Joints: Planar discontinuities involving no relative displacement of the adjacent blocks. Joints develop during the exhumation of rocks following erosion of the overburden. Joints result from contraction and expansion due to cooling and decompression respectively.


Stylolites and stylolitic joints: Stylolitic joints are discontinuities that result from a deformation mechanism called "pressure-solution". They form through dissolution along the surface of the joint due to compressive stress acting across the joint. Dissolution is triggered by stress concentration at the contact between grains, which put into solution molecules detached from minerals and clastic grains. Stylolitic joints are often darker than the surrouding host rock as iron-coated insoluble particles accumulate in the joint. In section, stylolitic joints are made of tooth-like cones. The cones axes can be either perpendicular to the joint or can make an angle, but they always point in the same direction to that of the maximum stress, and that's pretty cool...