Rain transporting and depositing fine to medium grain quartz sand.
Is it an unusual or standard natural structure and deposition of watery material?
Are there any similar examples on any solar system planets, asteroids, comets, space object?
If deposits of chemical elements did come down from the skies, perhaps mixed with water in the denser atmosphere or in a dirty liquid deluge from another planet like Saturn, Jupiter, Venus, dry Mars, then some might look like these sandy castles?
A larger version of the sandy water running down your thumb constructions, when a child at the seaside.
The sudden formation within the last 5000 years of the great quartz sandy deserts like the Sahara? That 3 millennia BCE is according to one geologist theory. Similar to plasma mythology and Electric Universe models and predictions.
Even if this is just nature's fun East Anglia chalky sand castles, for some it is intriguing micro fluvial geology.