Stone Tool Analysis: Undulations
Compression force operates down the length of the flake, stiffening it, and shearing force separates the flake from the core; these two forces operate at right angles to each other, and when approximately balanced, the crack travels parallel to the core face. The balance shifts back-and-forth as the crack grows however, favouring one force until pulled back by the other force, and this results in an undulating crack path.
We see this as concentric ripple-like features that are concave in the direction of flake propagation, like waves radiating from a stone thrown into a pond. As the crack continues to grow and the energy available begins to drop, the crack becomes more unstable, and the undulations tend to become more pronounced. This is why undulations are usually more pronounced towards the distal end of a flake or flake scar and are rarely visible on a bulb of force. Undulations can be very pronounced and dramatic on unstable cracks, as is often seen in wedging-initiated flakes.