Introduction

During the last glaciation, the Laurentide Ice Sheet (LIS) created the spectacular glacial landscape of northern North America. This landscape preserves a detailed record of the former ice-sheet size, subglacial bed conditions and ice-sheet behaviour. This chapter discusses the distribution of glacial landsystems that were created along the southern margin of the LIS in the northern USA during the late Wisconsin Glaciation (Fig. 6.1). As we show, landforms and sediments along the southern margin of the LIS are not randomly distributed but are arranged in patterns, which suggest that climate, topography, bed conditions and the resulting ice-sheet dynamics combined to yield distinct landsystems.

A difficulty in interpreting glacial features is the time-transgressive nature of the landscape. Ice-sheet conditions evolved and we must distinguish between landforms developed during cold conditions of ice advance and stability during the Last Glacial Maximum (LGM), and those that formed during deglaciation, when water was abundant and the ice margin was more dynamic. We define a landsystem as 'a genetically related set of landforms and sediments within a distinct region'. Our definition of landsystem contains numerous depositional environments (such as subglacial and ice marginal) that are created by a single ice lobe during a restricted time interval (such as a glacial phase). As genetic processes change with time and environments migrate laterally, one landsystem is superimposed on another. Therefore, to speak of a particular area as containing one specific landsystem is misleading as numerous depositional environments, during multiple phases, produce a suite of landforms and sediments. Commonly, an area is dominated by one landsystem, but includes older (palimpsest) or younger features (superimposed). Our experience and our landsystem maps reflect this, in that we find that two or three landsystems are commonly superimposed even in a small area such as is covered by a 7.5 X 7.5 minute USGS quadrangle. Nevertheless, the purpose of thematic mapping is to generalize and show patterns within the complexity of nature.

We seek to understand the southern LIS by:

1. mapping the distribution of landforms and sediments and classifying them (as landsystems)

Figure 6.1 The maximum extent of the southern margin of the Laurentide Ice Sheet. The Last Glacial Maximum extent was probably never a continuous ice margin because each lobe reached its maximum position at slightly different times and began retreating at different times. For example the Des Moines lobe and other lobes in the western region reached their maximum extent sometime after 14,000 l4C years BP, whereas most of the lobes in the Great Lakes region reached their maximum positions earlier at about 21,000 l4C years BP. Lobes in New England may have reached their maximum even earlier by about 23,000 l4C years BP. Ice flow directions and ice divides are after Dyke and Prest, 1987; Veillette et al., 1999.

Figure 6.1 The maximum extent of the southern margin of the Laurentide Ice Sheet. The Last Glacial Maximum extent was probably never a continuous ice margin because each lobe reached its maximum position at slightly different times and began retreating at different times. For example the Des Moines lobe and other lobes in the western region reached their maximum extent sometime after 14,000 l4C years BP, whereas most of the lobes in the Great Lakes region reached their maximum positions earlier at about 21,000 l4C years BP. Lobes in New England may have reached their maximum even earlier by about 23,000 l4C years BP. Ice flow directions and ice divides are after Dyke and Prest, 1987; Veillette et al., 1999.

2. interpreting conditions of landform-sediment genesis, and

3. deducing characteristics of the ice sheet from those features.

We restrict most of our discussion to landforms created during the maximum extent of the LIS. We also discuss how in many areas, landforms and sediments created during deglaciation changed significantly from those created during advance and while ice was at its maximum position. This indicates that glacier-bed conditions also changed significantly between the LGM and deglaciation.

Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

Get My Free Ebook


Post a comment