Digital Varves

Varves of the Month for 9/1/2009 - 9/30/2009

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 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:

  • September 2009 NE6897-6905: (This month) Distal varves
  • August 2009 NE6853-6864: 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 two successive core images that have been stitched together. The images were chosen because of the summer layer detail that they show. This is a distal sequence 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 29.1-28.2 feet. The top of this interval is 74.3 ft and 337 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. After a sporadic match following the major flood event at ~NE6810 the Perry Hill and NEVC records again match well above ~NE6905. 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 (2.2-5.6 cm) with only one varve (NE6903) over 4.0 cm. The varves show features common to ice-distal varves. In terms of a definition, ice-proximal varves are composed of sediment almost entirely derived directly from glacial runoff and the 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 varves with clear melt season subdivisions are rare. In this month's varves both the early and late melt season layers are poorly developed or absent. The change to distal varves up section is a natural consequence of ice recession. The varves were deposited 329-337 years after recession of the ice sheet when the ice-front had receded approximately 93 kilometers north of the PHS core site to Newbury, VT and North Haverhill, 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 in normal distal varves. Unlike in areas to the south the distal varves of the Perry Hill core sites are thicker than transitional varves lower in the section (see June and July 2009 Varves-of-the-Month). For example, distal varves near Hartford, Connecticut (see Varves-of-the-Month for 4/27/2008) are much thinner and more clayey with winter layers as thick or thicker than summer layers. The upper Connecticut Valley had more rapid ice recession and annual meltwater production was higher. The upper Connecticut basin also has higher relief, which promotes paraglacial erosion following deglaciation, and in the Perry Hill area the valley is narrow, which forces sediment accumulation in a confined area rather than spreading it over a wider basin. The stacked graded summer units represent melting or storm events. More rhythmic, possibly diurnal, units are not as common as lower in the section. Many of the graded units that contain fine sand appear to be derived from sediment delivered to the lake by tributaries and then remobilized by bottom currents. Correlative ice-proximal varves further north in the Connecticut Valley are less sandy than the distal varves shown here. Sand became more prevalent at the Perry Hill sites as prograding deltas delivered more sandy sediment further into the lake basin where it was picked up by strong down valley currents. A major source of sand is the Sugar River Valley just to the north at Claremont.

Subdivisions of the melt season are hard to define in the distal varves shown here. The late melt season unit appears to have faded into a gradation between the summer and winter layers or is replaced by a set of light gray silt partings that appear to impinge on the winter layer. Near the bottoms (lowest centimeter) of several of the varves on this image (NE6899, 6900, 6906) there are a few coarse to medium sand grains that are sprinkled among the first graded layers of the summer. These zones can be recognized when the image is magnified by the small spots or holes left by the grains as they were removed during scraping of the core. These grains appear to be drop sediment possibly left by river or shore ice. The freezing of these grains into shore or bank ice and also the delivery of sediment from deltas to the surface of lingering lake ice in the early summer all seem like possible mechanisms for creating a rafted load early in the melt season. The rafting of sediment by icebergs would be problematic given the long distance the icebergs would have to travel and the complete lack of such features lower in the core in ice-proximal varves. The icebergs would also have to pass through narrow valley constrictions at the mouths of the White (White River Junction) and Sugar (Claremont) rivers where deltas were prograding into the lake. An Eolian (wind-blown) origin is also possible although some of the grains are very coarse for eolian sediment.

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 winter or non-melt season layers are dark bluish- to greenish-gray clay beds where NEVC numbers are posted on the image. Some of the winter beds have a very dark color at their upper surface that may be related to fine organic accumulation. All of the winter clay beds on this image are thin (a few millimeters) and have very subtle normal grading from silty clay at the bottom to nearly pure clay at the top. In some cases the thickness of clay layers is hard to evaluate as a winter layer thickness because it appears that initial clay deposition is interrupted by sand and silt laminations, creating a split clay layer (NE6902, 6904). Clay deposition appears to begin in some years prior to the end of runoff and melting events, making the clay bed a combined late summer and winter layer. The splitting of some clay layers and their otherwise uniform thickness negates a meaningful correlation between winter and summer layer thickness.

Past Varves of the Month...