When extension occurs to an angle to the layering, necking and thinning in stronger layers can initiate via the development of shear fractures. The boudins evolve with a strong asymmetric shape, and each boudin rotates during extension, while developing a sigmoid shape. Asymmetric boudinage can also evolve from shearing when the shear zone cut through a layered rock formation. In this case the sense of shear can be inferred by the sigmoid shape of the boudins.

Left: A competent carbonate layer in a softer pyritic phyllite was submitted to oblique lengthening (i.e., NNW-SSE). A series of shear fracture develops, one of then evolving into an extensional fracture in which pyrite accumulates. The photo is ca. 8 cm long.
Right: Example of asymmetric boudinage of a quartz vein in a micaschist. This asymmetric boudinage is here due to shearing. The shape of the sigmoids and the angle between shear bands and the quartz vein suggest a top to the right sense of shear.