The Quaternary history of the Cordilleran Ice Sheet fringe : Ashley Lake area, Montana

Abstract

The Quaternary glacial history of the Ashley Lake area, approximately 20 km west of Kalispell, MT, was deciphered using classic techniques of glacial geology. Unlike the Laurentide Ice Sheet, the multiple advances of the Cordilleran Ice Sheet have not been well defined. Ashley Lake nestles within the central Salish Mountains, an area interpreted by some previous workers (e.g., Alden, 1953; Richmond, 1986; cf. Waitt and Thorson, 1983) as surrounded by, but not covered by the fringes of the lastglacial Cordilleran Ice Sheet. This study attempts to establish a foundation for the Quaternary history in this region for the Cordilleran Ice Sheet.Specific interests were to define the spatial and temporal attributes of the prelastglacial maximum, lastglacial maximum, and lastglacial recession. The techniques employed include: 1) mapping of glacial features to reconstruct glacial margins and iceflow directions; 2) mapping of the distribution of erratic boulders to determine ice sheet maxima; 3) radiocarbon dating; 4) tephrochronology to numerically date deposits; and 5) measurement of weathering rinds to determine the relative age of deposits. Abundant glacial landforms combined with erratics allowed a reconstruction of ice margins. Moraines, direct overflow channels, lateral meltwater channels, kames and glacial depressions are among the most important landforms mapped. Radiocarbon and weathering rind dating proved to be very limited in their usefulness. No reliably dateable, closely limiting carbon samples were found. Weathering rinds were too thin and inconsistent to provide for meaningful relative dating. Tephras known to exist in the region were found in ice marginal bogs and provided a minimum date for deglaciation. The spatial retreat of the Cordilleran Ice Sheet here was determined and it was found that local topography greatly affected the lastglacial recession and therefore the lastglacial advance. The lastglacial maximum inundated the entire Ashley Lake area and its tributaries. The pre-lastglacial maximum was even more extensive and covered everything in the area except the highest ridges. The information gathered in this field investigation can now be applied to our knowledge of the Cordilleran Ice Sheet to finetune its spatial attributes.