Digital Varves

Varves of the Month for 4/1/2009 - 4/30/2009

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

Scale bar in cm.

Click on image to download original image file

Since February 2009 and continuing through this year is 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 and have very similar varve stratigraphy. The complete section contains 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 at the bottom of this page). 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 indicated on the varve plot and below with the oldest varves at the bottom of the list:

  • April 2009 NE6652-6655: (This month) Return to ice-proximal varves
  • September 2008 NE6640-6644: Ice-proximal/transitional varves + flood event
  • March 2009 NE6631-6635: 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 70.9-70.0 feet. The top of this interval is 32.5 ft and 87 varves above the bottom of the varve section in the PHS core 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 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 (9-10 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 below them (see Varves-of-the-Month for Sept. 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 and thicker varves takes place over an interval of about 10 years (see varve plot) and is likely due to an increase in current velocities and sediment transport associated with an increase in meltwater production. The varves were deposited exactly 84-87 years after recession of the ice sheet at the PHS core site when the ice-front had receded approximately 24 kilometers north to Hartland, 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 thicker more ice-proximal varves (see February 2009). The varves occur about a decade after a flood event (see Varves-of-the-Month for Sept. 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) mark an abrupt change from the nearly pure clay of the previous winter's layer. The early melt season units are olive gray and internally have thin normally graded rhythmic units that may be diurnal cycles. The olive color and finer grain size of these units distinguishes them from the main melt season unit (M) above.

The main melt season unit (M) has a distinctly lighter gray (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). This suggests that during the main melt season meltwater was moving through the area in pulses, possibly in response to strong diurnal variations in meltwater production and occasional spikes due to storm runoff and glacial flood events. Near the top of the main melt season units in each complete varve shown on the image (NE6653-6655) there is a relatively thick light gray fine to medium sand layer grading upward to olive gray clayey silt (green bars). These units are by far the coarsest layers of each summer and may represent large precipitation (rain) or melting events late in the summer. The sandy layer in NE 6653 even has ripple cross-bedding.

The late melt season unit (L) is here the least distinct and the thinnest of the three summer units (E, M, and L) and is composed of mostly clayey silt to silty clay units interspersed with occasional lighter gray silt to fine sand partings that appear to represent late summer storm or melting events. These units tend to be thin or absent in fully transitional (ice-proximal to ice-distal) varves and the late season unit is missing from NE 6652. The late melt season units also have the olive gray color seen in the early melt season units that is related to increased clay deposition. Silt and fine sand partings that are regularly spaced and that could be easily interpreted as diurnal units in more ice-proximal varves (see February 2009) are absent.

Most of the varves show a sharp break from the silty deposits of the summer layer to the very clayey winter layer (W). None of the winter (or non-melt season) beds on this image are 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. All of the clay winter beds are thick (over 1.0 cm), representing some of the thickest clay beds in the Perry Hill cores.

Past Varves of the Month...