Digital Varves

Varves of the Month for 7/1/2009 - 7/31/2009

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 (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:

  • July 2009 NE6804-6818: (This month) 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 five successive core images that have been stitched together. The images were chosen because of the summer layer detail that they show and the major flood event that occurred in this interval. 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 43.8-41.4 feet. The top of this interval is 43.6 ft and 248 varves above the bottom of the varve section at the PHN core site 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 top of the 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 (W) are corrected 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.

The varves shown here are moderately thick (3.7-1.2 cm) with three thick varves (ranging from 7-27 cm, NE6808-6810) associated with a flood event. The flood units were difficult to capture in the hollow stem auger sampling system because of their non-cohesive sandy texture and they may be slightly compressed in the core. In the center of the image is the join between two sections of core liner in the 5-ft sampler. The non-flood event 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 composed of silty sediment directly from the glacier. 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 236-250 years after recession of the ice sheet when the ice-front had receded approximately 68 kilometers north of the PHN core site to just south of Fairlee, VT and Orford, NH. This is approximately the position that the receding ice front had when the flood event occurred in the Connecticut Valley. 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 although not as many as in more ice-proximal varves lower in the section. The stacked graded units represent melting events, with some of them possibly being diurnal, especially near the top of the summer layer where they are often rhythmic (see NE6805-6807). Some of the graded units may also be derived from sediment delivered to the lake by tributaries and then remobilized by bottom currents. This is thought to be a minor effect in this month's varves because this tends to make the varves sandy. 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 very thin or indistinct early melt season units as compared to ice-proximal varves (see February to May 2009) and as a result are not annotated as in previous Varves-of-the-Month. The early melt season units, when they occur (NE6805, 6807-6809), are an olive gray color and very thin. The late melt season units are absent. In some cases the late season unit appears to have faded into a gradation between the summer and winter layers (NE6804, 6809, 6811, 6812, 6815, 6816) or is replaced by a set of light gray silt partings (NE6805, 6806, 6810, 6813, 6818).

Top: The Mink Brook varve section with flood events near Hanover, NH as photographed back in the 1930's by Richard Lougee (from Lougee, 1935). On the outcrop are Richard Lougee (above) and James W. Goldthwait (below). The white line to the right and the level of Lougee's neck mark the base of the flood events described in the text. The flood events are recorded at Mink Brook by varves that have sand beds and have an approximate total thickness of 3.5 meters on the photograph. Bottom: 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.

All of the winter (or non-melt season) beds on this image are thin (a few millimeters), have very subtle normal grading from silty clay at the bottom to nearly pure clay at the top, and have a nearly uniform thickness, even in the flood event varves (NE6808-6810). The uniformity of the winter layer thickness negates a meaningful correlation between winter and summer layer thickness. None of the winter layers are split by fine sand or silt partings after the initiation of clay deposition (greenish-blue color). When clay deposition began it was an irreversible phenomenon that was not interrupted by further pulses of silt or fine sand.

Varves NE6808-6810 represent a large flood sequence that has been recognized at several other places in the Connecticut Valley and as far north as Hanover, NH where Antevs recorded two of the flood event varves (NE6807-6808) as having thicknesses of 1.7 and 3.5 meters (Antevs, 1922, p.58-59). Antevs' upper Connecticut varve plots show the thickness of these couplets further south near Claremont, NH (see varve plot) and not the full thickness that they have further north. Pictures of the flood event varves where published by Lougee in the May 1935 (see photo below) and November 1957 issues of the Dartmouth Alumni Magazine. Lougee attributed the flood varves to the lowering of Lake Hitchcock in the Connecticut Valley to the level of his Lake Upham (Lougee, 1935, 1957). It is now known from many delta measurements and other varve occurrences in the Connecticut Valley that lake level did not drop at the time of the flood event. The exact cause of the flood event is not known but is probably related to the catastrophic release of water from a glacial lake impounded in the White River basin. Given the approximate position of the receding ice front in the White River Valley at the time of the flood event and the topographic configuration of the White River valley at Bethel, VT it is possible that the flood represents the release of water from an ice-dammed lake west and southwest of Bethel. Other possibilities that have been considered are that the flood events may be from a subglacial source in the Connecticut Valley or that they are related to a large esker complex between Hanover and Lyme, NH. While a subglacial origin has not been eliminated it still lacks evidence and the esker complex occurs in an area that was entirely deglaciated decades prior to the flood event.

In each flood varve at the Perry Hill sites the flood units make up the bulk of the summer layer and are almost featureless medium sand to fine sandy silt with very subtle normal grading. The bottom of each flood unit starts with a medium to fine sand bed that in NE6808 and 6809 has ripples. The occurrence of these units in multiple years indicates that there were large releases of water perhaps at an ice dam that failed sequentially allowing a lake to drain to progressively lower levels in successive years or as a lake that drained and refilled in three successive summers. In each year the flood unit was deposited after the melt season had begun. This release after the beginning of the melt season may have been triggered by increasing subglacial water pressure during the early melt season that opened tunnel outlets. An astonishing thing is that varve deposition after the flood events returned almost immediately to the exact depositional conditions that existed prior to the flood years. Varves NE6811-6812 are only slightly thicker than the norm from prior to the flood.

An as yet unresolved aspect of the flood events in the Perry Hill cores is that they appear to be offset from and occur over a shorter duration than the same flood events in Antevs (1922) New England Varve Chronology. The Perry Hill flood event record starts 2 years later and over 3 years as compared to the 5 years of flood events in Antevs' chronology. The record shown by the Perry Hill cores matches exactly the record of two other new core records south of Claremont, NH collected in 2008 (Claremont Junction and Weathersfield, VT) that are also offset from and have a shorter flood history than Antevs' record. All of the new varve core records match Antevs' NEVC both well above and well below the flood events but matches are very poor leading up to and just after the flood events.