Digital Varves

Varves of the Month for 12/1/2010 - 1/31/2011

Varves from Newbury, Vermont

Scale bar in cm.

Click on image to download original image file

This month's varves are thin gray non-glacial varves from outcrop cores at Newbury, Vermont. The varves were collected in 1996 and the split core was partially dried to make the summer and winter layers more easily discernable. The varves on this image are plotted below in years of the the North American Varve Chronology (NAVC - AM years). The varves on the image were deposited ~13,300-13,100 yr ago when receding ice and glacial melting no longer significantly influenced varve deposition in the valley at Newbury. The receding ice front was in southern Quebec at this time.

Plot of the NAVC (AM) varve years 7550-7650 showing the thickening of varves associated with a potential drop in lake level (at red bar). Click on image for larger version (~1MB JPG)

The NAVC in the Connecticut Valley shows abrupt changes in varve thickness that are maintained for at least a few decades. These positions in the varve chronology may represent times of lake level change or if varve thickness is not maintained for very long they may represent flood events into the Connecticut Valley from the release of water in tributary lakes. After the recession of ice in the upper Connecticut Valley lake water persisted in the upper valley long after the drainage of Lake Hitchcock and sediment from these lower unnamed lake stages has been cored at Newbury, VT and North Haverill and Hanover, NH. At this point not much is known about the spillways and chronology of these postglacial lake stages and no names have been given to them.

The varve record from Newbury, as well as a varve record from Occum Pond in Hanover, NH, show a prominent and abrupt change to thicker varves at AM 7605 (see red bars on image and graph) that appears to represent a sudden drop in lake level. Prior to the event non-glacial varves are very thin but show a marked change to thicker and sandier varves at AM 7605. The event shown in the core occurs across the entire basin and its associated changes in varve lithology and thickness are maintained for many decades. The change in varve thickness and lithology closely coincides with the beginning of the Intra-Allerød Cold Pulse, however climate change is not thought to be the cause of the single year change in varve thickness and lithology. The response of non-glacial varves is dependent on a complex series of processes, such as changes in vegetation and erosion rates, which are not likely to produce the rapid change seen in the core and varve record.

Past Varves of the Month...