Swedish geologist, originator of the varve-counting method used in geochronology, first published in a paper entitled, "A Geochronology of the last 12 000 years". De Geer observed that lake beds consist of couplets of laminated sediments (varves) light-colored silt layers alternating with strata of darker clay. These represent annual accumulations and thus provide a means for dating the sediments simply by counting the number of varves present.