I took a random scoot up the side of Bridge Mountain in Zion Canyon last week. My immediate goals were to reach the base of the Navajo sandstone for some much needed exertion and to scour the talus slopes for dino-prints.
I followed the base of the Navajo ledge eastward into Pine Creek Canyon where I terminated the scoot on the upper switchback right at the entrance to the Zion tunnel. Along the way I took photos of the geologic contact between the Navajo and Kayenta formations, with the intent of illustrating how they interact to bring about the canyon scenery we see today.
Basically it goes sumpfin' like dis here:
The porous and crumbly Navajo sandstone overlies the denser & less permeable Kayenta shale which creates a prominent spring line in Zion Canyon. Famous landmarks that are a direct result of this "contact" are the Weeping Rock, Emerald Pools, Grotto and the hanging gardens of the Narrows. Kayenta is a Navajo word that literally means "place of the springs". This friable shale layer also erodes more quickly, once exposed, causing the softer slabs of sandstone above to overhang. Eventually these precariously perched slabs violently collapse into the canyon depths below causing the thunderous rock falls that Zion is rightfully famous for. In this way the canyon gradually widens as layer upon layer peel off from the walls and are then carried away by the waters of the Rio Virgin.
Simple enough, ain't it?
Cracks form in an over-hanging slab prior to collapse.