The Evidence: Varved Sediments
Virtually all of Vermont’s bedrock is coated in a layer of till pasted down underneath the advancing glacier. In the Champlain Valley, the glacial till is immediately blanketed by extremely fine grain sediments stacked in layers repeatedly alternating between silts/clays. As described in the HjulstrÃ¶m Curve, tiny particles (clays and medium-sized silts) will stay entrained (in transport) in water unless the water becomes nearly motionless. Even wind blowing over the surface of the lake causes enough turbulence to keep these sediments from settling out. If, however, you were to freeze the surface of a lake it would reduce the flow velocity within the lake enough that all of the suspended sediments would settle out. The heaviest of these small sediments (silts) would settle first and the lightest (clays) would settle last. Every year that the surface froze over you’d get another varved layer of sediments, again with silts settling first followed by clays. If we could count the number of layers we could get an estimate of how long the lake lasted!