Old Red Sandstone
The ORS rocks were deposited in a series of low-lying terrestrial basins, with lakes and rivers. These basins were between ranges of the Caledonian mountains. The ORS was laid down in this non-marine terrestrial environment. Thick deposits of sand and mud, 11,000 metres (36,000 feet) deep, often stained red by oxidized iron minerals, collected as the basins sank down.