Evidence for Holocene stability of steep slopes, northern Peruvian Andes, based on soils and radiocarbon dates
Radiocarbon dating and soil relationships indicate that landscapes in highaltitude glaciated valleys of the northern Peruvian Andes have been remarkably stable during the Holocene. Radiocarbon dates show that deglaciation was underway by 12 ka, and that slopes and alluvial fans at the bases of slopes were essentially stabilized by at least 8 ka. The soils consist of fine-grained loessial A horizons overlying Bw horizons in gravelly till or alluvial-fan gravel. Following deglaciation, widespread gullying took place in till on the steep (maximum angle: 37°) sideslopes of most valleys; the eroded material was deposited as fans at the bases of the slopes. Loess was then deposited as a fairly uniform blanket across most elements of the landscape. Soil formation began during or following loess deposition, and because soil-profile morphology is sufficiently similar at most sites, soil formation has been a dominant process during much of the Holocene. This remarkable stability, especially for such steep slopes, is attributed to a combination of tight packing of the till, permeability of the capping loess, rapid revegetation following ice retreat, and roots from the present grassland vegetation and possibly former forests.