Flow of water through joint blocks that exhibit cavernous weathering was modeled for the Ferrar dolerite in Bull Pass, Antarctica. A peculiar moisture regime allows an analytical solution under steady-state, saturated conditions. The presence of surface coatings on the top surface of the blocks causes the matric potential gradient within the blocks to be horizontal near the surface, deflecting the flow of migrating water toward the uncoated sides during evaporation. Because weathering of the rock interior is proportional to the moisture flux, the extent of rock weathering will be similar along similar contours of the matric potential gradient. As granular disintegration occurs, it will also follow the lines of equipotennal, and cavernous weathering will result if those lines are concave.