Digital Varves

Varves of the Month for 9/1/2008 - 9/30/2008

Connecticut Valley Varves, Perry Hill Basin (PHS), Charlestown, New Hampshire

Scale bar in cm.

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This image shows ice-proximal varves from glacial Lake Hitchcock in the Connecticut Valley of southern New Hampshire. The site (PHS) is in a deep varve-filled basin at the west flank of Perry Hill in Charlestown, NH just south of Claremont. The sample was collected in 2007 and is from a Central Mining Equipment continuous sampling system running inside a hollow-stem auger at a depth of 73 feet. Curvature of the beds in the image is due to coring of the sediment. Red lines on the image define the boundaries between each annual layer. The numbers in the winter layer of each varve are years in the New England Varve Chronology (NEVC) of Ernst Antevs (1922) with yellow indicating the numbering system of the lower Connecticut varves and blue numbers the numbering system of the upper Connecticut varves. The varve sequence in Perry Hill Basin was a critical core section for closing a gap in the NEVC between the lower and upper Connecticut varve sequences. For more on closure of the Claremont Gap go to Closure of the Claremont Gap.

The varves shown here are relatively thick and were deposited in an ice-proximal environment exactly 72-76 years after recession of the ice sheet when the ice-front had receded approximately 20 kilometers to the north to near Windsor, VT. The ice recession rate in the Connecticut Valley at this time was approximately 270 m/yr. The summer or melt season layers (lower parts of each varve) are composed of a stack of graded muddy silt to silty fine sand beds. The winter or non-melt season layers are very dark bluish-gray clay beds where varve numbers are posted on the image. The summer layer in the thickest varve (NE 6311) is darker than the other much thinner summer layers, probably indicating a much muddier water column and more rapid deposition. Varve 6311 may represent a summer season with abnormally high melting or a summer in which there were many heavy precipitation and runoff events. An alternative is also the release of water (flood event) from an ice-dammed lake in a tributary valley that drained to Lake Hitchcock. A flood event may have significantly raised suspended sediment concentrations in the lake for the melt season. Varve 6311 is also the varve with the thickest winter clay bed, perhaps being a function of a greater volume of clay introduced in the summer. After the events of varve 6311, deposition in the following year appears to return to the pattern established prior to varve 6311.

All of the varves have an olive-brown tinted muddy silt unit that marks the beginning of melt season deposition. The best examples are in the bottoms of varves 6310 and 6312. This unit is distinct from the winter (non-melt season) layers beneath them that are composed of nearly pure clay. The muddy silt unit represents deposition in the late spring and early summer when melting is occurring but is not enough to generate the strong lake floor currents that will transport coarser sediment later in the summer. Varve 6311 is again different than the other thinner varves on the image in that the muddy silt unit is thinner than in the other varves and the lower few centimeters of the melt season layer has a prominent fine sand bed grading upward into olive-gray mud. The melt season layer of varve 6311 also shows a fining (darkening) upward trend in its upper half that is not present in the other varves.

The upper parts of summer layers in all the varves appear to transition to winter layers with a sequence of rhythmic light silt to fine sand and dark mud layers. The best examples are in varves 6309-6311. Late melt season deposition of this type suggests a waning of bottom currents and diurnal deposition.

Past Varves of the Month...