Gravity is the dominant force in any canyon, it’s what causes the water to flow which carves down through the earth’s sloping surface, cutting through rock and carrying the debris ultimately out to sea. The younger rock high up on the canyon’s rim erodes and along with the power of the freezing and thawing of ice fractures and splits away rock to tumble down into the canyon creating a slope of unconsolidated rock which itself is worn away by the river. The falling rock varies widely in size with massive rocks like this one bounding down until slope and friction bring it to a stop.