Bog is spongy land covered with peat. Bogs of great extent exist in some of the northern parts of the world. A very considerable part of the surface of Ireland is occupied with them. The Bog of Allen (see ALLEN, BOG OF) is the most extensive in the British Islands. The Solway Moss (q.v.), on the western border of England and Scotland, is about 7 miles in circumference. Chatmoss (q.v.) in Lancashire is 10 sq. m. in extent. There are large bogs in some of the Hebrides and the Orkneys. The swamps of the east of England are in general not peat-bogs, but consist chiefly of soft mud or silt.
The general surface of a bog is always nearly level, but it is usually varied with rushy tussocks rising above the rest, and having a rather firmer soil. By the continued growth of peat, the surface of a bog is gradually elevated; that of Chatmoss, for example, rises above the level of the surrounding country, having a gradual slope of 30 or 40 feet from the centre to the solid land on all sides. Occasionally, the quantity of water becoming excessive owing to continuous rains, a bog bursts, and pours a terrible deluge down the course of a stream, causing great devastation, not only by the force of its torrent, but by the enormous quantities of peat which it deposits upon meadows and cultivated fields, as has happened in some memorable instances in Ireland. In December 1896 a large bog of 200 acres burst at Rathmore, near Killarney; several lives were lost, and many miles of country laid waste. The depth of a bog is sometimes more than 40 feet. The spongy mass of which it is formed shakes on the least pressure. Sometimes it is impossible to traverse it; in other cases, it is possible only for those who are well accustomed to it, a false step being a plunge into a quagmire, in which a man sinks as in a quicksand. Safety is sometimes insured by 'pattens'—boards fastened upon the soles of the feet—a method which has been also employed to enable horses to work upon the surface. Tradition reports that at the battle of Solway in 1542, a fugitive troop of horse plunged into the moss, which instantly closed upon them.
One of the remarkable phenomena of peat-bogs is the frequent presence of roots and fallen trunks of trees, in a good state of preservation, many feet below the surface (see BOG-OAK ORNAMENTS), and this frequently where no trees exist on the surface now. Hence it is inferred that in many instances these morasses originated in the decay or partial destruction of ancient forests (see the article PEAT). Bogs are popularly divided into two classes—Red Bogs and Black Bogs; the decomposition of the vegetable matter in the former being less perfect, and the substance, consequently, more fibrous and light than in the latter. There is indeed no precise line of distinction, and all intermediate conditions occur. The most extensive and deepest bogs are the red bogs; black bogs are more numerous. See BOG PLANTS.
There can be no doubt that much of the land now occupied by bog is capable of being rendered very productive, whilst the effects of extensive bogs upon the climate are always injurious. The reclaiming of shallow mountain bogs is comparatively easy, and in some cases it is effected by a very simple and inexpensive drainage, and by throwing them at once under cultivation in a manner analogous to that known in Ireland as the lazy-bed method of planting potatoes—the soil upon which the bog rests being partially digged up and thrown over its surface. Great difficulties, however, attend the reclaiming of red bogs; the chief being in some cases caused by the low situation of the bog, and the want of fall for drainage. Another serious obstacle is presented by the spongy substance of red bogs being extremely retentive of water, so that a deep ditch only drains a very narrow strip on either side of it. Drainage is certainly effective only when the drains are cut through the peat into the underlying soil, and this operation is often too costly. Where a good soil is known to exist below, the peat is sometimes removed bodily; thus it has been floated off, as in the long-continued operations at Blair-Drummond, on the banks of the Forth. A portion of the peat taken from the upper surface is not infrequently burned in heaps upon the spot, the ashes becoming a manure, and assisting in the formation of a soil.
Of course, the first essential in the reclaiming of bogs is drainage. The method of effecting this must be varied according to circumstances; but very frequently, after a general outlet with sufficient fall has been secured, wide open drains are cut, by which the bog is divided into strips, which again are traversed and subdivided by smaller drains. When these drains begin to serve their purpose, the surface of the bog sinks, and their depth is reduced; they are then often deepened, and at last a permanent system of covered drains emptying themselves into open ditches is thus formed, and fits the land for all the purposes of agriculture. It is, however, often ploughed before this state of things is attained. Lime, calcareous sand, clay, and other manures are applied, according to circumstances, to promote the conversion of the peat into useful soil. Sometimes the first crop taken from the ploughed bog is a crop of oats; sometimes it is found preferable to begin with rape, turnip, or the like. In some places in the north of Ireland fiorin grass (see BENT-GRASS) has been sown on bogs in process of being reclaimed, and enormous crops have been obtained. See WASTE LANDS.