Stone Tool Analysis: Conchoidal initiation
Conchoidal initiations are caused when force is applied away from the edge of the core. This causes a cone-shaped crack to form, called a Hertzian cone. If the cone is initiated near the edge of the core, and the angle between the struck platform and the exterior face of the core is less than about 90 degrees, the crack induced by Hertzian cone can change shape and run down the core face.
Hard hammers are typically used to apply force away from the edge of the core. Hard hammers concentrate the force in one small spot, initiating the tiny circular crack that grows into the Hertzian cone. Conchoidal initiations are most common with the used of hard hammers, but can also occur with soft hammers if the blow is struck away from the edge of the core and the force is sufficiently concentrated to cause a tiny circular crack to start. Conchoidal initiations are typical of pressure flaking because of the way the force is concentrated by the small end of the pressure tool. Conchoidal initiations also occur when soft punches are used in indirect percussion.