See these thin little squiggly gray lines in the rock in the photo above? These are what you call stylolites.
Stylolites are caused by pressure dissolution, meaning that a large amount of stress was applied to the rock in a direction perpendicular to which to stylolites are formed. Because of this pressure applied, there’s a dissolving zone, or stylolitic seam, with some type of fluid present removing the original rock and leaving insoluble rock elements like clay and iron oxides behind (which are what the visible gray lines in the photo are).
Most stylolites are found in carbonated rocks like limestones, dolostones, or marbles.
These particular stylolites were found along a window sill of my dorm from freshman year, so geology is really always all around you!
Photo by author
Emily Zawacki, Ph.D. 