Introduction

For most individuals the Laurentide Ice Sheet (LIS) refers to Antarctic-size ice sheets that extended from the general area of the Great Lakes in the USA northward to the Canadian Arctic

Laurentide Ice Sheet

coast, and westward from the uplands and fjords of Baffin Island and Labrador to the foot of the Rocky Mountains (Fig. 40.1). However, the late V. Prest insisted that the term was applicable only to the ice sheet during the last glaciation. In this section I will adopt the more general usage for an ice sheet that had an

Figure 40.1 Outline of the Laurentide Ice Sheet as depicted in Dyke & Prest (1987b) and the outline today (Dyke et al., 2002). The large bold lettering refers as follows: CIS, Cordilleran Ice Sheet; IIC, Innutian Ice Sheet; FB, Foxe Basin ice divide; LC, Labrador ice divide; KC, Keewatin ice divide. The positions are approximate and shifted through time. Also shown are examples of long distance erratic plumes (see text for discussion), specifically the Dubwant red bed sandstones and the 'omars' (Prest, 1990) of northern Ontario (oolitic jaspers and greywackes).

Figure 40.1 Outline of the Laurentide Ice Sheet as depicted in Dyke & Prest (1987b) and the outline today (Dyke et al., 2002). The large bold lettering refers as follows: CIS, Cordilleran Ice Sheet; IIC, Innutian Ice Sheet; FB, Foxe Basin ice divide; LC, Labrador ice divide; KC, Keewatin ice divide. The positions are approximate and shifted through time. Also shown are examples of long distance erratic plumes (see text for discussion), specifically the Dubwant red bed sandstones and the 'omars' (Prest, 1990) of northern Ontario (oolitic jaspers and greywackes).

area of approximately 12 x 106km2, a central thickness estimated to be in the range of 3-4 km, and sufficient mass to cause a worldwide fall in sea-level of around 70m (Denton & Hughes, 1981; Paterson, 1972). Given the estimated volume of the ice sheet, computed in various ways but invariably resulting in an estimate ca. 70+ m of equivalent sea-level, then the global history of sea level deduced from the variations in the S18O (Shackleton, 1973; Shackleton & Opdyke, 1973) must be dominated over the past 2.5 Myr by the growth and retreat of this ice sheet. The Antarctic Ice Sheet is about the same size as the former LIS but all indications are that it has been much more stable and has certainly not participated in the dramatic 100 and 41kyr glacial cycles that are characteristic of the global ice-volume record (Raymo, 1992) (see Andrews, this volume, Chapter 21). Thus the LIS has to be considered a major component in the Cenozoic glaciations of our planet.

In this section: (i) I will briefly outline the history of exploration and thought that resulted in the concept of a vast North American Ice Sheet, the major unit of which is the LIS. I will then go on to address some critical issues in respect to this ice sheet, namely: (ii) the erosional history of the ice sheet; (iii) the nature of the ice sheet's bed and evidence for glacial transport; (iv) evidence for a complex growth and retreat. Because this is a brief survey readers should be aware of several major compilations that should be consulted for additional details. In particular the 1989 compendium Quaternary Geology of Canada and Greenland (Fulton, 1989) covers many regional and ice-sheet wide topics. Recent compilations of note include those by Dyke et al (2002) and Dyke (2004); these are especially concerned with the chronology of the last glaciation and deglaciation.

Enormous strides have been made in understanding the spatial and temporal variability in glaciological processes under and marginal to the LIS since my involvement, which started in 1959. In a general chronological sequence the major elements aiding this have been:

1 the aquisition of 1: 50,000 aerial photographs and the provision of 1: 250,000 topographic maps, starting in the late 1940s onward;

2 the development and usage of radiocarbon dating in the late 1950s;

Figure 40.2 Redrawn map showing Tyrrell's concept of the ice sheet at the turn of the last century (1898). Note the overall similarities with the more modern concepts on Fig. 40.1.

Figure 40.2 Redrawn map showing Tyrrell's concept of the ice sheet at the turn of the last century (1898). Note the overall similarities with the more modern concepts on Fig. 40.1.

3 the military and national infrastructure in the Canadian north (e.g. DEW Line and Weather Stations), which enabled field parties to be supported;

4 the LANDSAT imagery of the 1970s;

5 the interaction, starting in the late 1970s, between glacial studies on land and offshore marine studies on glaciated margins and adjacent deep-sea basins, largely promoted by the Bedford Institute of Oceanography;

6 the recent use of cosmogenic exposure dating and isotopic studies on provenance to aid in understanding ice-sheet chronology, erosion and transportation.

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