Field and Laboratory Studies of Patterned Ground in a Colorado Alpine Region
INSTAAR Occasional Paper 49
1992, 44 pp. (cost: $5)
Sorted nets, sorted stripes, earth hummocks, and frost boils were studied from 1961 until 1969 on Niwot Ridge and in the Boulder City Watershed, Colorado Front Range. The study produced maps, profile descriptions, and fabric data for representative patterns, measurements of modern activity, and observations on age and paleoenvironmental significance. Field evidence and laboratory studies suggest that most of the patterns formed rapidly, by upward mass displacement of plugs of fine earth in response to density imbalances in the thawing soil; some of the patterns then continued to evolve slowly due to circulatory overturn driven by vertical frost heaving in their centers.
The occurrence of large-scale sorted nets and stripes on well-drained knolls and ridgecrests at altitudes as low as 3245 m implies that icy permafrost existed at least 100 m below modern timberline during the late Pleistocene. Sorted nets have developed at even lower altitudes during the past century, but only in microenvironments where autumn saturation does not require the presence of a frozen substratum. Moisture-induced changes in frost intensity during the Holocene caused areas of patterned-ground activity to contract and expand; the general trend in the Front Range during the past 25 yr has been toward revegetation of frost-patterned ground.PDF (6 MB)