Latest pleistocene and early Holocene fluctuations of glaciers in the Canadian and northern American Rockies

Abstract

In the Canadian and northern American Rockies small-scale fluctuations of glaciers during and after late Wisconsinan deglaciation have been identified using stratigraphy of superimposed glacial deposits, geomorphology, and records of glacial advance in lake and peat sediments. Several authors have proposed that valley-glacier readvances of relatively modest scale followed the late Wisconsinan maximum. Evidence for some of these is difficult to evaluate from the literature and no deposits recording such readvances have been numerically dated. Several minimum-limiting radiocarbon dates for deglaciation suggest ice had retreated to within a few tens of km of present limits by ca. 12 ka BP, and to very near present limits by ca. 11 ka BP. One or more minor glacial advances, or stillstands during recession, evidently occurred in valley heads prior to 11 ka BP. Part of the evidence consists of moraines a few km downvalley from subsequently emplaced Little Ice Age and Crowfoot moraines. The Piper Lake moraine in the Mission Mountains of Montana is overlain by Glacier Peak G tephra, and thus has a minimum age of 11,200 14C years BP. The Crowfoot Advance, a minor advance of about the same magnitude as Little Ice Age advances, occurred between ca. 11,300 and 10,000 14C years BP and is thought to be related to the Younger Dryas climatic reversal. Crowfoot moraines have been found from Jasper National Park on the north to the Mission Mountains on the south. Crowfoot ELAs were an average of 40 m lower than modern ELAs around the Canada-U.S. border. There have been past proposals of significant early Holocene glacial advances in the Rockies, i.e. after Crowfoot time but before the LIA, but recent evidence indicates that glaciers then were less extensive than at present.