Look up to the granite walls, spires, and cliffs all around you! The rock's story begins about 100 million years ago with granitic magma deep within the earth. This molten material was formed from older continental rock, as it was over-ridden by the westward-moving North American Plate. About 35 million years ago, dramatic uplift began that eventually raised the Sierra Nevada to its current height. Within the last several million years, fluctuations in temperature produced periods of glaciation over much of the Northern Hemisphere.

In mountain areas, rivers of ice, sometimes thousands of feet thick, flowed down canyons and stream courses. In Yosemite Valley, where the granites contained vertical cracks and joints, the moving ice masses tore away the fractured rock and moved it down the Merced River canyon. The ice in this Valley was once over 5,000 feet deep and covered everything you can see except the top 800 feet of Half Dome and the top of Eagle Peak just west of Yosemite Falls. Approximately 14,000 years ago, a warming trend began. The receding ice left behind a dam of rock and gravel across the western end of the Valley. As the glacier continued to melt away, a shallow lake formed. Imagine swimming to the next stop where you will discover what happened to that ancient lake.