Exfoliation joints or sheet joints are surface-parallel fracture systems in rock, and often leading to erosion of concentric slabs. (See Joint (geology)). General characteristics of exfoliation joints Commonly follow topography. Divide the rock into sub-planar slabs. Joint spacing increases with depth from a few centimeters near the surface to a few meters Maximum depth of observed occurrence is around 100 meters. Deeper joints have a larger radius of curvature, which tends to round the corners of the landscape as material is eroded Fracture mode is tensile Occur in many different lithologies and climate zones, not unique to glaciated landscapes. Host rock is generally sparsely jointed, fairly isotropic, and has high compressive strength. Can have concave and convex upwards curvatures. Often associated with secondary compressive forms such as arching, buckling, and A-tents (buckled slabs) Formation of exfoliation joints Despite their common occurrence in many different landscapes, geologists have yet to reach an agreement on a general theory of exfoliation joint formation. Many different theories have been suggested, below is a short overview of the most common. Removal of overburden and rebound This theory was originally proposed by the pioneering geomorphologist Grove Karl Gilbert in 1904. The basis of this theory is that erosion of overburden and exhumation of deeply buried rock to the ground surface allows previously compressed rock to expand radially, creating tensile stress and fracturing the rock in layers parallel to the ground surface. The description of this mechanism has led to alternate terms for exfoliation joints, including pressure release or offloading joints. Though the logic of this theory is appealing, there are many inconsistencies with field and laboratory observations suggesting that it may be incomplete, such as: Exfoliation joints can be found in rocks that have never been deeply buried. Laboratory studies show that simple compression and relaxation of rock samples under realistic conditions does not cause fracturing. Exfoliation joints are most commonly found in regions of surface-parallel compressive stress, whereas this theory calls for them to occur in zones of extension. One possible extension of this theory to match with the compressive stress theory (outlined below) is as follows (Goodman, 1989): The exhumation of deeply buried rocks relieves vertical stress, but horizontal stresses can remain in a competent rock mass since the medium is laterally confined. Horizontal stresses become aligned with the current ground surface as the vertical stress drops to zero at this boundary. Thus large