LAKES, SHEAR & MULTIPLE TILLS: AN EXAMPLE OF A COMPLEX ICE MARGINAL POSITION IN THE EASTERN CATSKILLS

A thick (27m) exposure along a prominent ridge transverse to the Beaver Kill Valley near Willow, NY in the Eastern Catskill Mountains documents a complex sequence of glacial sedimentation at an ice margin. Although the study site is within a rugged upland setting the Hudson Lobe of the Laurentide Ice Sheet has traditionally been assigned as the source of Wisconsin age deposits in the region.

Detailed stratigraphic, sedimentologic and structural analysis reveals an uncharacteristic sequence inclusive of eight discrete diamicton units, interpreted as deformation tills interlaced with glaciofluvial and glaciolacustrine deposits. Diamictons are generally matrix supported, clast rich, overconsolidated and range in thickness from 0.24m- 7.0m. Diamictons exhibit pervasive deformation including folds, shearing, boudins and measurable fabrics.

In mid section of the exposure is a 4m thick sequence of varves composed of laminated silty-fine sand and clay. The majority of varves within the exposure do not display the deformation observed within the diamictons that occur above and below the varve section. Oversized clasts as large as 0.3m occur within the varve units and some couplets contain sandy horizons that contain gravel sized material. Diamictons below the varves display recumbent folds, clast plowing and boudins. The lower most diamictons contain a higher percentage of crystalline lithologies whereas diamictons in the upper section display a higher percentage of local Devonian sedimentary rocks.

The valley morphology upstream from the Willow Moraine displays a flat low relief plain 0.75km in width composed of sand, downstream from the moraine the valley develops a narrow 0.15 km wide sinuous expression, armored with cobbles and boulders. The upstream valley is interpreted as a proglacial lake basin that developed with ice retreat. Assignment of the varves is complicated by the existence of another large proglacial lake in the Esopus Creek valley the trunk stream to the Beaver Kill. The Willow Moraine is consistent with a model of active dynamic ice behavior. Evidence of valley glaciers have been proposed for other locations in the Catskill Mountains at present there is not enough data to substantiate if the Willow Moraine developed as an independent valley glacier or as a sublobe of the Hudson Lobe continental ice.