Like a ripple on a pond or a blot on a page ironstone concretions spread out across the layers of sandstone in Coxley Quarry so I guess that they must have formed after the sandbanks were laid down, perhaps during the process of solidification.
There’s a sausage-shaped patch of pure white sand a couple of feet across which is encased in a rusty crust. It looks as if the iron has been leached out by a mineral-rich solution and I guess that this then bubbled upwards through the sand because above the lens of white there’s a knobbly network of weathered-out rusty chambers.
There are also rolled pebbles of ironstone. What seems to have happened here is that a rubbery crust of iron-rich gunge has formed on the bed of the prehistoric river then a strong current has dislodged it and trundled it into an ironstone Swiss roll. Iron is deposited when river water, rich in iron salts, meets brackish water.
The surface of the Earth would be lacking in colour if it wasn’t for iron-rich minerals which range through ochre yellows, rusty reds and mineral greens to the fool’s gold of iron pyrites.