A tool mark is an impression, cut, scratch, gouge, or abrasion made by a instrument in contact with an object. In forensic science, detectives look for a gouge in the windowsill made by a burglar’s crowbar or the scratch of a key on a car door. Earth scientists look for an erosion impression or gouge made in sedimentary rock by a solid object such as a mudstone fragment, a shell, or bits of waterlogged plant material—twigs or branches. Tool mark also refers to the traces of abrasion made by an object dragged across a rock layer during glaciation or sedimentation. These marks include grooves and prod, skip, and bounce marks, each deriving its shape from the indentation of the object on the surface. At a crime scene, detectives rarely ﬁnd the crowbar—just the impression left in the windowsill wood or on the car door. Likewise, earth scientists rarely ﬁnd the tools on the bed surface—only their imprint or trace on the rock.