The floor of Yosemite Valley is littered with piles of rocks that crumbled off the park's iconic cliffs. These rockfalls happen all the time, because Yosemite's granite walls are riddled with cracks produced by geologic stress. Scientists know the rock falls can be triggered by things like earthquakes, rainfall, and freezing and thawing ice. But many falls occur without an obvious cause. Now researchers think that heat may be the culprit. "We noticed that there had been a bunch of rock falls that were happening in the summertime, on particularly hot days." Brian Collins, a research civil engineer with the US Geological Survey. "And we noticed when we looked at the timing that they were happening in the afternoon, when we thought the temperatures were at their hottest." Collins and Greg Stock, Yosemite's park geologist, wanted to know if small rock movements, induced by changing temperatures, might weaken cracks and contribute to rock falls.