Digital Varves

Varves of the Month for 2/1/2009 - 2/28/2009

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

Scale bar in cm.

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This month's varves are on three successive core images that have been stitched together. They are ice-proximal varves from glacial Lake Hitchcock Connecticut Valley of south-central 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 (varves of the month from September and October, 2008 were from higher in the Perry Hill Basin stratigraphy). The sample was collected in the summer of 2007 with a Central Mining Equipment continuous sampling system running inside a hollow-stem auger at a depth of 91-92 feet. This depth is 20.5 ft and 22 varves above the bottom of the drill hole where varves rest on till. Red lines on the composite image define the boundaries between each annual layer (bottom of summer (melt season) layer / top of winter (non-melt season) layer). The yellow lines define divisions within each varve. The numbers in the winter layer of each varve (W) 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 critical to 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 in NEVC

The varves shown here are relatively thick (8-12 cm) although not as thick as most adjacent couplets in this part of the section. They were chosen because they would fit on a high-resolution image of reasonable size and because of the summer layer detail that they show. The varves were deposited in an ice-proximal environment exactly 22-25 years after recession of the ice sheet when the ice-front had receded approximately 7 kilometers north to near the mouth of the Sugar River in Claremont, NH. The ice recession rate in the Connecticut Valley at this time averaged approximately 280 m/yr, very rapid for a terrestrial glacier where most of the ablation is by melting. The summer or melt season layers (lower parts of each varve) are composed of a complex stack of graded muddy silt to fine sand beds. The summer layers have the typical structure of ice-proximal varves with three components: early (E), main (or nival - M), and late (L) melt season units. Varve 6259 is thin for this part of the varve section and does not have a well-developed late melt season layer. The winter or non-melt season layers (W) are dark bluish- to greenish-gray clay beds where NEVC numbers are posted on the image.

The early melt season units (E), which mark an abrupt change from the nearly pure clay of the previous winter's layer, are marked by highly rhythmic normally graded layers of light olive gray silt to medium olive gray clayey silt. The olive color and finer grain size of these units distinguishes then from the unit above. The very regular nature of the graded bedding suggests diurnal variations in early melt season discharges of meltwater from the glacier. The main melt season unit (M) has a distinctly grayer (less olive) appearance and more silt and fine sand than other parts of the summer layer. The contacts between different units in the main melt season layer are also not as sharp or distinct as in other parts of the summer layer. This suggests that during the main melt season sediment and meltwater input to the lake was not exhibiting a strong diurnal variation and sediment was deposited by a current that maintained a relatively uniform flow with occasional spikes due to storm runoff, melting, or glacial flood events. The late melt season unit (L) is composed of mostly dark clayey silt to silty clay units interspersed with white silt to fine sand partings and beds that appear to represent storm runoff or melting events. The late melt season units also show a hint of a return to the olive gray color seen in the early melt season units that is related to increased clay deposition. In some varves the late melt season unit has a few partings that are regularly spaced and may represent diurnal oscillations, but here rhythmic layering is not nearly as well developed as in the early melt season layers. It is not uncommon for a prominent white silt to fine sand parting or bed to mark the end of summer deposition followed by a rapid gradation to winter (W) clay (see varves 6258 and 6260-6261). These summer-ending units sometimes have the coarsest sediment of the entire summer layer and are not limited to ice-proximal environments. They may be the result of a very late melt season storm, melting event, or fall overturning but their origin remains a mystery.

Past Varves of the Month...