The mechanism by which valley incision limits the elevation of mountain peaks in a collision zone. At the same time as the colliding plates push the mountains upward, rivers and glaciers carve deep valleys that increase the slope of mountainsides. When the slopes exceed a threshold value, landslides (see mass-wasting) reduce the slope to the angle of repose while removing material from higher levels. A balance is reached at which landslides maintain a constant mountainside slope while also constraining the increase in mountain elevation.