Study of patterned ground at Trinchera Peak, Colorado.

Abstract

In 2000, patterned ground was discovered on Trinchera Peak (4120 m), Cu1ebra Range, Sangre de Cristo Mountains, southernmost Colorado. Subsequently, another site of similar patterned ground was discovered on a ridge (3905 m), just north of Trinchera Peak. An initial overview of the study sites revealed the patterned ground is better formed at Trinchera Peak than at Site 2. At Trinchera Peak the most distinct polygons range between 3 and 5 meters in diameter. The polygons are sorted with the margins of the polygons composed of stones and the polygon centers consisting of finer sediment and turf. The patterned ground of Trinchera Peak is remarkably similar to patterned ground at other sites around the world. This preliminary study is an attempt at determining a minimum age of the patterned ground formation. Methods included photographing the site, recording measurements of lichens (Rhizocarpon geographicum), collecting soil samples, and recording soil temperatures. Lichen measurements from both patterned ground study areas were compared to lichen growth rate curves developed by Benedict (1967) from the Indian Peaks area in the Colorado Front Range. The median lichen diameters at Trinchera Peak and Site 2 indicate a minimum time range for patterned ground activity to be -AD 1350 for Trinchera Peak and -AD 1750 for Site 2. Using only the five largest lichens of both study sites a minimum age of patterned ground formation was calculated to -AD 850. These lichen ages are considerably younger than expected, thus lichenometry results may suggest episodes of snowkill rather than periods of patterned ground activity. Additionally, lichen dates obtained may be erroneous due to differences in substrate, climate, moisture, and length of the growing season between the Indian Peaks Region of Colorado and Trinchera Peak. Advanced soil development, large lichens, and the relatively mild climate of the Holocene indicate the polygonal patterned ground may have developed initially as far back as the Pleistocene. Variability in lichen sizes, frost boil, and current revegetation of Trinchera Peak indicate periods of patterned ground activity as well as episodes of snowkill during the Holocene. Patterned ground activity may have occurred during the Neoglaciation (prior to AD 850) and then been partly reactivated during the Little Ice Age (-AD 1300-1900). The recent climatic warming trend has led to revegetation and stability of Trinchera Peak patterned ground.