the preparation of the Congress as vice-chairman of the preparatory committee, and from May 1907 - as chairman of the executive committee. Within the framework of the congress, he delivered the now classic lecture "A geochronology of the last 12000 years". He began this lecture with such words "Geology is the history of the earth, but hitherto it has been a history without years." At the congress De Geer formally introduced the term varve defining it as any annual sedimentary layer, and also proposed that the term geochronology be restricted to varve dating, other existing techniques being less accurate and precise. By then, observations of the stratigraphic relationship between varved sediment and recessional moraines, and the correlation of varve sequences between geographically distant sites, added more compelling evidence to De Geer's essentially circumstantial speculation. The geological community accepted that the couplets were unlikely to represent any period other than the year. In addition, before the start of the Congress, De Geer conducted an excursion for 65 delegates from 14 countries, within the framework of which he examined the Dicksonfjorden on Spitsbergen. Main research areas De Geer's early studies of raised beaches, used to reconstruct glacio-isostatic sea level changes, and his mapping of glacial moraines to reconstruct the extent of the last Scandinavian ice sheet and its pattern of deglaciation (the particular type of moraine he studied is now referred to as De Geer moraine), were well received. However, De Geer is most famous for discovering varves and pioneering their use in geochronology. During fieldwork in 1878, De Geer noticed that the appearance of laminated sediments deposited in glacial lakes at the margin of the retreating Scandinavian ice sheet at the end of the last ice age, closely resembled tree-rings. In his best known work Geochronologia Sueccia, published in 1940, De Geer wrote "From the obvious similarity with the regular, annual rings of the trees I got at once the impression that both ought to be annual deposits" (1940, p. 13). While this observation was not new, De Geer was the first geologist to exploit its potential application. De Geer called these annual sedimentary layers varves and throughout the 1880s further developed his theory, publishing a brief outline of his discovery in 1882, which he followed with a presentation to the Swedish Geological Society in 1884. It was not until 1910, at the International Geological Congress, that De Geer's pioneering work