STRUCTURES RESEMBLING ICE-WEDGE CASTS SUGGEST DISCONTINUOUS PERMAFROST DURING LAURENTIDE RETREAT, CENTRAL NEW YORK STATE

Structures resembling ice-wedge casts occupy siltstone bedrock joints on upland divides 10 kilometers south of the last major Laurentide advance into the eastern Susquehanna drainage of central NYS. They are similar in scale and structural configuration to ice-wedge casts found in Arctic permafrost. No similar features are found within glacial or post-glacial deposits.

As with classic periglacial ice-wedge casts, these range from 30 cm to 1.5 m in width near the surface and taper downward to depths of 2 to 3 m where they narrow to thin seams. However, confinement within bedrock joints precludes polygonal distribution.

Bedding in the host siltstone is sharply upturned at the wedge tops in marginal deformation that diminishes with depth. The extent of deformation appears to be directly proportional to wedge thickness. In all cases the deformation fades laterally within two meters of the wedge.

Vertical sorting within all wedges exhibits coarser clasts near the top, fining downward. Several wedges contain vertically oriented zones suggesting multiple phases of development by filling from above accompanied by collapse of overlying lodgment till. Fragments of host bedrock dominate the coarse clastic filling, whereas the finer fraction contains erratic lithologies similar to those found in overlying till.

Paleoclimate interpretation suggests discontinuous permafrost in uplands that predates Laurentide readvance circa 18,000 years.