* Correspondence address: Department of Physics, Simon Fraser University, Burnaby, BC, Canada V5A 1S6. E-mail addresses: olian.@sfu.ca (O.B. Lian), shicock@julian.uwo.ca (S.R. Hicock). Quaternary International 68 } 71 (2000) 147 } 162 Thermal conditions beneath parts of the last Cordilleran Ice Sheet near its centre as inferred from subglacial till, associated sediments, and bedrock Olav B. Lian*, Stephen R. Hicock School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand Department of Earth Sciences, The University of Western Ontario, London, Ont., Canada N6A 5B7 Abstract Subglacial sediments and bedrock associated with the last (Fraser) glaciation were studied in a mountain valley and on a mountain plateau (ca. 1200 m elevation, near the south-central spreading centre of the last Cordilleran Ice Sheet) in order to gain information on subglacial thermal conditions. Our study indicates that as ice advanced it was probably initially coupled to, or dragged along, a largely frozen and immobile substrate that fractured. As glaciation proceeded, subglacial till was deposited mainly by lodgement. In most places, however, "eld evidence suggests that, as till thickened and pore water content increased, the till experienced ductile deformation. At some sites there is evidence that pore water increased to a point where till #owed as a viscous slurry while eroding, entraining, and transporting pieces of soft bedrock. In places, brittle deformation was superimposed on ductily-deformed till, a transition that we associate with subsequent till dewatering and sti!ening. A deformable substrate would have contributed to extending ice-#ow, thus increasing #ow velocity while decreasing ice surface pro"les. If this situation was wide spread, then the interior of the Cordilleran Ice Sheet would have consisted of local accumulation areas that were drained by rapidly #owing glaciers (ice streams?). This is supported by previously recorded ice-#ow patterns inferred from large-scale ice #ow indicators, and by other evidence of rapid ice movement along the southern fringe of the ice sheet.  2000 Elsevier Science Ltd and INQUA. All rights reserved. 1. Introduction Over the last decade there has been an increasing emphasis on understanding the nature of ancient ice sheets with the hope that models of growth and decay, developed for late Wisconsinan ice sheets can be used to better understand ancient climatic #uctuations that are recorded elsewhere (e.g., Clark et al., 1995, 1996 and references therein). Furthermore, and perhaps more im- portantly, information about ancient ice sheets can be applied to predicting the future behaviour of the Green- land and other Arctic ice masses that in#uence global sea level and present-day climate in the northern hemisphere. There has been much recent research on developing quantitative models of the last Cordilleran Ice Sheet (CIS; e.g., see review by Jackson and Clague, 1991). Present computer models (simulations) are based largely on land-surface elevation and on the knowledge of local net mass balance, the latter being derived from empirical functions (e.g., Roberts, 1991). However, ice sheet models are limited by the size and nature of the data base, and most models do not generally take into account informa- tion related to subglacial thermal conditions (Murray, 1997), even though this parameter is crucial for modelling ice #ux and ice-surface elevation (Booth, 1986, 1991; Boulton, 1996; Clark et al., 1996; Marshall et al., 1996; Murray, 1997). The reason for this lack of information about former subglacial conditions is that until recently (e.g. Boulton and Jones, 1979; Boulton and Hindmarsh, 1987; Hart and Boulton, 1991; Hicock and Dreimanis, 1992; Hart, 1994; Hicock and Fuller, 1995; Hicock et al., 1996) the data needed to distinguish between various till types was not available, and as a result subglacial till was generally thought of as a passive and immobile substrate. It has only been since the mid-1980s that its importance in governing the nature of ice #ow has been realised (Murray, 1997; Hindmarsh, 1997). Recent reviews and discussions of the importance of this `paradigm shifta 1040-6182/00/$20.00  2000 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 0 0 ) 0 0 0 4 0 - 9