The rocks that form the backbone of the Sierra Nevadas formed when an arc-shaped chain of volcanoes erupted where the Sierra Nevadas now stand. Rising through older rock, molten lava erupted. The volcanoes grew, but even as they grew, erosion was wearing them away and the once-deep granite began to be exposed at the earth's surface. Eventually so much of the upper part had worn away that the surface of the ancient range had a low elevation of just a few thousand feet.

Later, the continental crust east of the Sierra Nevadas began to stretch in an east-west direction. The crust broke into a series of valleys and mountain ranges. Over time, the range that we now know as the Sierra Nevadas began to rise along its eastern edge. Not long after the Sierra Nevadas' uplift began, the earth cooled, marking the beginning of the Ice Age. Glaciers grew in the Sierra highlands and made their way down former stream channels, carving U-shaped valleys. The sheer walls and hanging valleys of Yosemite National Park are a product of this chilly past.