The same incredible geological forces that first formed Yosemite are still at work in the park today. Perhaps nowhere can you see more dramatic evidence of that geology than right in the Yosemite Valley.
While walking around the valley floor, sheer walls rise to the north and south. Upon closer inspection, you'll notice that littered around the base of the valley's cliffs are piles of granite talus. These are the result not just of erosion, which weathers the rock, but also California's plate-tectonic activity and earthquakes, which can cause granite to fracture along joint lines and rocks to come suddenly crashing down on the valley floor.
Water is another factor in Yosemite Valley's rockfalls. Melting snow in late spring causes the valley's famous waterfalls to become more powerful. Year-round the currents of rivers and streams that originate in the Sierra Nevada high country can carry rocks and boulders right over the tops of the falls. During winter, the freezing and expanding of water into ice can also cause rocks to fracture through the stress, then to tumble down the sides of cliffs and over waterfalls.
To get a good look at evidence of rockfall in any season, just follow the easy walking trails that lead right up to the base of the valley's famous waterfalls, including Yosemite Falls and Bridalveil Fall. Check out our safety tips about avoiding rockfalls, especially in Yosemite Valley.