Geologists working in different regions are studying sea levels, peat bogs and ice core samples by a variety of methods, with a view toward further verifying and refining the Blytt–Sernander sequence. This is a classification of climatic periods initially defined by plant remains in peat mosses. Though the method was once thought to be of little interest, based on 14C dating of peats that was inconsistent with the claimed chronozones, investigators have found a general correspondence across Eurasia and North America. The scheme was defined for Northern Europe, but the climate changes were claimed to occur more widely. The periods of the scheme include a few of the final pre-Holocene oscillations of the last glacial period and then classify climates of more recent prehistory.