Downslope Soil Movement in a Colorado Alpine Region: Rates, Processes, and Climatic Significance

Abstract

Soil-movement rates, processes, and land-forms were studied above timberline on the east slope of the Colorado Front Range. Maximum rates of downslope movement measured in sorted stripes, turf-banked lobes and terraces, and stone-banked lobes and terraces at nine experimental sites on Niwot Range ranged from 0.4 to 4.3 cm/yr. Rates of displacement were strongly influenced by differences in moisture availability and gradient, but were relatively unaffected by differences in soil texture and temperature. Movement is currently confined to the upper 50 cm of soil; columns of small cement rods placed in vertical drillholes showed no downslope displacement below this depth after four years of burial.Periodic measurements of downslope movement and frost heaving during the 1965-66 annual freeze-thaw cycle suggest that solifluction is a more effective process than frost creep in the saturated axial areas of turf-banked lobes in wet sites, but is less effective than frost creep at their edges. Potential frost creep at one experimental site exceeded theoretical values calculated from heave and slope measurements. Retrograde movement was larger than anticipated. At a site where frost creep is the dominant movement process, stone-banked lobes moved three times as rapidly as the finer-textured soil between them; at a site where shallow solifluction is important, stripes of coarse debris moved only half as rapidly as stripes of fine material.Turf-banked lobes and terraces are the result of intense solifluction beneath a cover of vegetation; they form where downslope movement is impeded, and are normally associated with a decrease in gradient such as occurs on concave lower slopes. The shape of the front (linear or lobate) is determined by the uniformity of moisture distribution parallel to the contour of the slope. At least two generations of turf-banked lobes and terraces occur in the Niwot Ridge area. Terraces with gentle, subdued fronts and patterned treads date from the late Pleistocene, and were used as camping and butchering areas by prehistoric man as long ago as 7,650 ± 190 radiocarbon years. Most bear an Altithermal soil: five dates for the soil range from 5,800 ± 125 to 5,300 ± 130 BP. Lobes and terraces with overhanging fronts and unpatterned treads postdate the Altithermal interval: radiocarbon and stratigraphic evidence suggest that they formed late in the Temple Lake Stade of Neoglaciation. The front of one small Neoglacial turf-banked lobe has advanced at an average rate of 0.19 cm/yr during the past 2,340 ± 130 radiocarbon years.Stone-banked lobes and terraces are caused by frost creep and are favored by an absence of vegetation; they commonly develop where moving sorted stripes or blockfields encounter a decrease in gradient. Sorting is partially inherited, but is accentuated as the lobe or terrace moves downslope. Stone-banked lobes and terraces in the Niwot Ridge area developed late in the Temple Lake Stade of Neoglaciation. A series of radiocarbon dates from the buried A horizon in one stone-banked terrace suggests that its front has advanced at an average rate of 0.34 cm/yr during the past 2,470 ± 110 radiocarbon years. Movement was slow during the Temple Lake-Arikaree* interstade (2,650 to 1,850 BP), a time of soil formation and intense cavernous weathering, and during the Arikaree glacial maximum, when lichen measurements show that the slope was covered with an insulating blanket of perennial snow. Disappearance of the snowbank led to an eight-fold increase in the rate of terrace advance between about 1,150 and 1,050 BP.The widespread occurrence of stone-banked lobes and terraces, sorted polygons, and sorted stripes on the treads of turf-banked lobes and terraces suggests a general decline in the availability of moisture and the effectiveness of solifluction since the end of the Middle Stade of Pinedale (Late Wisconsin) Glaciation. During the latest Pinedale and earliest Neoglacial ice advances frost creep replaced solifluction as the dominant movement process on many slopes. Neither process is particularly effective today, except in specialized microenvironments that are saturated with meltwater in the autumn.