I'm that nerd. Structure like this comes from the soil containing 2:1 clays - clay is made of sheet like chains of silicone and oxygen

Depending on its structure, two sheets can bind together imperfectly, leaving a gap in the middle. Water gets in the gap and forces the sheets to expand (and contract when it dries out). Over decades to centuries you get structure forming. You get prismatic structure because hexagons are the most efficient shape in nature, so shear planes form when clays smush against themselves - it's the same mechanic with basalt columns, but there is volcanism driving it