At http://eos.tufts.edu/varves/Geology/chronology.asp … an interesting outline on varve chronology and its usefulness in assessing the Late Glacial period and the beginning of the Holocene. The use of varve sequences to establish time lines in sedimentology sequences and for correlation is vital to interpretation of geological beds. Varves are laid down on an annual basis – showing up seasonal changes. They are especially useful in glacial environments as they can establish how long it took for deglaciation events to occur. They can also represent vital evidence of marine incursions – and sea level changes.