Ice‐wedge casts of Wisconsinan age in Eastern Nebraska

• Published in: Permafrost and Periglacial Processes
• Main Author: Wayne, William J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1991
• Subjects: Ice; permafrost; Permafrost and Periglacial Processes; wedge*
• Online Access: http://dx.doi.org/10.1002/ppp.3430020305; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fppp.3430020305; https://onlinelibrary.wiley.com/doi/pdf/10.1002/ppp.3430020305

Abstract Sand‐filled, wedge‐shaped structures beneath a thin layer of aeolian sand penetrate a pre‐Illinoian till and gravel in northeastern Nebraska. Interpreted to be relic thermal contraction crack wedges, they provide the first definite evidence in this State of the former presence of permafrost. The wedges are 5‐7 m apart, 1.8‐2.8 m deep and 0.6‐0.9 m across at the top, taper downward to a crack and intersect to form polygons. They are filled with medium sand that contains mostly rounded and frosted grains. Vertical fabric is present in each wedge. Ventifacts lie along the top of the till, which is covered by 0.3‐0.4 m of interlaminated medium and fine sand that grades upward into 1.0 m of Peoria Loess. These sand wedges formed in thermal contraction cracks, in dry, windswept areas 25‐50 km south of the Late Wisconsinan ice margin, where snow cover was minimal, so that blown sand fell into the open cracks. They probably required mean annual temperatures of —6 °C to —8 °C or lower, along with strong winds and rapid drops in temperature. Yardang‐like topography, beyond the limit of Wisconsinan till across central South Dakota and northeastern Nebraska, is further evidence of exceptionally strong winds parallel to the ice margin during the Late Wisconsinan glacial maximum, between 22 and 18 ka.