Digital Varves

Varves of the Month for 2/1/2010 - 2/28/2010

Varves along Starrucca Creek near Lanesboro, Pennsylvania

Scale bar in cm.

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This month's varves are from an area of northeastern Pennsylvania along Starrucca Creek about 5 km east of where it enters the Susquehanna River at Lanesboro (Susquehanna, Pa 7.5-minute quadrangle). The varves here are overlain by till (Braun, 2001), which judging from the lightly consolidated nature of the overridden sediment was probably deposited by a minor readvance during the last deglaciation. The varve core in this image was collected in 2003 by Duane Braun, Jonathan Nichols, and Jack Ridge along Starrucca Creek at the eroded toe of a slump block of varved sediment. The varve section was disrupted by faults and the varve record from this exposure could not be matched to potentially contemporaneous varve records of the Hudson Valley or New England.

Jack Ridge, Jonathan Nichols, and Duane Braun (left to right) collecting core samples at the Starrucca Creek varve section using 3-inch PVC pipe and a homemade drop-weight core driver.

Yellow 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 previous year). The numbers are local numbers of varves in the section. Winter clay beds are a dark reddish brown color as much of the lacustrine clay deposited in this valley was derived from the glacial erosion of Devonian red shale. Summer layers are a complex stack of muddy silt to fine sand micro-graded units. These units likely represent runoff or storm events from the receding glacier and where they are regularly repeated suggest diurnal deposition. Summer layers in the varves show a subtle upward fining with the lower part of summer layers generally being lighter in color (sandier) than units higher in the summer layer. The varves are 2.5-6 cm thick and their complex summer stratigraphy is typical of ice-proximal varves. Small faults cutting through the varves on this image where formed during the coring of the section or slumping that occurred at this exposure.

Braun, D.D., 2001, Surficial geology of the Susquehanna 7.5-minute Quadrangle: Pennsylvania Geological Survey, 4th series, Open-File Reports 00-01.

Past Varves of the Month...