Ice wedges in permafrost, Lower Yukon River area near Galena, Alaska
DOI:
https://doi.org/10.26485/BP/1962/11/8Keywords:
ground-ice masses, polygonal network, thermal contractionAbstract
Foliated ground-ice masses exposed in vertical banks of the Yukon River occur as wedges or vertical or inclihed dikes 4 mm to 3 m wide and 1 m to 5 m high. Individual wedges are part of a polygonal network of ice enclosing polygons or cells of frozen ground 1 m to 30 m in diameter. The dominant foliation or lineation of bubbles and minute soil particles is subparallel to the sides of the ice masses. Some sharp-walled, clear, white ice-filled veins 1 to 5 mm wide trend parallel to or cut across the prominent foliation. Many of the planes of foliation are faulted, sheared, bent, or otherwise deformed by minor movement and adjustments within the ice mass.
The ice crystals are 1 to 100 mm in diameter, colorless, and rather equidimensional. The tops of the wedges have ice nipples or projections 10 to 150 mm wide extending upward 50 to 300 mm into the overlying frozen sediments Extending upward 50 to 600 mm from these ice projections are ice-filled fissures 2 to 10 mm wide, some of which extend to ground surface.
The sediments adjacent to the wedges are pushed up, and in many places overturned. The upturning may affect the sediment 1 /2 to 3 m on either side of the masses. Foliated ice masses cross-cut other ground-ice and sediment layers. Ice-filled cracks extending from the surface downward into the ice wedge, the cross cutting of sediments and older ice masses by ice wedges, and the foliation support Leffirtgwell's hypothesis that ground-ice masses form in seasonally recurring thermal contraction cracks in permafrost - cracks produced by great seasonal temperature changes.
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