This photograph shows some extremely thick (10-15 cm) varves (annual layers created in lake sediments) in Hanover, New Hampshire. The layers show up because of a change in grain size of the sediment from course (sand) in high-discharge times like spring flooding and fine sediment (silt or clay) when the lake and inflow are calm in winter. At the end of the last ice age the Laurentide Ice Sheet, which covered most of Canada and the northern United States, was retreating northward and lakes formed along its edges from all of the meltwater. The Connecticut River valley was dammed by sediment toward the south and a large lake formed: Glacial Lake Hitchcock! If you drive along the modern-day Connecticut River, you may notice beautiful terraces on either side of the river, these were different lake levels, and varves can be correlated from different locations along the river valley to see if the lake burst through its sediment dam and flooded areas in southern Connecticut.
For a map: