Geer , Gerard Jacob de

(1858–1943) Swedish geologist

Geer came from a noble Swedish family in Stockholm and both his father and brother served as prime minister of Sweden. He graduated from Uppsala University in 1879 and worked initially with the Geological Survey on the problem of raised beaches before taking up an appointment as professor of geology at Uppsala in 1897. In 1924 he became the first director of the Stockholm Geochronological Institute.

Geer originated the varve-counting method for dating the geological past in years, a system that gave unprecedented accuracy in age determinations. In 1878 he had begun a study of the Quaternary Period in Sweden and soon became aware of the layered deposits, known as varves, laid down in glacial lakes. Seasonal differences in the material deposited enabled individual years to be identified, the summer layer consisting of light-colored coarse-grained material and the winter layer of dark-colored fine material, and Geer noticed the analogy of the varves to tree rings.

He tried to see if the sequence of varves from one region would correlate in any way with those of other areas and found that this could be done for most parts of Sweden. However, this would only allow him to say that two samples came from the same time without being able to say whether that time was a century or a millenium in the past. He was able eventually to establish a base year at 6839 bc from which point individual years could be counted in either direction.

Geer's work was a major breakthrough although it was soon to be overshadowed by radioactive dating and was limited to certain glaciated areas. In his later years, Geer tried to apply his techniques and to establish correlations with other areas of the world, but with varying success.