Digital Varves

Varves of the Month for 10/1/2008 - 10/31/2008

Connecticut Valley Varves, Perry Hill Basin (PHN), 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 south-central New Hampshire. The site (PHN) is in a deep varve-filled basin at the west flank of Perry Hill in Charlestown, NH just south of Claremont (and just north of the site of last month's varves). 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 72 feet. This is the same depth as last month's varves but these varves are older because till at this site is shallower by about 20 ft and the bottom varves that drape the basin are at a higher elevation at this site. Red lines on the image define the boundaries between each annual layer (bottom of summer over preceding top of winter). 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. More on closure of the Claremont Gap.

The varves shown here are relatively thick and were deposited in an ice-proximal environment exactly 55-57 years after recession of the ice sheet when the ice-front had receded approximately 15-16 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 varves show the typical structure of ice-proximal varves with a summer layer that is made of three components: early, main, and late melt season units.

The early melt season unit is marked by thin olive gray silty clay laminae that are sometimes barely distinguishable from the underlying winter layer in terms of color. The early unit has noticeable amounts of silt and fine sand as compared to the winter bed beneath it which is composed of nearly pure bluish-gray clay. The early unit may also have beds of some of the coarsest sediment of the summer layer in the form of fine sand beds as occurs in the bottoms of varves 6623 and 6624. These sand beds appear to represent early summer melting events. The early melt season unit in the bottom of varve 6625 is thinner than in the other varves. The main melt season unit is generally a very light gray (when dried) laminated clayey silt with small amounts of fine sand and occasional darker units of higher clay content. The late melt season unit is composed of laminated silt beds that have distinct white fine sand partings, especially in their tops. In some varves the late melt season unit has partings that are very regularly spaced and may represent diurnal oscillations. It is not uncommon for one of the white partings to mark the end of summer deposition and it is usually followed by a rapid gradation to winter clay. White fine sand partings mark the end of the summer layers in varves 6623 and 6625. The white partings may represent the closing stages of summer deposition at a time when there is less suspended clay and silt in the water column than earlier in the summer.

Past Varves of the Month...