The striking difference between the canyon walls is due to the amount of sunlight hitting these walls, which in turn determines the amount of freeze-thaw erosion occurring on each wall. The continuous cycle of freezing and thawing water, expanding and contracting within the canyon walls, weaken the rocks, causing them to break off or erode. Since the south-facing wall receives more direct sunlight throughout the day, any moisture that falls there quickly evaporates. Without any moisture, freeze-thaw erosion is less likely to take place. The north-facing wall is in shadow much of the day.
Water evaporates much slower in the cool shade.
The water that remains on the north-facing wall freezes and thaws throughout the winter, which increases the erosion that takes place.