GEOLOGY OF THE KERRY GEOPARK
[jee'âlujee]A science that deals with the history of the earth as recorded in rocks. |
Rain, wind and sun can weather everything including rocks. Flowing water can remove continents and the plates that make up the earths surface are constantly in motion.
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These rocks can be found on Knocknagantee (676m) |
600 million years ago
600 million years ago (mya), Ireland as we know it today did not exist and where we stand today was an ocean (the Proto-Atlantic) with large land masses to the north and to the south. Over time these two land masses moved towards each other, crumpling up the sea floor between them. They crashed together about 500 mya and continued to push against each other. Caught in this vice the rocks between and at the edges of the two land masses were crumpled, crushed and folded against each other. This intense crumpling pushed the rocks up into a mountain range known as the Calendonide, to the north of where South-West Kerry is located today. The rocks which made up this mountain range consisted of marine sediments originally laid down in horizontal layers on the sea floor between the two land masses. As they were raised up and crumpled and crushed, the layers were contorted into various shapes and sometimes sheared and cut.
400 million years ago
It was 400 mya. 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 the study area 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 differeing 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 study 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 Valencia Slate Formation (source of the important 19th century slate industry on Valentia Island). Next is St. Finan's Sandstone Formation which are 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.
370 million years ago
Eventually however all of the great mountain range (the Calendonide) was eroded away and the sea came in again from the south. It was now 370 mya and much of Ireland basked in a shimmering warm shallow tropical sea. Sea life was abundant and as time passed the remains of corals and shellfish built up on the sea floor and became consolidated into limestone. In time this sea also retreated.
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300 million years ago
In Central Europe another collisional event occurred towards the end of the Carboniferous (300 mya). The ORS and its overlying limestone were folded and thrust upwards into mountains. This was the Amorican fold belt which stretches from Dingle Bay to central Europe . The mountains of the Iveragh Peninsula as we know them today, were formed during this uplifting. The ORS and its various formations was folded into upward and corresponding downward folds, anticlines and synclines.
The landscape of the study area reflects the fact that both the ORS and the carboniferous rocks were folded along east-west axes i.e. the whole lot were given a push from the south. Tension cracks in the rocks are visible on the beach at Derrynane at the southern end of the pier. These are uni-directional cracks in the sandstone rock which have been filled with white quartz. They demonstrate the pressure to which the rocks of the area were subjected during their formation. The cracks taper at both ends with the maximum yield directions perpendicular to these.(David Howard)
Folding and Faulting
Examples of folding and faulting in the rocks are common throughout the study area. A large dipping fold is clearly visible to the south from Coomacista car park. At Castlecove East a good example of a large fold structure can be seen from the main roadway (N70). The individual layers within the rock represent periods of stable and consistent formation while inter-layer periods represent some environmental changes. The entire study area consists of anticlinorial and synclinorial rock formations. This area at Castlecove East is an example of an anticline several hundred metres across.
About one third along the main road to the pier on Lamb's Head is found a good example of a fault. Even though these faults occurred hundreds of millions of years ago, they maintain a weakness in the rock formation which the sea erodes forming a cave or a bay as a result. Caves and inlets, large and small found along the study area coastline frequently owe their origins to faulting.
Metalliferous ores were formed along the joint and fault planes during this Amorican folding period and include deposits of copper such as those found on Coad mountain and Valentia Island . They are often accompanied by quartz, which is highly visible in the rock face and especially evident at Coad.
After the mountains of ORS covered by carboniferous limestone had been raised, a prolonged period of weathering followed which resulted in the gradual erosion of the less resistant limestone. Limestone in the region is now confined to a narrow coastal strip between Kenmare and Blackwater Bridge. The upper layers of the ORS were also eroded and so in cross section, as one walks across the peninsula, you pass over the various formations.
20 million years ago
The youngest rock to be found in the study area is a dolerite dyke of Tertiary age at Hog's Head. This is an igneous rock formed from molten rock approximately 20 mya. It intruded into the surrounding rocks cutting its way vertically upwards as a 5 metre wide body. This type of rock is not seen anywhere else on the peninsula but geographical evidence traces it from Smerwick Harbour on the Dingle Peninsula to Glengarriff in West Cork.(David Howard). The dyke can be viewed clearly from the main road at Loher.
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The ice age
Global cooling in Europe began in the Tertiary Period culminating in the Ice Age about 1.7mya. During the Ice Age the climate alternated between cold and warm stages and Ireland 's last cold stage only ended 10,000 years ago.
One of the later events during the Cork/Kerry glaciation was the formation of a great ice centre in the upper part of Kenmare Bay , around the Templenoe area. The major glacier moved westward and was joined by several glaciers which broke through the mountains from the north and by smaller corrie glaciers. The entire ice mass moved along the southern edge of the study area. Evidence of its movement can be found in the smoothing of the rocks and the scraping of their surface (striae) at Lough Fadda and Coongar Harbour near Tahilla.
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Ice striae |
Glacial Breaches
The ice which pushed through the mountains from the north, breaking through from one valley to another formed glacial breaches. The Glenmore Breach and Staigue Breach are evidence of this type of movement.
Corrie Lakes
The evidence for the many small mountain glaciers is found in the presence of small deep lakes high up in the mountains. During the cold periods snow fell on the sheltered lee of the hills and turned to ice. Eventually the accumulating ice could no longer hang on to the hillside and began to flow down into the old river valleys. The continuous movement of ice in and out of the hollows, along with the pebbles and rocks which it carried, gradually gouged out and deepened the depressions leaving a corrie or cum filled with water when it melted. This type of feature is very common within the study area.
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Corrie lake at Coomcallee |
As the ice moved down the river valleys as valley glaciers, loaded with rock debris, it straightened out the valleys and deepened them. The landscape to the north-east of Sneem contains classic examples of corries and valley glaciation.
Some of the Corrie Lakes in the Study Area
Coomrooanig
Lough Sallagh (Esknaloughoge)
Lough Coomcallee
Lough Nambrackdarrig
Lough Coomcurrane
Slievenashaska Lough
Coomavanniha Lough
Eagles Lough
Lough Coomnacronia
Lough Coommen
Lough Sallagh (Finnararagh)
As the corries were sculpted out and the valleys were deepened the resulting rock was carried forward by the glacier. When the ice melted it was dumped. Evidence of this is found in the cross-sections of the coastal cliff between Hogs Head and Waterville which consist of moraine or glacial till. This dumped material was also responsible for blocking existing water flow channels and creating lakes. Lough Currane is the largest of such moraine locked lakes on the Iveragh Peninsula . Glacial till deposits can also be seen a little east of the Derrynane Hotel on the N70.
Kenmare Bay is also a product of the ice. It is a broad-mouthed drowned valley or ria formed in a major synclinal valley. The valley was formed by earth movements in the late Teritary Period but it only became submerged to approximately its present depth before the growth of the Quaternary Period ice sheets.
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The Old Ballroom, Killarney Road, Sneem, Co. Kerry | kerrygeopark@sccird.com | Tel: 064 75824