THE QUATERNARY HISTORY OF THE CORDILLERAN ICE SHEET FRINGE, ASHLEY LAKE, MONTANA

The Quaternary glacial history of the Ashley Lake area, approximately 20 km west of Kalispell, MT, was deciphered using basic 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 last-glacial Cordilleran Ice Sheet. This study attempts to establish the 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 last-glacial recession, the last-glacial maximum, and the pre-last-glacial maximum. The techniques employed include: 1) mapping of glacial features to reconstruct glacial margins and ice flow directions 2) mapping of the distribution of erratic boulders to determine ice sheet maxima 3) radiocarbon dating to numerically date deposits 4) tephrochronology to assign numerical dates to deposits and 5) measurement of weathering rinds to relatively date deposits.

Abundant glacial landforms combined with erratics allowed a detailed reconstruction of ice margins. Moraines, ice-marginal channels, spillways, kames and ice-marginal bogs are among the most important landforms mapped. Radiocarbon and weathering rind dating proved to be very limited in their usefulness. Very little dateable carbon was found. Weathering rinds were too thin and inconsistent to create a meaningful relative dating equation. Tephras known to exist in the region were found in ice-marginal bogs and provided a minimum date for deglaciation.

The last-glacial recession was largely affected by variations in local topography and a detailed spatial retreat was determined. The last-glacial maximum inundated the entire main valley and its tributaries. The pre-last-glacial 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 fine-tune its spatial attributes.