Stone Tool Analysis: Umbo
In Hertzian cone formation, the area in direct contact with the indentor—usually less than 2-3 mm in diameter—is depressed until the stone fails and a crack starts. The tiny circular crack is sometimes visible in fine-grained stone, and is referred to as a ‘ring crack’. Often only half of the circle is visible where it is exposed when the flake detaches from the core. This semi-circular feature is referred to as the ‘umbo’.
The presence of the ring crack or umbo define the PFA. An umbo is sometimes confused with the platform lip seen in bending initiations, but an umbo’s curvature in plan view is smaller in diameter and more pronounced than a platform lip. Also, the profile of an umbo is nearly vertical relative to the platform surface, whereas a platform lip curves outward slightly towards the flake’s ventral surface. Umbos are diagnostic of conchoidally-initiated flakes, although not all conchoidal initiations will display a recognisable umbo.