Ireland's relief is not especially favorable. The island suggests a very irregular and shallow bowl, with a muchbroken rim of low, rounded mountains surrounding a central plain. The plain itself is broken by mountains. No part of the country is over 40 miles from mountains. The rim consists of a combination of the old Caledonian and Hercynian mountain systems, while the depression is filled with Carboniferous limestone. Evidences of recent Alpine mountain-building appear in the break between Ireland and Great Britain; in the accompanying eruptions of basalt that are represented by the Plateau of Antrim in the northeast; in the structural depression of Lake Neagh in that same region; and in the uplift and revival of some of the old mountain systems in the south, especially the sandstone Mountains of Kerry in the far west which attain elevations of 3,000 feet. The Irish mountains are mostly knobs of especially hard and resistant rock, such as the granite uplands of Wicklow, south of Dublin; Connemara, in the center of the west coast; and Donegal in the northwest. All mountains are bad for Ireland. Not only are altitudes above a thousand feet or even less too cool for agriculture, but mountains promote cloudiness and rainfall, especially on their western and seaward slopes. Such conditions are disastrous to agriculture in so marginal a country.
Another handicap of Ireland is that much of the present topography is the result of the erosion of soluble limestone, with its consequent unusual forms of relief. The lowland consists almost wholly of limestone, and in many cases this results in vast areas of bog. The central plain is a typical karst region characterized by underground drainage, and numerous sink holes and other undrained hollows. The largest of these hollows are occupied by numerous lowlying lakes not more than about a hundred feet above sealevel; thousands of smaller hollows contain bogs. Thus large parts of the lower land are useless for cultivation.
The Ice Age did not improve matters much. Glaciers of local origin covered the mountains, making them smooth and rounded, and carrying away a great part of the soil. Some parts of the lowland have profited from the glacial drift which was thus carried down, but in many places the number of lakes and bogs has been greatly increased.