Digital Varves

Varves of the Month for 6/1/2008 - 6/30/2008

Connecticut Valley Varves at Canoe Brook, Dummerston, Vermont

Scale bar in cm.

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This image shows glacial varves from the Connecticut Valley of southern Vermont. The winter or non-melt season layers (dark, top part of each annual layer) are labeled for New England varves 6039-6047 (lower Connecticut varve sequence, Neweeklyw England Varve Chronology numbering system of Antevs, 1922). These varves were deposited about 350-400 years after ice recession at Canoe Brook, when the receding ice margin was 40 km to the north near Charlestown, New Hampshire. Sedimentation here is still dominantly controlled by the input of sediment from the receding glacier.

Summer or melt season layers (light-colored units) are composed of a stack of micrograded units of fine sand and silt. Most of the varves have sandy and sometimes lightly oxidized layers marking the beginning (bottom) of each summer layer and a gradation of summer layers into winter layers. Gradation from summer to winter is best developed in varves 6040-6042, and 6044. The coarse layers that begin most summers are the result of rapid meltwater release from the glacier and adjacent land surface in the early summer, perhaps assisted by lake overturning. The coarse event at the beginning of NE 6044 has a clayey cap associated with it that could represent the resettling of clay disturbed by the first bottom currents crossing the lake floor as the summer begins. The summer initiating event may be instigated by the summer's first precipitation or melting event. Occasionally, as in NE 6043, 6044, and 6046 there are sand partings that are at or near the top of the summer layer and they may represent a precipitation or melting event at the end of the summer. Summer layer thickness is greater than winter layer thickness and also varies much more than winter layer thickness in these varves. In this image summer and winter layer thicknesses do not appear to vary proportionally with the two thickest summer layers (6041 and 6044) having relatively thin winter layers. Both varves 6043 and 6046 have relatively thin summer layers but relatively thick winter layers. The gray spot in the base (center) of the summer layer of varve 6042 is a carbonate concretion formed at the sandy parting in the center of the summer layer. These calcium carbonate cemented features are common in the varves of the Connecticut Valley.

Past Varves of the Month...