Digital Varves

Varves of the Month for 6/4/2010 - 6/30/2010

Varves from Charlestown, NH

Scale bar in cm.

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This month's varves are from a core in the Clay Brook Valley in Charlestown, NH and are in the interval of varves 6050-6150 of the New England Varve Chronology. Unfortunately the short length of the varve sequence, their prodeltaic environment, and problems with core recovery have prevented us from matching the varves precisely.

The varves were deposited high on the side of the Connecticut Valley in a small sub-basin that was later filled by sand and gravel from a prograding delta. The varves in the sequence are generally very thick and sandy. A distinctive fine to medium sand bed marks the beginning of the melt season (summer) layer in each varve and is the coarsest sediment deposited during the summer. This event, appearing to be a burst of water and sediment from the glacier, probably does not represent the beginning of the melt season but instead the first currents that were energetic enough to pour sediment into this high elevation basin. This could represent the first warm spell of the early summer that creates high runoff from snow and ice melting. Alternatively, it could represent the passage of a threshold where meltwater storage in the glacier finally builds pressure to the point where it suddenly forces open new subglacial passageways. For the first time during the summer the meltwater system dumps a large discharge of water into the lake and one that is capable of forcing sandy and muddy water into the Clay Brook basin. Immediately above the initial summer sand bed is a layer slightly darker than later parts of the summer layer, perhaps due to the settling of reworked flocculated clay from the top of the underlying winter layer. The remainder of the summer layer is finer and dominated by micaceous fine sand and silt that shows frequent laminations representing diurnal or melting events during the summer. The summer layer grades into the winter layer, which is composed of distinctive slatey-blue clay. The winter layers are relatively thin for varves this thick, a characteristic of winter layers deposited in relatively shallow water or in prodeltaic environments.

Past Varves of the Month...