| 1. Symposia / Historical Safaris
3. Research 4. Surveys
7. Folklore 9. Contact us 11. Links |
 A collision between the North American and European continental plates,some 390 million years ago, resulted in a westward movement along the LeicFault of north Clare Island relative to the southern part of the island.These major movements of continental crust along the Leic Fault opened up deep fractures which allowed moltenrocks and hot gases of very unusual composition to rise from great depths. The mineralcontent and chemical composition of these rocks is very similar to a rock known as kimberlite. Kimberlites provide one of the main sources of diamonds, though a detailed study of these rocks on Clare Island has shown that there is very little chance of diamonds actually occurring in them. The most spectacular place to see these rocks, referred to as 'psuedo kimberlites' by Prof. Philips, is north westwards along the cliffs from the Signal Tower, at the top of the great chasm known as Aill Leathu (Allahan). Here the sea has cut into the west side of Croaghmore. Various forms of alterations, seen particularly at Leic and in the region of the lighthouse, have produced barite, graphite and fine grained silica, indicating the possible occurrence of gold mineralization similar to that proven on Croagh Patrick. Prof.Philips points out that the areas of interest for gold exploration are also some of the finest scenic areas on the islands, and he warns that"far-sighted wisdom in the use of natural resources may well be needed in the near future of Clare Island." The older rocks of the Ballytoughey Formation are overlain by at least 300metres of almost flat-lying sedimentary rocks some 330 million years old.These rocks are best seen in the low eastern cliffs in townlands of Maumand Capnagower. There are many relics of plant life preserved in these rocks. The most obvious features are small twigs, branches and roots of trees preserved either with coaly material or in castes of sandstone. A large number of grey shale horizons have yielded microscopic plant spores. Several of these spores are rare or absent in Europe but are well known in Canada. This is not surprising, according to Prof. Phillips, because these rocks were formed before the Atlantic Ocean separated Ireland from Canada. Somewhere in Canada is another piece of Clare Island. On this evidence, the evolution of most of the rocks which make up the hard surface of Clare Island had taken place 300 million years ago, but a lot more had to happen before the shape which meets the eye from the hill above Westport had finally formed. Croaghmore had to be pushed upwards. The Leic Fault had to form.Successions of ice: thawing, melting, moving, depositing their moraine,and scratching the rock surface had yet to come. On the top of the Big Hill there are relics of a cap of fine-grained sandstone of Lower Carboniferous age. These 330 million year old rocks are related to the flat sedimentary rocks seen in the cliffs of Maum and Capnagower. The uplift of the mountain, by some 600 metres along the Leic Fault, relative to the north of the island took place about 60 million years ago. There is growing evidence that most of Ireland's mountains were uplifted at this time as a result of the major earth movements and volcanic activity associated with the opening of the North Atlantic Ocean. The grooves on the fault plane at Leic are horizontal, not vertical, which means that the earth movements which formed them involved horizontal rather than purely vertical movements. Recent horizontal fault movement on the Leic Fault could also explain a very curious feature of the landscape of southern Clare Island. There is a large southwesterly sloping valley, known as Cummer, between Knocknaveen (Glen Hill) and Croaghmore. This valley does not continue north of the Leic Fault. It appears to be truncated by the fault in the same way that modern valleys are offset by very recent movement on the San Andreas fault in California. The rocks of the island are a natural continuation of those forming the hills around Louisburgh, Old Head and Croagh Patrick. The east-west axis of these hill has been offset about 5 kilometers northwestwards to form Clare Island. The island was pushed into the bay about 400 million years ago as a result of movement on the Maum Valley Fault, that is the fault responsible for the deep valley of Doolough, clearly visible from the island, and Maam Valley of Northern Connemara.
2. Signs of the last Ice-Age Further evidence of recent uplift comes from the low-lying rock platform which dominates the scenery on the south side of the island. This platform rises from about 8 metres above sea level in the harbour area, to about 40 metres above sea level at Toormore. This platform has younger deposits of boulder clay forming drumlins sitting upon it. At Toormore the southern slope of Croaghmore rises very steeply from this platform along what was probably an ancient sea-cliff line. This platform is but a mere three million years old. It has been tilted upwards to the west by far more recent earth movements, probably caused by the removal of the mass of ice that began to flow westward past Clare Island some15,000 years ago. The last landbridge between the island and the mainland was probably along the present sea-route to the island, that is, from Roonagh Head on the south side of Clew bay. The sea here is rarely more than 18 metres in depth. The exact date for the severance of the landbridge is in dispute. With the end of the last Ice Age and the final flooding of the landbridge, the last chapter of Clare Island's geological evolution is closed. The date of Clare Island's naissance, whether it was 10,000, 8,000 or 7,000 years ago, is crucial when reviewing the island's vegetational history. It was mainly over this last tie to the mainland that the bulk of the island's plant and animal species migrated.
|
