Slickensides are polished, grooved surfaces that occur along shear planes within the soil. These shear planes result from the shrink-swell action of smectite clays that accompanies cycles of wetting and drying. As Vertisols are wetted, the soil volume increases; the volume then decreases as the soils dries. Slickensides form along the internal shear planes as soil aggregates move past one another in response to these volume changes.