Exfoliation is a form of mechanical weathering in which curved plates of
rock are stripped from rock below. This results in exfoliation domes or
dome-like hills and rounded boulders. Exfoliation domes occur along planes
of parting called joints, which are curved more or less parallel to the
surface. Exfoliation domes are best developed in granitic rock. Rocks may break apart
in layers and may also expand.

Photo by G.K. Kilbert in Sequoia National Park, exfoliation in rock

Exfoliation sheets that are parallel to the valley in Yosemite National Park
photo from http://www.ux1.eiu.edu/~cfjps/1300/weathering.html