Digital Varves

Varves of the Month for 5/1/2009 - 5/31/2009

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

Scale bar in cm.

Click on image to download original image file

p>Starting in 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 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:

  • May 2009 NE6687-6691: (This month) 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. They are transitional (from ice-proximal to distal) 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 59.3-58.3 feet. The top of this interval is 44.2 ft and 123 varves above the bottom of the varve section 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 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 moderately thick (9-3.5 cm) and 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 not present 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 119-123 years after recession of the ice sheet when the ice-front had receded approximately 34 kilometers north to just south of White River Junction, VT and Lebanon, 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 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) and late (L) melt season 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 the NEVC (data files are available in Downloads). 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 and are very thin in this image, especially in the thinnest varves (NE6690-6691). The early melt season units are olive gray and have indistinct internal bedding. 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 sediment to this site. The olive color and higher clay content of these units distinguishes then 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 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. The main melt season (M) unit dominates the summer layer and can have very rhythmic internal bedding suggestive of diurnal cycles, especially in thinner varves (NE 6690-6691).

The late melt season unit (L) is composed of indistinct 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. Two of the varves, NE6686 and 6689, have a coarse graded sand bed in the late melt season unit that seems to mark the end of the summer layer and increases the thickness of the late melt season units in these years. All the other late season units are very thin by comparison. The summer ending sand beds are less common than lower in the section (See April 2009) where they seem to occur almost every year. 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. The varves show a sharp break from the silty deposits of the summer layer to the very clayey winter layer (W) marked by a silt to fine sand parting (NE6686, 6689, 6691) or a gradation from the late melt season unit to the winter layer (NE6687,6688, 6690). 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. Some of the clay winter beds are thick (up to ~1.0 cm) and the varves appear to show a good correlation between winter and summer layer thickness.

Past Varves of the Month...