Why does the Sierra Nevada have a gentle western slope and a steep eastern escarpment?

The Sierra Nevada's characteristic asymmetry is the result of its formation as an upfaulted, tilted block of the Earth's crust. The range is bounded on the east by a major fault zone, which facilitated the uplift and westward tilting of the entire mountain block.1

This geological process created the two distinct sides of the range:

• Eastern Escarpment: Because the block was uplifted along the eastern fault, it created an abrupt, steep-faced escarpment. This side has been further shaped by the erosive forces of wind, rain, temperature fluctuations, frost, and ice, resulting in a series of steep-gradient canyons.1
• Western Slope: The western side follows the geologic dip slope. Because the block is tilted toward the west, streams flow more gently down this flank, eventually forming massive alluvial fans that extend into the Central Valley of California.1

While this massive uplift began millions of years ago, a significant portion of the movement occurred within the last two million years, resulting in the dramatic relief seen today.1