stylolite

any of the small striated columnar or irregular structures within the strata of some limestones

A contact surface usually found between two calcareous rock layers, marked by a series of jagged interlocking up-and-down projections that resemble a suture or the tracing of a stylus. Stylolites are found in rocks that have been subjected to pressure and are formed by a process, called pressure dissolution, whereby insoluble material, such as clay and hydrocarbons, becomes concentrated along a surface while other more soluble material, such as calcite, is carried away.