The Old Red Sandstone lives up to its name! It is the oldest rock formation in the city and is marked by its strong red colour.
Though, it can also be seen in many other hues. In some areas the rock may be purple, green or grey. The change of colour is owed to the chemical composition of the rock, primarily through the oxidation of iron. It is the iron, indeed, that makes most of the rocks due to rusting (iron oxides such as hematite forming).
Looking at one of the few extensive outcrops of the sandstone in the city, try to imagine this 10 metres of rock extending 7 km / 4.3 miles further down into ground . It is difficult to imagine, so we have provided a little illustration of what that would look like if the rock layers extended 7 km up into air!
Geologists refer to the area that these sediments filled in as the Munster Basin. It describes the lowlands in what is now Southern Ireland in which sediments during the Devonian time period filled up. Back then, the sediments were mostly loose sand and mud, but over millions of years they turned into solid rock as seen in the quarries.