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Geology, soils & Paleobotany



The geology of Clare Island has been the subject of considerable study. Because of its complexity, and again due to its'islandness', it has become a favourite case study for geology students.The New Survey of Clare Island has again undertaken research into the rocksof the island. In recent years, a lively academic debate has taken place as to the age of some of the rock formations, as well as how these formations came about

   In what follows, we borrow heavily from a 'layman's' account of the geology written for us by Professor Adrian Philips1.

1. Leic and Ballytoughey Formations

Prof. Philips writes that "the dramatic and extraordinarily variedlandscape of Clare Island is a result of the concentration into such a small area of many different types of rocks which were formed over a timespan of about one thousand million years."He sees the rocks of the island as an exceptionally valuable manuscript.The rocks are also one of the primary natural resources influencing not only the present landscape but also past, present and future life on the island.

  Geologically, the island can be divided into three sections, each of which have been formed at different periods of time. Nature has drawn a line from the steep cliffs on the north side of the Big Hill to the sandy beach by the harbour. From sea, or from the mainland, this line is quite visible. The fault which forms the cliffs on the mountain is carried through the island. At the foot of the inland cliffs is a road known as the 'Leic Road'. The visitor who passes by the great cliff-face at Leic is traversing one of the major fault systems of Ireland. The Leic Fault can be traced north eastwards through Donegal to the Great Glen Fault system of Scotland. Vertical movement on this fault produced the dramatic cliff face at Leic where one section of the Earth's crust was pushed upwards relative to the other, while horizontal movement brought ancient rocks to the northern part of the island from 160 kilometers away to the northeast.


  • If you walk to the lighthouse on the northern tip of the island and from there move south-west along the cliffs you will pass through a series of steeply inclined craggs. These cliffs plunge nearly 150 metres into the sea. While movements on the Leic fault occured around 360 million years ago, the craggs at Ballytoughey were being formed 100 million years earlier.. The age of the Ballytoughy formation is under dispute. The youngest part of the island, the eastern part known as Capnagower and the area south east of Leic, was dumped from a glacier a mere 17,000 years ago. We can walk through Leic from Ballytoughey to Capnagower, traversing millions of years of geological time, all the while experiencing the illusion that the earth upon which we are walking is eternal, constant since the beginning of time. But when we write out all the digits we get some idea of the amazing chronological span of this geological evolution:
  • young hillocks and hummocks of 17,000 years compared with ancient cliff faces of 36,000,000 years.

Geological Map of the Ballytoughey Formation:

Geological Map of the Ballytoohy Formationlegend.gif (6066 bytes)


  • The oldest part of Clare Island forms low rock ridges within a two kilometer long and 200 metre wide belt of land in the southern part of the island between Kill and Portariff. These rocks are seen again to the east along the south side of Clew Bay. They extend as far as Manorhamilton in County Leitrim, forming the Ox Mountainsalong the way. These rocks were formed from layers of mud and silt, probably deposited on the bed of an ancient shallow sea. The age of these rocks is unknown but they were probably formed at least 600 million years ago and possibly more than  1000 million years ago.

These rocks form a link to the remote past. It is not, however, a lifeless past. Hidden in the rocks of Clare Island is also a record of past life.The oldest fossil animal ever found in Ireland comes from the beds of chertin the townland of Ballytoughey More in the northern quarter of Clare Island. It is the fossil of a sponge probably 540 million years old.

More recently, another fossil of slightly younger age was found  in 1997 by Dr. John Graham.

fossilYounger microfossils, some 470 million years old, have been found in younger layers of this same succession of rocks, known as the Ballytoughey Formation. These rocks are best seen near the lighthouse and along the northwestern cliffs of the island. Therocks of the Ballytoughey Formation were folded and refolded at least four times during major earth movements 460 million years ago.

South of the Leic Fault, the rocks are considerably younger than the folded crevices in Ballytoughey and almost half the age of the low rock ridge between Kill and Portariff. The sedimentary rocks south of Leic were laid down along the southern coast of the North American continent. At this time the southeast portion of Ireland was attached to the European continent and a major ocean lay between it and the part of Ireland that was attached to the American continent.

Above this second layer of rocks lies a third, up to 300 metres in thickness, of interbedded red and green siltstones, sandstones and beds of volcanic ash.These rocks contain numerous traces of evidence of deposition in very shallow water.

The main bulk of the Big Hill is formed by a major unit of sandstones, best seen at the west end of the island around the ruins of the old Signal Tower at Toormore. Along the steep cliffs on the west side, several horizons of calcareous siltstones provide a limey soil. It is here that the rare alpine flora find their unique island habitat.




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