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Variations in climate produced observable differences in the thickness of sediments, and, like the patterns of variation in tree rings, this allows matches to be made between deposits in separate lake beds. Palynostratigraphy Microscopic wind-blown pollen grains survive well in many soil conditions, and pollen that has accumulated in deep deposits - such as peat-bogs - can provide a long-term record of changes in vegetation; suitable samples may be collected from soils exposed by excavation, or from cores extracted from bogs. Dendrochronology It has been recognised since at least the fifteenth century that trees produce annual growth rings - their physiology was understood by the eighteenth century - and that they could be counted to calculate the age of a tree when it was felled. Key factors Click for a list of the key factors for Radiocarbon Dating 4.5.3.