of the rocks in the canyon walls helps to break the rocks down into smaller particles which can be eroded by stormwater deeper into the canyon and eventually into the Colorado River. The canyon continues to grow at an imperceptible rate.
The rate at which intervals erode are related to the hardness of the different lithologies; The metamorphic and granitic rocks near the base of the canyon and the well cemented sandstones weather at much slower rates than shales.