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IV.—On the Formation of the “Rock-Basin” of Lough Corrib, County Galway

Published online by Cambridge University Press:  01 May 2009

G. H. Kinahan
G. H. Kinahan
Affiliation:
Geological Survey of Ireland.
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Extract

Lough Corrib is a long irregular lake of various widths, but having a general bearing of about N.W. and S.E. Its N.W. portion is in a granite and metamorphic rock country, while the rest of it overlies Carboniferous rocks, principally limestone. The northern portion is deep, the southern shallow, and through the whole of it are scattered numerous islands which, in the former part, are generally composed of Boulder-clay, while those in the latter portion are nearly always rock. Its known natural outlets are two, one being over the barrier of metamorphic rocks at Galway (U on Map, Pl. XIX), and the other subterranean passage south of Castlegar (T on Map, Pl. XIX). On all sides of the lake are rocks extending under it, or, to use Professor Ramsay's term, it lies in a “Rock-basin.” What excavated this Rock-basin? I propose in this paper to consider.

Type
Original Articles
Information
Geological Magazine , Volume 3 , Issue 29 , November 1866 , pp. 489 - 495
Copyright
Copyright © Cambridge University Press 1866

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References

page 489 note 1 The accompanying Map (Pl. XVIII. and XIX.) is copied from the Index Ordnance Map of the county Galway, and on it, copied from the Chart of Lough Corrib, are the parts of the lake that are respectively below the 100 and 50 feet contour lines; the former being surrounded with a broken line, and the latter by dots. Not to crowd the map, all the places that are referred to in this paper are marked by letters, while other names are left out.

page 490 note 1 The passage to the Castlegar outlet was closed about fifteen years ago by the Board of Works, to facilitate the navigation of the Lough.

page 491 note 1 This barrier was lowered considerably during the navigation and drainage works previously mentioned.

page 491 note 2 History of H-iar Connaught, by Roderic, O'Flaherty, Esq. Edited by James, Hardiman M.R.T.A. Pages 28 and 29.Google Scholar

page 491 note 3 The depths given are those on the chart, which was made before the drainage and navigation works; now the lake is a little shallower.

page 492 note 1 Any one who can look so far back may bring in this name—“The Old Lough”—as evidence in favour of this theory, as the place may be so called from a tradition handed down from the Pre-glacial men “of the time when the lake did not exceed these limits.”

page 492 note 2 Full particulars about the Coole Lough water-basin are given in the Mem. Geol. Survey of Ireland, Ex. sheets 115 and 116 p. 7.Google Scholar

page 492 note 3 What might be considered in favour of a subterranean passage out of this gut is, that in one place there is a hole exactly the same depth as the isolated hole in “The Old Longh,” viz., 152 feet; and also that in the metamorphic rocks there are beds of limestone, one of which might possibly be so placed that it joined into the Carboniferous Limestone, and through it the passage ran. However, against it is the fact that in the neighbouring hills no subterranean passages occur, and the boundary between the metamorphic rocks, and the Carboniferous Limestones can be traced bv the streams that flow down the hills, taking the ground as soon as they come on the latter.

page 493 note 1 For particulars about primary and secondary striæ, see Paper by the author— “Notes on some of the Drift in Ireland”—read before the R.G.S.I. “Dublin Quarterly Journal of Science,” 10., 1866.Google Scholar

page 494 note 1 This would be much more marked on a geological map, on which the bog and alluvium were coloured, as then the original dimensions of this part of the lake could be seen.

page 494 note 2 The lie of the islands can be better seen on a larger map.

page 494 note 3 Although its superficial extent has increased in some places, it really has been greatly curtailed, for many of the large bays, as previously mentioned, are filled up hy bog and alluvium. These could not be defined until the geological maps surrounding the lake are published.

page 495 note 1 In places on tbe north shore of Galway Bay, peat with the roots of trees is found below high water mark; however, this does not prove that the land has sunk, because, at the present day, about two miles west of Galway, between Blackrock and Blake's Hill, there is a morass in which peat and trees are growing. This latter is divided from the sea by a barrier of sand and gravel, and in tbe places where peat is now found below high water mark, similar barriers may once have existed.