Development of Ideas About Holocene and Latest Pleistocene Glacier Advances in the North American Cordillera

Abstract:

It all started when Francois Matthes coined the phrase “little ice age” (LIA) in 1939 to explain cirque moraines in the Sierra Nevada.  Porter and Denton in the late 60’s promoted the concept that the LIA was part of a multi-millennium regrowth of glaciers called “Neoglaciation.”

A second set of small moraines found in a few cirques short distances downvalley of LIA moraines in the American Rockies began attracting some attention in the 1940s-50s. By the 1960s-70s there was much argument over the age(s) of these moraines. A proliferation of ages appeared in the literature in the 1970s-80s, but Thom Davis and Jerry Osborn in 1987 proposed that most or all these outer cirque moraines are actually Younger Dryas (YD) in age.

In Alberta in the 1970s, Brian Luckman began studying LIA deposits and Osborn began mapping outer cirque moraines (Crowfoot moraines). The two joined forces for an overview of Holocene glacial history in the Canadian Rockies in 1979.

Eric Leonard and Mel Reasoner began lake-sediment studies in the Canadian Rockies in the 1980s-90s. Reasoner and Osborn concluded using lake sediments that the type Crowfoot moraine is YD in age, and in Colorado and Wyoming, Davis, Reasoner, and Brian Menounos established YD ages of outer cirque moraines. Lateral-moraine stratigraphy, begun in the 1980s by June Ryder and Osborn in British Columbia, corroborated the evidence from lake sediments that minor advances and retreats punctuated a gradual Neoglacial expansion of glaciers that began 7 or 8 ka.

The era of cosmogenic dating began in the 1990s, with John Gosse’s work in the Wind River Range. Most, but not all, cosmogenic ages on outer cirque moraines, including those yielded by Shaun Marcott’s broad survey, are in support of the concept that such moraines are YD or pre-YD in age, although uncertainties resulting from production-rate questions remain. There have been various claims of early Holocene advances greater in magnitude than the LIA, but these have been dismissed in the literature and have not gained general acceptance.

Most recently, Menounos found so-called “monster moraines” well downstream of LIA moraines in northern British Columbia and the Yukon where there are no small Crowfoot-type outer moraines. New dates suggest these monsters are a different manifestation of the YD associated with the decaying Cordilleran Ice Sheet.