Geology

Flash floods in the desert

Valentia Slate is a sedimentary rock that formed 385 million years ago in an environment similar to Death Valley in California today. In a desert environment, occasional flash floods washed rocks, sand and silt from the eroding mountains into a nearby flood plain and these rivers then distributed the eroded material for tens of kilometres across the plain. In some places the material was composed of sand and gravel but further from the mountains, only the finer material made it that far. Each layer (called a “bed” by geologists) can, in simple terms, be linked to a flash flood one day 385 million years ago.

Much like parts of Death Valley today, a geological fault separated the lower land from the mountains and the lower land was sinking. As the land sank, over many millions of years, the sediment built up until it was several kilometres thick. The rate of accumulation has been estimated as an average of a tenth of a millimetre per year – this gives an impression of the very long periods of time involved. These rocks now make up most of South Kerry and are known as the Old Red Sandstone. The lowest part of the Old Red Sandstone that is visible today is called the Valentia Slate Formation and it is within these rocks that the Valentia Slate Quarry is located.

Flash floods in the desert

Valentia Slate is a sedimentary rock that formed 385 million years ago in an environment similar to Death Valley in California today. In a desert environment, occasional flash floods washed rocks, sand and silt from the eroding mountains into a nearby flood plain and these rivers then distributed the eroded material for tens of kilometres across the plain. In some places the material was composed of sand and gravel but further from the mountains, only the finer material made it that far. Each layer (called a “bed” by geologists) can, in simple terms, be linked to a flash flood one day 385 million years ago.

Much like parts of Death Valley today, a geological fault separated the lower land from the mountains and the lower land was sinking. As the land sank, over many millions of years, the sediment built up until it was several kilometres thick. The rate of accumulation has been estimated as an average of a tenth of a millimetre per year – this gives an impression of the very long periods of time involved. These rocks now make up most of South Kerry and are known as the Old Red Sandstone. The lowest part of the Old Red Sandstone that is visible today is called the Valentia Slate Formation and it is within these rocks that the Valentia Slate Quarry is located.

Death Valley

Plate tectonics

After this vast amount of sediment was deposited, the forces of plate tectonics folded and faulted the flat-lying layers of sediment. This is why the beds of the Old Red Sandstone are no longer flat lying and are folded. At the quarry, the beds are dipping at about 30 degrees. However, the pressure from the plate tectonics also caused the flat clay minerals in the rock to align and recrystallise themselves into what is now the “grain” of the slate. This grain is what the slate now splits along.

Fossils at Valentia

The Old Red Sandstone formed during the Devonian. At this time in Earth’s history, a type of animal that lived in the water gradually evolved to come out of the water and walk on the wet silt recently washed down by the flash floods. These animals, called “tetrapods”, were a type of large lizard and lived more than 300 million years before T-Rex evolved. On a day about 385 million years ago, one of these tetrapods walked on a bed of freshly deposited silt and made a clear set of footprints that can still be seen today at Dohilla and at other locations on Valentia. This is the oldest known evidence of the evolutionary transition of vertebrate animals from the water to the land. For this reason, the trackway on Valentia is very important scientifically and much-visited by geologists, evolutionary biologists and everyone with an interest in how life on Earth developed.

Volcanoes on Valentia

The area around the quarry and Valentia Harbour have volcanic rocks from volcanoes that were active at the same time as the Old Red Sandstone was deposited. On Beginish it is possible to see the same kind of rocks and patterns that are so familiar from the Giants Causeway but the Beginish rocks are older by more than 300 million years!

Plate tectonics

After this vast amount of sediment was deposited, the forces of plate tectonics folded and faulted the flat-lying layers of sediment. This is why the beds of the Old Red Sandstone are no longer flat lying and are folded. At the quarry, the beds are dipping at about 30 degrees. However, the pressure from the plate tectonics also caused the flat clay minerals in the rock to align and recrystallise themselves into what is now the “grain” of the slate. This grain is what the slate now splits along.

Fossils at Valentia

The Old Red Sandstone formed during the Devonian. At this time in Earth’s history, a type of animal that lived in the water gradually evolved to come out of the water and walk on the wet silt recently washed down by the flash floods. These animals, called “tetrapods”, were a type of large lizard and lived more than 300 million years before T-Rex evolved. On a day about 385 million years ago, one of these tetrapods walked on a bed of freshly deposited silt and made a clear set of footprints that can still be seen today at Dohilla and at other locations on Valentia. This is the oldest known evidence of the evolutionary transition of vertebrate animals from the water to the land. For this reason, the trackway on Valentia is very important scientifically and much-visited by geologists, evolutionary biologists and everyone with an interest in how life on Earth developed.

Volcanoes on Valentia

The area around the quarry and Valentia Harbour have volcanic rocks from volcanoes that were active at the same time as the Old Red Sandstone was deposited. On Beginish it is possible to see the same kind of rocks and patterns that are so familiar from the Giants Causeway but the Beginish rocks are older by more than 300 million years!
Tetrapod trackway Valentia
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