The climate was hot. Deserts developed over huge areas. The rocks which had been uplifted from the sea bed into the atmosphere began to be eroded and carried down the mountainside by torrential mountain streams. This was the Devonian period of Geological Time. At the base of the slopes the eroded materials were deposited in great fans of debris, which consolidated over the millennia to form Old Red Sandstone (ORS). Red is the colour of desert rocks, due to the presence of the iron oxide, haematite. Haematite provides the cement that holds what otherwise would be a lose pile of rock debris together and makes a rock out of it. Old Red Sandstone However not all the ORS within Kerry Geopark is red in colour. Green and grey coloured varieties also exist. This difference in colour is due to the fact that these rocks were laid down in differing environmental circumstances. Many of the ORS rocks consist of conglomerates i.e. rocks made up of boulders and pebbles of other rocks cemented together. However beds of finer-grained red sandstone were deposited in rivers and fine sands and muds were deposited in many temporary shallow lakes at this time. As would be expected of a desert climate, many were deposited as wind blown sand. Within the Geopark area different formations of ORS were deposited dependent on the conditions that prevailed when the sediments were laid down. This resulted in a variety of colours dependent on the original sediments that made up the rock. The lowest of the formations are the purple siltstones of the Valentia Slate Formation (source of the important 19th century slate industry on Valentia Island). Next is St. Finan's Sandstone Formation which is grey coloured rocks. The Ballinskelligs formation comes next, which are purple sandstones. Then the Transition Formation, which is found in the Sneem area. The sediments of these formations vary from fine-grained fluvial facies with a dominance of siltstones and fine sandstones at the western end of the peninsula where the sediment size rarely exceeds that of fine sand to a coarse-grained fluvial facies at the eastern end characterised by coarse and pebbly sandstones. The old red sandstone is very thick - approximately 5,200m around the Sneem area.