Digital Varves

Varves of the Month for 11/1/2008 - 11/30/2008

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

Scale bar in cm.

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This month's varves are from a drill core in sediments of glacial Lake Hitchcock in the Connecticut Valley of southern New Hampshire. The drill site (ALD) 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 60.5-61.5 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 have a thickness typical of the Connecticut Valley of New Hampshire and Vermont (1.5-4.0 cm) and were deposited in a transitional (ice-proximal to ice–distal) environment approximately 425 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 6174-6179 were deposited the ice-front had receded approximately 30 kilometers north to Charlestown, NH. 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 varves seen here were deposited during the period of end moraine construction while the ice front was in North Charlestown.

This month's image shows the typical structure of transitional varves with summer layers dominated by dark clayey silt layers of the early melt season followed by thicker light silt to fine sand layers of the main melt season. Late melt season layers are not well developed as in more ice-proximal varves. The winter or non-melt season layers are dark bluish to greenish-gray clay beds marked with varve numbers on the image. Summer layers appear to grade into the winter layers with occasional light silt to fine sand partings that represent either individual storm events or diurnal layers (see tops of varves 6174, 6178, and 6179). One relatively thick graded, fine sand to silt layer in the top of varve 6179 appears to represent a late summer storm or meltwater/flood event that interrupts the usual more orderly gradation from summer to winter. In this part of the Aldrich Brook section there appears to be a high correlation between summer and winter layer thicknesses suggesting that winter layer thickness is largely controlled by the volume of clay delivered to the lake during the main melt season.

A conspicuous feature is the contorted bedding in varve 6177. This type of disturbance is sometimes common in varve sections, especially when deposition occurred next to a steep valley side. The thickness of this couplet could not be used for the purpose of correlation because it is not an accurate representation of deposition controlled by seasonal weather patterns. It is also important to recognize two other aspects of this bedding disturbance. First, there are no missing varves in the section. This was confirmed by matching the varves to records of the NEVC and a duplicate core at the site taken ~20 ft away. The contorted layers represent the addition of material from an up slope mass movement event without the complete elimination of varve 6177 which is still capped by a clay bed. A second aspect of the fold in varve 6177 is that, although slightly compressed, it has a repeat of the pattern in varve 6176 below. It appears that up slope from the core site varve 6176 underwent mass movement failure and was moved to the core position as a contorted layer without the complete destruction of its internal bedding. At the core site an undisturbed section of varve 6176 is still preserved beneath the folded units in varve 6177.

Next month you will see more of the Aldrich Brook section and some changes that occur in the varve stratigraphy over time and have been linked to the rates and patterns of ice recession as well as climate change.

Past Varves of the Month...