Digital Varves

Varves of the Month for 6/1/2009 - 6/30/2009

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

  • June 2009 NE6751-6768: (This month) 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. They are transitional (from ice-proximal to distal) varves 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 47.5-46.0 feet. The top of this interval is 39.0 ft and 200 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 the top of the winter or non-melt season layer from the previous year). The yellow lines define divisions within 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 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 moderately thick (4.4-1.2 cm) but noticeably thinner than varves shown previously from lower in the section. The varves also show features common to varves transitional from ice-proximal to ice-distal varves. Some of the features found in ice-proximal varves are thin or missing while the varves are generally thicker than distal couplets. In terms of a definition, ice-proximal varves are composed of sediment almost entirely derived directly from glacial runoff and the melt season or summer layer is easily split into three units representing the early, main, and late melt seasons. All parts of an ice-proximal melt season layer are clearly recorded during summer deposition. 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 absent with the main melt season layer dominating. The change from more ice-proximal to transitional varves up section is a natural consequence of ice recession. The varves were deposited exactly 183-200 years after recession of the ice sheet when the ice-front had receded approximately 55 kilometers north to just south of East Thetford, VT and Lyme, 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 although not as many as in more ice-proximal varves lower in the section. Some of the summer layers have conspicuous medium to fine sand beds (NE6753, 6755, 6758, 6760, 6761, 6762, 6766) that may represent exceptional melting or glacial flood events. Some of these units may also be derived from tributary runoff events during rain storms. 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 (E) melt season units and thin or absent late melt season (L) units as compared to ice-proximal varves (see February 2009).

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. 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 and are very thin in this image, especially in the thinner varves near the top of the image. The thinness of these layers suggests that bottom currents are very weak in the beginning of the summer and in some years transport very little or no sediment to this site. Varves NE6753 and 6758 stand out as exceptions where silty clay beds in the top of the early melt season unit might be mistaken for a winter layer if not for their higher silt content than a normal winter layer. The early melt season units are olive gray and internally have indistinct internal bedding that is more pronounced in some of the thicker varves lower in the image. The olive color and higher clay content of these units distinguishes them from the main melt season unit (M) above.

The main melt season unit (M) has a distinctly grayer (less olive) appearance and more silt and fine sand (less clay) than other parts of the summer layer. The internal layers of the main melt season units are graded and have distinct contacts with each other. 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.

Of the 18 varves on this image the late melt season unit (L) is absent in 10 of the varves and weakly developed in most of the others. When present the late season unit may contain rhythmic light gray silt partings (NE 6751, 6752, 6760, 6762) suggestive of diurnal pulses or storm events. Most of the late season units are thin (NE6756-6759) and grade into the winter layer. 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. When the late melt season unit is absent the varves usually show an abrupt change from a light gray silt parting to the winter layer and in one case a gradation (NE6755). None of the winter (or non-melt season) beds on this image are split by fine sand or silt partings after the initiation of clay deposition (greenish-blue color) and they all have very subtle normal grading from silty clay at the bottom to nearly 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 less than1.0 cm thick and the varves show a fair correlation between winter and summer layer thickness.

Past Varves of the Month...