Rivers Without End

The rivers and wadis flowing from the high lands to the west and north drained into the centre of this desert where they eventually soaked into the sand, or spread out and evaporated. They did not get as far as the coast in the south, and the sediment they carried was deposited, baking in the sun, in vast sheets across the plain. Silt and mud cracked as it dried, hot winds whipped dry sand into banks and dunes. Subsequent layers swept across in seasonal floods, filling in cracks, flowing around dunes and sand ripples. Year after year the sediment flowed in and built up in depth. This area was a subsiding basin, known now as the Munster Basin, accumulating sediment and sinking into the earth’s crust as it did so. It would eventually become the deepest known terrestrial sedimentary basin in the European area, with over 7,000 metres of sediment at the deepest point.

That is... seven kilometres deep.

During the Middle Devonian and into the Late Devonian, between 390 and 360 million years ago, the flood channels, wadis and sediment fans crept out further and further into the basin, and these longer drainage systems resulted in a greater sorting of sediment, and rounding of particles by erosion during transport. Vast quantities of sediment were deposited on the surface of this basin and built up to become a large buried body of siliceous rock, lying in horizontal layers, one over the other. The sediment ranged from coarse sand to finer rounded sand, with silts and muds becoming more prominent in the quieter reaches of the channels and farther positions of the fans. The sediments were sorted by weathering and erosion in the desert environment, resulting in a high proportion of the more stable and resistant mineral silica. The aridity of the environment meant that iron and other metal oxides were also deposited, and oxidation could take place in the presence of air, which, over time, gave rise to the colouring of the rock, the reds, browns and purples we see all around us today.

Some of the sedimentary rocks in the deeper parts of the basin underwent changes, metamorphosed by the heat at depth. This is thought to have exceeded 300°C as a result of the thinner crust caused by the extension that gave rise to the basin. So these older rocks are harder and more brittle as a result – this is how and when the local slates quarried at Valencia, Rosscarbery, and Drinagh were formed and why so much of the rock in the area is as sharp as steel, hard on the hands, on the ploughs, and resistant to erosion.

NEXT - Palaeoecology of a Devonian Flood Plain - with all this warmth, water and sediment,conditions were ripe for a population explosion.