Exfoliation is a type of physical weathering that occurs from rock pressure at depth.
Whether the rock has cooled from a melt at depth or whether the rock has been carried to deep depths, the rock is under pressure which causes it to contract. When the rock reaches the surface through erosion and tectonics, there is a relaxation or release in pressure which allows the rock to expand. The joints that develop parallel to the surface are called exfoliation joints. These can be seen in the local mountains, in Yosemite, in areas where once deep-seated rocks are now exposed at and near the Earth's surface. Frost wedging is another type of physifcal weathering.