Minecraft wiki entry for Basalt_pillar, associated text: Basalt pillars have their starting points in the ceiling of an area, then have a pillar of between one and three blocks wide in both the X and Z axis going all the way down to the ground, where a flat clump of basalt can be found around a 3×3 area from the center of the pillar, as if a vertical lava flow, like those commonly found in the Nether, had solidified. Possibly due to blue ice next to omnipresent lava going over a soul soil ceiling, as with the "basalt generator" design.