a system of scientific concepts concerning the repeated development of glaciers that covered enormous areas of the earth.

The idea that ancient glaciers in mountains were distributed more extensively than at present was expressed by researchers of the Alps as early as the end of the 18th century and the beginning of the 19th. But the birth of glacial theory dates to the 1840’s, when scientists associated the origin of erratic blocks scattered over large areas in Europe and North America (fragments of rock whose primary deposits were hundreds and thousands of kilometers away from the given locality) with the activity of ancient glaciers. The glacial genesis of deposits containing such blocks was established for the foothills of the Alps by the Swiss scientists J. Venez, J. Charpentier, and J. L. Agassiz; for northern Europe by the German geologist A. Bernhardi (1832); and for Scandinavia (1840) and North America (1846) by J. L. Agassiz. However, even until the middle of the 1870’s, a majority of scientists felt that the deposits that contained erratic blocks were marine sediments, among which the blocks were scattered at some time by icebergs. These views, which were termed drift theory, were developed most fully in the 1830’s by the English geologist C. Lyell. Great contributions to the discrediting of drift theory and the strengthening of the idea of continental glaciation were made by the Russian scientists K. F. Rul’e (1852), G. E. Shchurovskii (1856), F. B. Shmidt (1867), and P. A. Kropotkin (1876); the English scientist J. Geikie (1879); and the Swedish scientist O. Torell (1875). Their works contributed to the rapid and universal recognition of glacial theory.

Glacial theory is mainly concerned with the last stage of the earth’s history, for a greater part of which, the Pleistocene, extensive glaciation was particularly characteristic. The works of the founders of glacial theory are based on ideas of monoglaciation, but in the 20th century an overwhelming majority of researchers shifted to views of polyglaciation—that is, the theory of a multiplicity of glaciations. According to this theory, there was an alternation of cold (ice ages or glacials) and warm (inter-glacials) intervals of time several times during the Pleistocene. As a result of this alternation the sheets of continental ice in the middle latitudes first formed and then melted away completely. The work of A. Penck and E. Brückner on the history of glaciation in the Alps (1909) made a particularly great contribution to the establishing of polyglaciation. In Russia and the USSR the introduction of polyglacial concepts is associated with the names of S. N. Nikitin, A. P. Pavlov, G. F. Mirchink, S. A. Iakovlev, I. P. Gerasimov, K. K. Markov, and A. I. Moskvitin. According to modern concepts, repeated fluctuations of the climate are considered a scientific fact, and the disputes are chiefly over the degree of their abruptness and number and time of appearance.

A repeated recurrence of ages of great cooling of the climate and the development of vast continental glaciation have been established not only in the last period of the earth’s geological history but also in the much more remote geological past (Ice Age). Thus, glacial theory has acquired general geological significance, having turned from a particular concept into a component of the general theory of climatic changes in the earth’s geological past.