Digital Varves

Varves of the Month for 8/1/2009 - 8/31/2009

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 for 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 that rest 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 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:

  • August 2009 NE6853-6864: (This month) Transitional varves
  • July 2009 NE6804-6818: Transitional varves + flood event
  • June 2009 NE6751-6768: Thinning transitional varves
  • May 2009 NE6687-6691: Thick transitional varves
  • April 2009 NE6652-6655: 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. The images were chosen because of the summer layer detail that they show. This is a transitional (from ice-proximal to distal) sequence from glacial Lake Hitchcock in the Perry Hill Basin (PHN 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 34.6-33.3 feet. The top of this interval is 69.2 ft and 296 varves above the bottom of the varve section at the PHS core site where varves rest on till. Red lines on the image define boundaries between each annual layer (bottom of summer or melt season layer resting on top of the preceding winter or non-melt season layer). The yellow lines define the top of the summer layer and bottom of the winter layer in each varve. The numbers adjacent to the winter layer 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 Antevs' lower and upper Connecticut varve sequences. For more on closure of the Claremont Gap go to:Closure of the Claremont Gap.

This month's varves are moderately thick (2.0-5.4 cm) with all except three varves under 4.0 cm. The varves show features common to varves transitional from ice-proximal to ice-distal. Some of the features found in ice-proximal varves are thin or missing while the varves are generally less sandy than distal couplets. In terms of a definition, ice-proximal varves are composed of sediment almost entirely derived directly from glacial runoff. The melt season layer, or summer layer, is easily split into three units representing the early, main, and late melt seasons. Ice-distal varves have a large contribution of sediment from tributary drainages that is remobilized by lake bottom currents and the entire assemblage of melt season subdivisions is usually not present with the main melt season layer dominating. In this month's varves both the early and late melt season layers are very thin and poorly developed or absent. The change from more ice-proximal to transitional varves up section is a natural consequence of ice recession. The varves were deposited 285-296 years after recession of the ice sheet when the ice-front had receded approximately 81 kilometers north of the PHS core site to Bradford, VT and Piermont, NH. 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 summer layers are slightly sandier and thicker than varves in images over the last two months. The stacked graded units represent melting events, with some of them possibly being diurnal, especially near the top of the summer layer where they often have a consistent thickness and are very rhythmic (NE6855, 6856, 6858, 6860, 6862). Some of the graded units may also be derived from sediment delivered to the lake by tributaries and then remobilized by bottom currents. This becomes more prevalent up section as prograding deltas delivered more sandy sediment further into the lake basin where it was picked up by strong down valley currents. Some laminations in the summer layer are discontinuous (see NE6857, 6858, 6862) possibly as a result of bioturbation by crawling or swimming organisms. Small scale erosion and load deformation (or possibly bioturbation) can be seen at the base of the summer layer in NE6856. Two of the varves on this image (NE6857 and 6860) have mid-summer layers with a high clay content that might be mistaken for winter layers if measured on an outcrop or while the sediment is still wet or moist. There was a distinct advantage in studying and measuring these varves in a partially dried core. The clayey mid-summer layer in NE6860 appears to be the fine top of a graded unit that begins with rippled fine sand and represents a large melting event or minor flood. In the case of NE6857 the clayey mid-summer unit is at the top of a massive silt bed that may also represent a melting or flood event but of smaller magnitude. Neither of these events stands out on Antevs' NEVC record, which was measured further north, and the events on the Perry Hill record may have a more local origin. This suggests that they represent large storm or snow melt discharges from local tributaries.

A match of the Perry Hill Basin cores (PHN, PHS) vs. the upper Connecticut (UC) varves of a corrected NEVC (data files are available in Downloads). Note the different thickness scales of the varve records: PH on left, UC on right. 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 summer layers shown here have very thin or indistinct early melt season units as compared to ice-proximal varves (see varves from past months lower in the section) and as a result the early melt season units are not annotated separately. Varves NE6857 and 6860 are two varves that do have noticeable but thin olive gray early melt season layers. Late melt season units are absent except in varve NE 6864. In some cases the late season unit appears to have faded into a gradation between the summer and winter layers (NE6854, 6860, 6863) or is replaced by a set of light gray silt partings that appear to impinge on the winter layer (in all other varves not mentioned above). The dark brown spot near the base of NE6861 is an oxidized organic fragment (wood fragment) that unfortunately was too small for a radiocarbon age.

Past Varves of the Month...