ABSTRACT
The extent and thickness of former glaciers and ice sheets has most frequently been determined by accurate field mapping of landforms of glacial erosion and deposition. Patterns of former ice flow direction are often reconstructed through measurements of the distribution of glacial erratics and by glacial striae orientations (Plate 1). The principal morphological features that have been used to define the positions of former ice margins are lateral and terminal moraines, fluvioglacial outwash plains, ice-marginal meltwater channels as well as the extent of drift sheets. Within the outermost limits of a particular glaciation, the presence of linear moraines may either indicate stages in the overall retreat and thinning of the ice or they may represent periods of glacier readvance. In many instances, individual landform assemblages are used to define the former extent of glaciers and ice sheets. It should be stated also that, in many instances, it is difficult to estimate the size of former ice sheets and valley glaciers since ice-marginal moraines may be absent. This can arise when the ice was relatively clean (and hence could not deposit any debris) or when the ice margin did not exist in a particular area for a sufficient length of time to allow the construction of moraines. The vertical extent of former ice sheets and glaciers is frequently aided by the distribution of periglacial landforms. In land areas located beyond or above the area directly affected by glacier ice, the boundaries between glaciated topography and the lower limit of frost- shattered bedrock and patterned ground phenomena, known as trim-lines, can be used to define the surfaces of former ice masses (Ballantyne 1990).
