The Blytt–Sernander classification, or sequence, is a series of North European climatic periods or phases based on the study of Danish peat bogs by Axel Blytt (1876) and Rutger Sernander (1908).
The classification was incorporated into a sequence of pollen zones later defined by Lennart von Post, one of the founders of palynology.
Geologists working in different regions are studying sea levels, peat bogs, and ice core samples by a variety of methods, intending to further verify, and refine the Blytt–Sernander sequence.
For example, recent peat core samples at Roskilde Fjord and Lake Kornerup in Denmark identified 40 to 62 distinguishable layers of pollen, respectively.
There is no reason, for example, why the north Europeans should stop using bronze and start using iron abruptly at the lower boundary of the Subatlantic at 600 BC.