A pressure-dissolution structure commonly found in chalks and other carbonates. The stylolite appears as a jagged line on the rock surface. Increasing pressure causes the carbonate ions to dossolve and migrate. This does not occur along a straigh line, bit along crystal boundaries, which in turn gives rise to the jagged appearence of the stylolite. See http://www.geo.lsa.umich.edu/~vdpluijm/animations/stylolite.mov for a video of formation.
Stylolite in Upper Cretaceous Chalk, Flamborough Head.