It is in a small way a sort of vegetable pump which raises water a few inches or so. Stem and leaves and branches possess little cistern cells, which act both as capillary tubes raising the water and also retain it. The stems are upright and develop many branches, so that they become a close-ranked or serried carpet of upright moss-stems squeezed together, which floats on the surface of the water. Each moss-stem is growing upwards and dying off below. In consequence, the bottom gets filled up by dead mossy pieces, which accumulate there, while the live moss-carpet remains floating on the surface of the loathly, black, peaty water.
In many peat-mosses the water gets entirely filled up, but that does not stop the formation of the peat-moss. It is now resting on the water-saturated remains of its forefathers, and if water is abundantly supplied it goes on developing.
Thus in these lowland or estuarine peat-mosses the moss eventually occupies the water, and goes on growing. After this it develops like the moorland mosses which cover most of the Lowlands and Highlands of Scotland. They cover the hills, and it looks exactly as if some giant had plastered all those hills with a layer of six to ten feet of black peat from 1250 feet upwards.
The soil would at first be covered by a saturated moss-carpet of Sphagnum and other mosses. Rainwater falling upon it was all retained, and very little could get away, for the Sphagnum carpet is just like a huge sponge soaking up and retaining the water.
But it sometimes happens in these great upland mosses that there are enormous falls of rain which continue for days. Then the water collects under the living moss-carpet and over the dead peat. It may be gathered together in such quantities that the carpet of living peat above it bursts, and a deluge of peaty water overflows the surrounding country, destroying and spoiling everything that it encounters.
The worst of these inundations of black mud that has happened in recent years was in December, 1896, near Rathmore, where 200 acres of bog burst and a horrible river of mud overflowed the country for ten miles. Nine people perished, and enormous destruction was caused.
There have been many other cases. In 1824 Crowhill Bog, near Keighley, burst; and in 1745, in Lancashire, a space a mile long and half a mile broad was covered by peaty mud. There was also a case in 1697, where forty acres of bog at Charleville burst in the same way.[153]
Attempts have often been made to calculate the rate of growth of such peat-mosses. A great many of them began to develop on the mud left by the ice-sheet when the glaciers retreated at the end of the Ice Age. Those mosses are therefore probably 200,000 years old. Some of our Scotch mosses are twenty to twenty-five feet in depth, which gives a foot in 10,000 years. By calculation of the weight of the peat formed, Aigner made out that a certain moss was 20,600 years old, and was growing at the rate of two inches in a century.
But in Denmark ten feet has been formed in 250 to 300 years, and in Switzerland three to four feet of peat-moss has been formed in twenty-four years.
This shows quite distinctly that there is no regular rate of growth, and indeed it is obvious that much must depend on the climate, on the rainfall, on the drainage, and other circumstances.