We present a novel geologically-based method to generate vertical walls of rocky cliffs, crags or promontories. Our method procedurally generates a distribution of fractures in the bedrock to create a set of tiling blocks defined as implicit volumetric primitives. Blocks are in turn implicitly replicated over the vertical parts of the terrain and combined together to obtain a consistent volumetric representation of the fractured bedrock patterns using generalized union and blending operators. Our framework provides multiple levels of control: in addition to automatically generated blocks, the geometry of specific ones can be prescribed by the user using implicit primitives or construction trees, the shape of the blocks can be controlled by several parameters , and the placement rules may adapt according to the underlying geological strata and geometry of the terrain.