Boudins form due to a process called boudinage. Boudins form when lengthening affects a layered rock formation involving competent layers embedded into a less competent, easily deformable, host rock. Upon extension, the stronger layers lengthen via heterogeneous thinning leading to the development of pinch and swell structures (i.e. thinning of the strong layer is periodic). Amplification of thinning in the pinched regions eventually led to the segmentation of the stronger layers into boudins separated by necks.

The strength contrast and the direction of extension with respect to the layer impose a strong control on the geometry of the boudins