Abstract

Polygonal patterned ground in a tundra valley near Sawtooth Peak, Boulder County, Colorado, is unique in the Front Range because of its large size and its resemblance to fossil ice-wedge polygons. The polygons occur in a terminal moraine of latest Pinedale age (about 15,000 to 10,000 B.P.), at an altitude of 3,390 m. They range from 10 to 25 m in diameter and are outlined by shallow, vegetated troughs that show no surface indication of sorting. The polygonal ground pattern is attributed to thermal contraction cracking and ice-wedge formation during a late Pinedale or early Holocene cold interval in which permafrost existed at least 350 m below its present lower altitudinal limit. Frost-sorted rubble accumulated in troughs above the melting ice wedges; evidence of sorting, however, is concealed by humus-rich loessal or slope-wash sediments that bury the stones and fill interstices between them to form wedges of silty material as deep as 1.0 m. Organic matter in the humus-rich silt is believed to have eroded from an Altithermal A1 horizon during early Neoglaciation; radiocarbon dates of 5750 ± 110 and 5765 ± B.P. apply to formation of the Altithermal soil and are maximum ages for its erosion and redeposition, but they have no bearing on the time of patterned ground formation. Stones in the polygon borders have experienced no important frost sorting since the close of the Triple Lakes stade of Neoglaciation, which ended prior to 2855 ± 90 B.P. in the Sawtooth valley.

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