the result of violent changes in the earth's surface, most of which happened millions of years ago. The Appalachian Mountains and the Alps of Europe are examples of folded mountains. Folded mountains were formed of rock layers, squeezed by great pressure into large folds. In many cases, molten lava, coming with great pressure from below the earth’s surface, lifted the rock layers resulting in dome shaped mountains e.g. the Black Hills of South Dakota. Block Mountains are the result of breaks, or faults, in the earth’s crust e.g. the Sierra Nevada Range of California is a block that is 400 miles long and 80 miles wide. Many mountain ranges have been formed by more than one of the ways described. The Rockies are mountains made by folding, faulting, doming, and even erosion of lava