Digital Varves

Varves of the Month for 3/1/2009 - 3/31/2009

Connecticut Valley Varves, Perry Hill Basin, Charlestown, New Hampshire

Scale bar in cm.

Click on image to download original image file

Beginning last month and continuing through this year will be a series of images that show the progressive change in varve deposition that occurred over time in the Perry Hill Basin. The Perry Hill Basin core sites, PHS and PHN which is 0.5 km further north, are in the same varve-filled basin at the west flank of Perry Hill in Charlestown, NH just south of Claremont. The sites have a very similar varve stratigraphy. The core sections span over 500 varves from ice-proximal varves at the base resting on till to very ice-distal varves at the top of the section that fade into sediment in which varves are difficult to discern because winter layers are erratically preserved. In the following months we will show varves that get progressively younger and more ice-distal as ice receded further and further north from the Perry Hill Basin. Varves-of-the-Month for September and October 2008 were also from Perry Hill Basin cores (see the archive below this month's image). The general type and relative ages of the varves in the New England Varve Chronology (NEVC) in past Varves-of-the-Month images from the Perry Hill Basin are shown in the varve plot and are also indicated below with the oldest varves at the bottom of the list:

  • September 2008 NE6640-6644: Ice-proximal/transitional varves + flood event
  • March 2009 NE6631-6635 (This Month): Return to ice-proximal varves
  • October 2008 NE6622-6625: Ice-proximal to transitional varves
  • February 2009 NE6590-6593: Thick, ice-proximal varves

This month's varves are on three successive core images that have been stitched together. They are ice-proximal varves from glacial Lake Hitchcock in the Perry Hill Basin (PHS core) of the Connecticut Valley of south-central New Hampshire. 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 75-73.8 feet. The top of this interval is 28.7 ft and 67 varves above the bottom of the varve section where varves rest on till. Red lines on the image define the boundaries between each annual layer (bottom of summer or melt season layer resting on the top of the winter or non-melt season layer from the previous year). The yellow lines define divisions within each varve. The numbers in the winter layer (W) of each varve are years in the upper Connecticut varves of the New England Varve Chronology (NEVC) of Ernst Antevs (1922). A plot of the Perry Hill varves vs. Antevs' upper Connecticut varve record is shown below. The varve sequence in the 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.

The varves shown here are relatively thick (5-10 cm) and show features common to ice-proximal varves although these features are not as distinct as in varves near the bottom of the section (see Varves-of-the-Month for February 2009). The images were chosen because of the summer layer detail that they show. The varves also have more ice-proximal characteristics than varves immediately above and below them that have been shown as previous Varves-of-the-Month (Sept. and Oct. 2008) and the interval shown this month appears to be a subtle return to characteristics generally associated with more ice-proximal conditions. This return to more ice-proximal conditions, and also slightly thicker varves, is likely due to an increase in current velocities and sediment transport associated with an increase in meltwater production from the receding glacier. The varves were deposited exactly 64-67 years after recession of the ice sheet from the PHS site and when the ice-front had receded approximately 18 kilometers north to Windsor, VT. The ice recession rate in the Connecticut Valley at this time averaged approximately 280 m/yr, which is 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 fine sand to muddy silt beds. 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 summer layers have the typical structure of ice-proximal varves in the upper Connecticut Valley with three components: early (E), main (or nival - M), and late (L) melt season units, although the early and late melt season units are not as distinct as in more ice-proximal varves (see February 2009). The varves occur between two major flood events, the lower of which (NE6611-6612 on varve plot) represents the catastrophic escape of impounded water from the Sugar River Valley. This flood event does not appear in Antevs' upper Connecticut sequence on the varve plot because of the configuration of the receding ice sheet that forced the flood event to bypass the Connecticut Valley north of Claremont. The later of the two flood events, which is likely related to the release of water from a tributary north of Windsor, VT (Varves-of-the-Month for September 2008).

A match of the Perry Hill Basin cores (PHN, PHS) vs. the upper Connecticut (UC) varves of the NEVC (data files are available in downloads). Note the different thickness scales of the varve records: PH on left, UC on right. The PH records extend further back in time than the NEVC record, which begins at NE 6601 with very thick ice-proximal varves, the first 15 of which do not match other varve records in the region. Gaps in the PH records due to incomplete core recovery are interpreted through matching of the PH cores to each other, the NEVC, and other core records. The yellow boxes show the positions of this month's and previous Varves-of-the-Month from the Perry Hill Basin.

The early melt season units (E), which mark an abrupt change from the nearly pure clay of the previous winter's layer, are less distinct than lower in the section (see February 2009). These early melt season units are olive gray and thinner than lower in the section (see NE 6635-6636) and internally have much thinner and fainter (see NE6632, 6633, 6636), or absent (see NE6634-6635) normally graded rhythmic units that are thought to be diurnal cycles. The olive color and finer grain size of these units distinguishes them from the main melt season unit above.

The main melt season units (M) have a distinctly grayer (less olive) appearance and more silt and fine sand (less clay) than other parts of the summer layer. The contacts between different units in the main melt season layer are here very sharp as compared to in more ice-proximal varves lower in the section (see February 2009) although NE 6633 still has some gradational contacts. The sharp contacts suggest that during the main melt season meltwater was moving through the area in pulses rather than as a gradually varying continuous flow, possibly in response to strong diurnal variations in meltwater production and occasional spikes due to storm runoff and glacial flood events. Overall the main melt season units may show a very subtle fining upward with darker bands being more prevalent near the top of each unit (see NE6634-6635) but this does not occur in every varve and non-discernable grain size trends are more common.

The late melt season unit (L) is here the least distinct and the thinnest of the three summer units (E, M, and L). It is composed of mostly dark clayey silt to silty clay units interspersed with light gray silt to fine sand partings that appear to represent late summer storm or melting events. The late melt season units also show a return to the olive gray color seen in the early melt season units that is related to increased clay deposition. The late melt season unit has very few partings that are regularly spaced and that can be identified as diurnal oscillations, which is different than in more ice-proximal varves (see February 2009) where rhythmic layering in the late melt season unit is more common. All of the varves shown here have a prominent light gray silt to fine sand parting or bed that marks the top of the late melt season layer (L) and the end of summer deposition followed by a rapid gradation to winter (W) clay. These summer-ending units are not limited to ice-proximal environments and may be the result of a very late melt season storm, melting event, or fall overturning in the lake but their origin remains uncertain.

None of the winter (or non-melt season) beds on this image shows any hint of being split by fine sand or silt partings and they all have very subtle normal grading from silty clay at the bottom to pure clay at the top. When clay deposition began it was an irreversible phenomenon that was not interrupted by further pulses of silt or fine sand. There appears to be a hint on this image that some paired winter and summer layers show a thickness correlation with the two thinnest varves (NE6633, 6635) having the thinnest winter layers. However, the winter and summer thicknesses on the other varves do not show a clear correlation.

Past Varves of the Month...