The Metal Ages of Europe thus must be understood as indigenous local inventions and…. When the meltwater reaches a lake the heaiest sediments sand and silt sink to the bottom quickly and eventually formed thick layers of light colored rock. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year. Bones from fossils absorb fluorine from the groundwater. This technique solely depends on the traces of radioactive isotopes found in fossils.