Well-developed sorted stone polygons (patterned ground) and other periglacial phenomena are situated near the summit of Trinchera Peak and on the adjacent mountain ridge, Culebra Range, south-central Colorado. We investigated the age and origin of patterned ground with various methods of lichenometry, as well as aerial photography, soil sampling, and temperature logging. For lichenometry, the FALL (fixed area largest lichen) and percent-lichen-coverage methods were employed. Based on the largest lichens from two study sites, we estimate that lichen growth began in the midninth century during the medieval climatic optimum; we conclude that stone polygons have been stable geomorphic features for at least the past millennium. Observations from soil and tree-rings support the notion of relative stability for patterned ground and lack of periglacial activity during this period. Recent mean annual air and ground temperatures at Trinchera Peak are in the range -4° to -6°C, which is not sufficiently cold to maintain periglacial processes. During the Little Ice Age (AD 1500-1900), cooler climate resulted in lichen snowkill events but was not cold enough to destroy all older lichens or reactivate periglacial processes of patterned ground. Keywords: patterned ground, stone polygons, lichenometry, aerial photography, soil temperature, Little Ice Age, medieval climatic optimum, Trinchera Peak.