Evidence for late‐Pleistocene permafrost in the New Jersey Pine Barrens (latitude 39°N), eastern USA

Relict sand wedges, up to 2.5 m deep and 0.4 m wide, are present in the Pine Barrens of southern New Jersey. They indicate the previous existence of permafrost. The wedges are composed predominantly of sand that shows evidence of wind transport and abrasion. Optically‐stimulated‐luminescence dating of infill material indicates that thermal‐contraction‐cracking and emplacement of the sand infill must have occurred during two separate periods during the Late Pleistocene. The most recent was in Late Wisconsinan times,∼15–18 ka. An earlier period of permafrost conditions is indicated by dates >55–65 ka. On both occasions, the Late‐Pleistocene ice sheets would have advanced as far south as northern New Jersey and strong winds would have occurred in the lower mid‐latitudes. The sandy soils of the Pine Barrens would have allowed the ice‐marginal periglacial zone to extend southwards into southern New Jersey. The sparse tundra vegetation on the sandy substrate, with its relatively high thermal conductivity, would have permitted deep frost penetration because the ‘thermal offset’ would have been minimized. A mean annual air temperature of between −3.0°C and −4.0°C is inferred. Permafrost was probably discontinuous and less than 10–15 m in thickness. Episodes of permafrost thaw are indicated by the widespread occurrence of deformed sediments (‘thermokarst involutions’) and by various small‐scale non‐diastrophic structures associated with bog ironstone beds. The presence of soil (ground) wedges in southern New Jersey and adjacent Delaware also suggest conditions of deep seasonal frost, probably when the most recent permafrost degraded. Copyright © 2003 John Wiley & Sons, Ltd.