Above: Killary Bay, a drowned valley. The 16 km valley was deepened by ice carving down through soft Ordovician slates. The beds of slate rise through a series of successions beginning with coarse, purple coloured river borne material. Then come a series of marine sandstones and mudstones, showing that the sea was becoming less deep over time. Right: The limstone shore of Lough Corrib. This is a landscape that can be Patricia Walsh, the driving force vanished Iapetus ocean. The enormous traced back to a time when Ireland as behind the nearby Petersburg Outdoor Laurentia continent straddled what is we know it now did not exist. Most of Centre at Clonbur, remarked that “In now the equator, and south of this, but the rocks rising up into the peaks west summer I have waked along the canal, moving slowly northwards, was another of Lough Mask were laid down over and in winter I have kayaked it.” continent, Avalonia. Eventually the 450 million years ago in the Ordovican The disappearing canal, and the continents collided, closing the Iapetus era, and at that time this area was part eroded limestone blocks that are so Ocean, and forcing what is now the south of a continental margin by the long prominent in walls around The Neale east of Ireland and Britain, up against and Cong, are just a few of the the deformed and crumpled edge of geologically interesting features that Laurentia. By the late Silurian, about Patricia can point to in the area. 410 million years ago, the two shores The lakes themselves lie on the welded together into what we now western edge of the Carboniferous see running right across Ireland as limestone plane, and the grey rock, the Iapetus Suture. That was the end while hard, is easily dissolved by of the Iapetus Ocean, and for a long slightly acidic rainwater. Where time afterwards the area we now there is limestone, there are usually know as Ireland, was just one small caves, but unlike the Burren in part of a greater landmass. This is County Clare, which is famous why the rocks of Newfoundland for its caves, the water table is so are so similar to those in north west high in south Mayo and Galway Ireland, and the Atlantic, that now that underground streams do not forms our western coast, did not have to drop, but continue to flow yet exist. The Atlantic only began to horizontally. Part of the charm of open more than 300 million years Cong, with its famous monastery, later, a process that continues, and Worn into a variety of shapes and patterns, the limestone is that it stands just above swirling will continue for a long time to is to porous that water flows from Lough Mask into Lough water than wells up from below. come. Corrib without reaching the surface. Whatever caverns exist in the area We can only imagine what the usually remain flooded, and one of ancient continent of Laurentia the exceptions is Poll na Gollum, the looked like, but it must have been Pigeon Hole, by Cong, with its 61 a barren rocky landscape with fast steps down into the limestone. running torrents carrying debris Lough Mask is hemmed in down from bare mountains to the to the west by some of the most Iapetus shore. These ocean coast ancient rocks in Ireland, and this sediments remain in Ireland’s rugged landscape is all part of what north west as a six kilometre deep is known as the Joyce Country. succession of rocks. Named from one of the old 14th These Ordovician rocks, altered, century clans, variously known as cracked, and folded, now form the Joys, Joyces or Seoighs, the area a broad east west synclinal belt is spectacular wilderness, and for that extends through the Partry, geologists, it is the next best thing to Mweelrea and Sheffrey mountains. a Silurian and Ordovician paradise. The lower end of this formation with
SCIENCE SPIN Issue 47 Page 24