PRECISE COSMOGENIC MEASUREMENTS IN SUPPORT OF YOUNGER DRYAS AGE FOR FOURTH OF JULY CIRQUE OURTER MORAINE, COLORADO FRONT RANGE, USA

South-facing Fourth of July cirque lies between 3400 and 3600 m elevation on the east side of the Continental Divide in the Colorado Front Range (CFR). The outermost moraine in the cirque was originally mapped by Mahaney (1973), who used relative dating (RD) methods, including lichen percentage cover and thalli diameters, vegetation assemblages, surface boulder weathering, and soil development to suggest that the deposit is early Neoglacial age, correlative to deposits assigned to the Temple Lake stade elsewhere in the Rocky Mountains, dated about 4.5 ka by Richmond (1965). Mahaney also mapped a smaller moraine upslope in the cirque that he assigned to Audubon age (middle Neoglacial), dating about 1.9 to 0.95 ka. Williams (1973) mapped the same deposits using similar RD methods, but concluded that the two moraines mapped by Mahaney were Bull Lake (pre- or early Wisconsinan) and late Pinedale (late Wisconsinan) in age. Williams also mapped a moraine higher in the cirque that she ascribed to the Temple Lake stade. Benedict (1985) ¹⁴C-dated peat underlying sand in a bog down valley from Fourth of July cirque that he attributed to the advance of a rock glacier in the cirque at about 3.5 ka. If there were ever an alpine glacial chronology begging for better numerical age resolution, Fourth of July cirque would be the poster child.

Decades later, Marcott et al. (2019) dated six boulders on the outer moraine at Fourth of July cirque that provided an average ¹⁰Be exposure age of 11.59±0.50 ka, compatible with the latter part of the Younger Dryas (YD) cool interval. For comparison, seven boulders (minus one outlier) from the type Temple Lake moraine in the southern Wind River Range, Wyoming, provide an average ¹⁰Be exposure age of 14.00±0.71 ka, demonstrably pre-YD in age (Marcott et al., 2019). Davis and Osborn (1987) suggested that all outer cirque moraines in western North America, 1-3 km down valley from Little Ice Age moraines, are likely YD age, and not Neoglacial age as previously believed. However, Marcott et al. (2019) produced 130 ¹⁰Be ages from 19 of these outer cirque moraines across the region, including the Fourth of July cirque moraine and three other moraines in the CFR, that fall into six age groupings, with three pre-YD, one YD, and two slightly post-YD. Within this context, the southern exposure of Fourth of July cirque as a location for a YD moraine seems all the more remarkable.