The power of gravity on Earth is inescapable. Hold your hand high above your head, and then relax your muscles. What happens? Now, hold a book in the air and release it. Again, what happens? The force of gravity tugs at both your arm and the book, pulling them down towards the center of the Earth.
The same force that pulls your arm down is also ever at work pulling rocks, boulders, dirt and dust downward. Whenever the opportunity presents itself, gravity pulls a rock lower and lower towards the lowest surface possible.
Rocks, dirt and soil lie on the side of a mountain or hill, apparently unmovable. For many hundreds or even thousands of years the rocks and dirt change very little. Over time, however, as small amounts of dirt and additional rocks are added to the pile, the weight and mass of the pile build up. In one sudden and grand event, the entire pile might move several hundred feet within only a couple of minutes or seconds, only to once again come to rest on the side of the mountain or hill, waiting for the next event.