20 people had minor injuries from flying, gravel size rock chips. Thirteen visitors were transported to hospitals for treatment. One person with a knee injury was held overnight. This rock fall occurred because of natural weathering and erosion processes. The basalt cliffs have many natural cracks and fractures in them.
Water gets into the cracks, freezes, expands and pushes the cracks apart.
The exposed tan colored area, where the rock slid out from, indicates there was a crack for a long time. The upper part where the rock broke from is darker and shows more recent fractures. When the rock became too heavy for the unfractured part to hold it up, it broke loose and fell due to gravity. This natural process of weathering and erosion has created the beautiful, bowl-shaped notch in the Multnomah Falls cliff. The basalt type rock which falls here is called brickbat and breaks into many small pieces when it falls.