plumose structures are used to determine propagation directions and, in some cases, the principal stress orientations. Shear fractures Some fractures that look like joints are actually shear fractures, which in effect are microfaults. They do not form as the result of the perpendicular opening of a fracture due to tensile stress, but through the shearing of fractures that causes lateral movement of the faces. Shear fractures can be confused with joints because the lateral offset of the fracture faces is not visible in the outcrop or in a specimen. Because of the absence of diagnostic ornamentation or the lack of any discernible movement or offset, they can be indistinguishable from joints. Such fractures occur in planar parallel sets at an angle of 60 degrees and can be of the same size and scale as joints. As a result, some "conjugate joint sets" might actually be shear fractures. Shear fractures are distinguished from joints by the presence of slickensides, the products of shearing movement parallel to the fracture surface. The slickensides are fine-scale, delicate ridge-in-groove lineations on the surface of fracture surfaces. Importance Joints are important not only in understanding the local and regional geology and geomorphology but also in developing natural resources, in the safe design of structures, and in environmental protection. Joints have a profound control on weathering and erosion of bedrock. As a result, they exert a strong control on how topography and morphology of landscapes develop. Understanding the local and regional distribution, physical character, and origin of joints is a significant part of understanding the geology and geomorphology of an area. Joints often impart a well-develop fracture-induced permeability to bedrock. As a result, joints strongly influence, even control, the natural circulation (hydrogeology) of fluids, e.g. groundwater and pollutants within aquifers, petroleum in reservoirs, and hydrothermal circulation at depth, within bedrock. Thus, joints are important to the economic and safe development of petroleum, hydrothermal, and groundwater resources and the subject of intensive research relative to these resources. Regional and local joint systems exert a strong control on how ore-forming hydrothermal fluids (consisting largely of , , and NaCl — which formed most of Earth's ore deposits) circulated within its crust. As a result, understanding their genesis, structure, chronology, and distribution is an important part of finding and profitably developing ore deposits. Finally, joints often form discontinuities that may have a large influence on the mechanical behavior (strength, deformation,