Columnar Basalt

I not only had a lovely evening walk along the beach, near Shag Rock, but I learnt something about Shag Rock's structure. Apparently the rocks are columnar basalt - it's fascinating! I've often wondered about the column-like shapes of the rocks that have been revealed since the earthquakes shook poor old Shag Rock apart. So, thanks to Wikipedia . . . . .

Columnar basalt

During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it can't easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns.