Stone Tool Analysis: Bending initiation
Bend initiations are formed when the force applied to the edge of the core causes the edge to bend until it breaks. This starts the crack which then runs down the face of the core.
A Hertzian cone is not formed in bending initiations, and diagnostic features of conchoidal fracture—such as eraillures, umboes, and points of force application—are not found on bend-initiated flakes. Bend-initiated flakes lack a true bulb of force, although a subtle bulb-like swelling can occasionally be present. The mechanics of how this swelling forms in the absence of a Hertzian cone is poorly understood, but the swelling is likely caused by a reorientation of the propagating crack to the core face similar to that seen in conchoidally-initiated flakes.