of water between the ice and the rock. (Water occurs there due to heat and pressure trapped under the heavy, insulating river of ice.) With this kind of lubrication, the ice tends to slide over, rather than dig deeply into, the bedrock. However, out at the margins of the glacier, colder air and thinner ice allow the glacier to freeze to the ground. Being “glued” to the sides of the valley, the passing glacier tears out pieces of bedrock and carries them away. This action is called glacial quarrying.
In order to account for the variations in canyon slope and width, you can look at how erosion reflects bedrock structure. In this canyon, the primary variable of structure is joints—cracks in the bedrock. Since joints are planes of weakness, then the more there are, the faster the rock is weathered (broken down) and carried away.
Where joints are mostly oriented vertically, glaciers break off column-like pieces from the valley sides, thereby undermining the canyon walls above. In such a place, the valley will be exceptionally flat-bottomed and steep-sided, like Yosemite Valley.
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