Digital Varves

Varves of the Month for 12/1/2008 - 12/31/2008

Connecticut Valley Varves, Aldrich Brook site, Westmoreland, New Hampshire

Scale bar in cm.

Click on image to download original image file

This month's varves are a follow-up from last month and are varves from the same drill core (ALD) of sediment from glacial Lake Hitchcock in the Connecticut Valley of southern New Hampshire. The site is on the top of a bluff on the east side of the valley just south of Aldrich Brook in Westmoreland, NH. The landforms represent an ancient lake floor that was slightly eroded at its surface by early river formation in the valley and is today partly covered by small sand dunes. The sample was collected in 2007 with a Central Mining Equipment continuous sampling system running inside a hollow-stem auger at a depth of 50-51 feet. Drilling at this site went to a depth of 82.5 ft with at least another 100 ft of varves beneath the drilled section. Red lines on the image define the boundaries between each annual layer (bottom of summer layer/top of winter layer). The numbers in the winter layer of each varve are years in the lower Connecticut series of the New England Varve Chronology (NEVC) of Ernst Antevs. The Aldrich Brook varve sequence was critical to closing a gap in the NEVC between the lower and upper Connecticut varve series. For more on closure of the Claremont Gap go to: Closure of the Claremont Gap in NEVC.

The varves shown here are thicker than last months varves from lower in the same section (4-10 cm vs.1.5-4.0 cm). (Note: The varves shown on this month's image are thinner than most varves in this part of the Aldrich Brook section (6213-6235) and the image is one of only a few from this interval in which more than one varve can be shown at this scale.) Like last month, the varves were also deposited in a transitional (ice-proximal to ice-distal) environment but slightly later in time, approximately 475 years after the local recession of the ice sheet (~14,800 yr BP). For more on calibration of the NEVC go to: Calibration of the NEVC. By the time varves 6225-6227 were deposited the ice-front had receded approximately 34 kilometers north to Charlestown, NH in the Perry Hill Basin (see varves of the month for Sept. and Oct. 2008 in the archive below). The first 250 yr of ice recession from Aldrich Brook was at a rate of ~ 90 m/yr. When ice recession reached Charlestown it slowed down in response to a cooling episode and the ice-front began to oscillate and construct several end moraines. The period of slow recession and end moraine building represented by varves shown last month was an interval of lower meltwater production and cooler conditions that slowed down ice recession and triggered minor readvances of the receding ice front. This month's varves were deposited about 15 years after the end of the moraine building episode and at the beginning of a period of very rapid ice recession (280 m/yr). Commonly varves get thinner as ice recedes and the primary sediment source moves further away. Here, a sudden increase in varve thickness, representing the early stages of rapid ice recession, is a response to increased meltwater and sediment delivery to the Connecticut Valley. Changes in varve thickness from last month's thinner varves to this month's thicker varves reflects a sudden change in melting rates brought on by a sudden warming of climate that lasted another 550 years.

This month's varves are transitional varves but with expanded detail and more ice-proximal affinities than seen in last month's varves. Summer layers are dominated by light silt to fine sand layers of the main melt season that have gradational changes in grain size. Also shown are darker clayey silt layers of the early and late melt seasons that have highly rhythmic laminations which may be diurnal events. Rhythmic laminations are very well developed in the bottom of varve 6226 following a dark early melt season layer. The winter or non-melt season layers are dark bluish to greenish-gray clay beds marked by varve numbers on the image. Late melt season layers appear to grade into the winter (non-melt season) layers with light silt partings that represent either individual storm events or diurnal layering (see tops of varves 6226 and 6227). In this part of the Aldrich Brook section there appears to be a poor correlation between summer and winter layer thicknesses in sharp contrast to last months varves from lower in the section. This suggests that in some years much of the clay delivered to the lake during the summer is carried out of the basin and it may be late summer events that largely control the volume of clay that eventually settles in the winter. This may also explain why winter layers in this month's varves are thinner than the winter layers in last month's varves in which total annual thickness is lower.

Past Varves of the Month...