Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/269

 7. The Northern Irish Ice-system. — The most conspicuous moraine that I have seen in the British Isles is at a point at the northern horn of Donegal Bay, at a place called Clogher. It is marked by dots upon the inch-scale Ordnance Map. It is there nearly three inches long, and it consists of at least six parallel ridges of angular stones. The largest of these are as big as small houses ; and they rest where they were tilted off the ice, like stones shot from the end of breakwaters at Plymouth and Holyhead. Above this conspicuous moraine is " Cruach Beg," a hill of puddingstone ("? pebble-beds of the Old Bed) which is glaciated up to the top, 860 feet. The marks run along the top and side of the ridge horizontally, parallel to the moraine, aiming over the sea at the low country about Lough Conn, and at the head of Clew Bay, beyond that low gap. Here is one side of the bed of a glacier as deep as the hill is high ; but the other side was over in Sligo, beyond Donegal Bay*.

Following Donegal Bay round the coast, and looking to striated rocks, ridges of drift, and all other marks known to me, it seemed clear that the whole area of the bay, and all the lowlands about it, from Barnes Gap and the hills about it round by Lough-Erne side and Ballyshannon, past Sligo to Loch Conn, were covered by a sheet of ice which bore heavily upon a hill-top 860 feet high, at Clogher, near the end of Donegal.

But the low lands about Lough Conn are glaciated as Sweden is ; and great stones, like those which are in situ about Clogher and Slieve Liag, are scattered about the low lands at which striae point from " Cruach Beg " and Teelin Harbour.

Clew Bay is like Donegal Bay. The low grounds are all made of long ridges and furrows of drift which point westward, as do glacial striae and other marks down to Achill Head, along the northern coast of the bay. From Barnes Gap, east of Donegal Bay, to the northern horn of Clew Bay, there was continuous ice moving seawards, as it appears to me. But that was not the limit of the Irish ice according to its marks.

Depth. — Vast time has elapsed since the local systems were united. The weather has worn out many tracks, and chiefly those which were highest and oldest. The ice was more than 2000 feet deep at many places ; but it must have been far deeper. In Connemara is Shan Folagh, a ridge of hard quartz standing apart from the rest of the group of mountains, and about 2000 feet high. On this isolated top the rock is well glaciated, chiefly from the north-east.

The Atlantic is on one side ; and the nearest block of ground of equal height in the other direction is in Antrim or in Scotland. Ice must have gone over this hill. I once thought it was drift-ice afloat ; I now think it was part of the ice which covered Ireland from Donegal to Gal way.

High marks. — Beginning with the smallest class of mountain- glaciers, Irish marks have led back to large river-glaciers, to small and large local systems, to a combination of several local systems in an estuary of glaciers in Donegal Bay, to the union of two estuaries in


 * Rubbings were shown.

p2