The Formation of the Physical Landscape

Terranes - the distinctive sets of rock material that are the basement for Ireland's geology.

This map identifies the distinct geological blocks that make up Ireland. The blocks orginated early in geological time and in different places. Major earth movements later brought them together. By approximetely 400 million years ago, the terranes in Ireland had reached their present arrangement.

Courtesy of the Geological Survey of Ireland 2006.

Terranes - the distinctive sets of rock material that are the basement for Ireland's geology.
Courtesy of the Geological Survey of Ireland 2006.

Terranes - the distinctive sets of rock material that are the basement for Ireland's geology.

This map identifies the distinct geological blocks that make up Ireland. The blocks orginated early in geological time and in different places. Major earth movements later brought them together. By approximetely 400 million years ago, the terranes in Ireland had reached their present arrangement.

Courtesy of the Geological Survey of Ireland 2006.

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One of the challenges we must confront to understand the physical landscape is to get our heads around the time-scale involved. The physical landscape we see today is the outcome of a combination of developments, some of which stretch back hundreds, even thousands, of millions of years. Three major sets of events and processes have been particularly influential. These are:

(2) denudation and weathering processes, both those operating at the present time and those which have operated, with stunning long-term effect, over tens of millions of years.

These events and processes in combination largely determine the composition of the landscape, in terms of its rocks and soils, and its shape or appearance (sometimes called its morphology).

The Valentia Tetrapod Trail

Tetrapod Trail

Amphibians are animals that can live on land as well as in water. The tracks at Valentia were made around 385 million years ago by a four-legged amphibian known as a tetrapod. At that time what is now Ireland was at or near a very large ‘supercontinent’ (comprising an amalgam of most of the present-day continents), and lay south of the equator. Since then, over hundreds of millions of years, and due to the slow rearrangement and movement of large sections of the Earth’s crust as a result of the processes known as ‘plate tectonics’, ‘Ireland’ has drifted north to its present position. The footprints the tetrapod made across the ripple-marked sandy shore became covered in muddy sediment and then were preserved as the sediment hardened and later transformed into siltstone and slate. Hundreds of millions of years later, they have been uncovered as the sea erodes the rocks along the Valentia shoreline. In the mid-1990s, a Swiss geologist, Iwan Stössel, recognised the scientific importance of the Valentia tetrapod site as a record of a major stage in the evolution of life on Earth, the time when vertebrate life left the sea and began to breathe air and walk across land. The rather inconsequential-looking track-marks preserved at this site are what may be the earliest fossil record, possibly anywhere in the world, of the activities of an amphibian animal. [For more information, see Matthew Parkes, The Valentia Tetrapod Trackway, which is available from the Geological Survey of Ireland, price €2; See also Kerri Westenberg, The rise of life on Earth: from fins to feet, National Geographic, 196, 1999, pp.114-127].

Copyright Arnold Horner.

Tetrapod Trail
Copyright Arnold Horner.

Tetrapod Trail

Amphibians are animals that can live on land as well as in water. The tracks at Valentia were made around 385 million years ago by a four-legged amphibian known as a tetrapod. At that time what is now Ireland was at or near a very large ‘supercontinent’ (comprising an amalgam of most of the present-day continents), and lay south of the equator. Since then, over hundreds of millions of years, and due to the slow rearrangement and movement of large sections of the Earth’s crust as a result of the processes known as ‘plate tectonics’, ‘Ireland’ has drifted north to its present position. The footprints the tetrapod made across the ripple-marked sandy shore became covered in muddy sediment and then were preserved as the sediment hardened and later transformed into siltstone and slate. Hundreds of millions of years later, they have been uncovered as the sea erodes the rocks along the Valentia shoreline. In the mid-1990s, a Swiss geologist, Iwan Stössel, recognised the scientific importance of the Valentia tetrapod site as a record of a major stage in the evolution of life on Earth, the time when vertebrate life left the sea and began to breathe air and walk across land. The rather inconsequential-looking track-marks preserved at this site are what may be the earliest fossil record, possibly anywhere in the world, of the activities of an amphibian animal. [For more information, see Matthew Parkes, The Valentia Tetrapod Trackway, which is available from the Geological Survey of Ireland, price €2; See also Kerri Westenberg, The rise of life on Earth: from fins to feet, National Geographic, 196, 1999, pp.114-127].

Copyright Arnold Horner.

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A unique record of the interplay of geological processes over hundreds of millions of years is seen on the Tetrapod Trail at Valentia Island, Co. Kerry. These tracks were made around 350 million years ago by a four-legged amphibian known as a tetrapod. At that time, this part of what is now Ireland, lay south of the equator. The fossil footprints denote the tetrapod's route across a ripple-marked sandy shore. As one of the earliest records of such tracks to be found anywhere in the world, the Valentia site is an important part of Ireland's geological heritage.

The Slieve Bloom Mountains

Slieve Bloom

These mountains represent another episode in the geological history of Ireland.

Courtesy of Arnold Horner 2006.

Slieve Bloom
Courtesy of Arnold Horner 2006.

Slieve Bloom

These mountains represent another episode in the geological history of Ireland.

Courtesy of Arnold Horner 2006.

Enlarge image

These mountains represent another episode in the geological history of Ireland. The rocks that make up this great upland area on the borders of Laois and Offaly include sandstones as well as rocks formed deep in the ocean; they date to the Devonian and Silurian periods, 350 to 440 million years ago. At the beginning of that time, the area that is now the north-west of Ireland and the area that is now the south-east of Ireland, were on opposite sides of a shrinking ocean known to geologists as the Iapetus Ocean. By about 410 million years ago, these two areas had collided and a great mountain-building phase was in progress. The present-day Slieve Bloom mountains lie along, or are very close to, the 'Iapetus Suture', the notional line that marks where these two areas with very different early geological histories amalgamated to create the area now known as Ireland.

The challenge in thinking about geology and the origins of the Irish landscape is to comprehend very long time periods and the great events that created features like Slieve Bloom.