After the latest glacial advance of the Quaternary Ice Age, the climate of north-west Europe did not simply recover to its present level, but underwent a series of fluctuations, at times becoming warmer and drier than at present, and again approaching glacial conditions. The pioneer in the investigation of these postglacial climatic changes was the Norwegian Axel Blytt, who as long ago as 1876 made out a succession of dry and wet periods, which he termed Boreal, Atlantic, sub-Boreal and sub-Atlantic. The existence of these four periods has been abundantly confirmed; they are best shown in the peat-bogs of Norway and Sweden, where they are represented by layers of tree-stools alternating with beds of peat, but they have now been connected with de Geer's geochronological time-scale derived from the banded glacial clays. It is found that the dry, mainly cool Boreal period extended from about 6500 to 5200 B.C., the moist warm Atlantic period from 5200 to 3000 B.c., the dry warm sub-Boreal from 3000 to 850 B.C., and the wet cool sub-Atlantic from 850 B.C., to about 300 A.D. Thus the Neolithic in north-west Europe falls partly in the Atlantic and partly in the sub-Boreal period; the Bronze Age entirely in the latter. The early Iron Age, on the other hand, falls mainly in the sub-Atlantic.
A succession of dry and wet periods can be recognized over a wide area in northern and central Europe and again in Scotland and Ireland. The Swiss lake-dwellings are strong evidence of a dry climate during much of the Neolithic, because when they were established the levels of the lakes must have been very much lower than at present, especially if, as seems probable, the dwellings were first built not in the waters of the lakes but on peat-bogs on their Neolithic shores. In Scotland James Geikie obtained a sequence similar to Blytt's, though he stressed the changes of temperature rather than of rainfall. Geikie named his stages Lower Forestian, Lower Turbarian (i.e. Lower Peat), Upper Forestian and Upper Turbarian. The Scottish peat mosses have since been examined more closely by F. J. Lewis1, who found that the Upper Forest Bed extended over the whole mainland of Scotland almost to Cape Wrath, rising in places to nearly 3000 feet above sea level, or far above the present limit of trees. In the Highlands however it is split into two layers separated by one to three feet of peat, indicating a break in the dry conditions. It is interesting that a similar break in the dry Neolithic climate is shown by the history of the Swiss lake-dwellings. The Upper Forest layer is not found in the Shetlands.