Ice-wedge Casts of Wisconsinan Age in Eastern Nebraska
Sand-filled, wedge-shaped structures beneath a thin layer of aeolian sand penetrate a preIllinoian 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 wedg...
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DigitalCommons@University of Nebraska - Lincoln
1991
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| Online Access: | https://digitalcommons.unl.edu/geosciencefacpub/502 https://digitalcommons.unl.edu/context/geosciencefacpub/article/1512/viewcontent/Wayne_PPP_1991_Ice_wedge_casts__FINAL_MS.pdf |
| Summary: | Sand-filled, wedge-shaped structures beneath a thin layer of aeolian sand penetrate a preIllinoian 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. Résumé Des structures en forme de coin, remplies de sable pénètrent sous une fine couche de sable éolien dans un dépôt glaciaire et des graviers pré-illinoïens dans Ie NE du Nébraska. Interprétées comme des fentes de gel fossiles formées par contraction thermique, ces structures fournissent la première évidence de l’existence ancienne d’un pergélisol dans cet Etat. Les coins sont distants de 5 à 7 m et s’intersectent pour former des polygones. Ils sont profonds de 1,8 à 2,8 m, larges de 0,6 à 0,9 m au sommet et s’amincissent en profondeur jusqu’à devenir des fissures. Ils sont remplis de sables de taille moyenne formés principalement de grains arrondis et mats. La disposition verticale des éléments est la même dans chaque coin. ... |
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