Buried Treasure
Ironstone is a seam (layer) of rock, usually found deep underground. Cleveland’s ironstone seams began to form 180 million years ago, at the bottom of a warm shallow sea that covered this area in the Jurassic period. At that time, huge reptiles and shellfish lived in the warm waters and dinosaurs roamed the land.
Settling on the seabed
Rivers carrying iron-rich sand and silt flowed into the sea, dropping tiny particles of iron onto the seabed. Over millions of years the layers of gritty, sandy sediments slowly compacted into rocks. Ironstone seams eventually formed where the iron particles had gathered.
Ironstone uncovered
Ironstone is usually blue-grey when it is first mined, or sometimes yellowish or even purple. It turns rusty red when the iron particles on its surface react with the oxygen in the air.