Landscapelevel classification

The swarm types we recognize are classified as follows (Plate 38.8).

1 The deglacial envelope. This swarm type subdivides into two components.

• Wet-bed swarms defined by flow traces (typically drumlins and flutes) with aligned eskers. If the wet-based zone has resulted from thawing of previous cold-based conditions, these swarms will also hold ribbed moraines. These 'classic' swarms are interpreted to represent inward-transgressive formation of flow traces (Boulton et al., 1985; Kleman et al., 1997), which become preserved as new areas are successively deglaciated.

• Frozen-bed deglaciation swarms. In these, the landform record consists solely of a see-through pattern of meltwa-ter traces overprinted on a relict surface. Marginal channels are dominant and eskers are small or lacking. The relict surfaces may be former subaerially developed non-glacial landscapes or may contain (usually non-aligned) flow traces from an older glacial event.

Note that in addition to the deglacial envelope from the last ice sheet there can exist older deglacial swarms in a particular area. This will be the result if the last deglaciation event was characterized by frozen-bed conditions, preserving any pre-existing landscapes, or only experienced a short period of warm-based conditions that did not completely reshape previous deglacial swarms.

2 'Event' swarms. These swarms are defined by landform systems with abundant flow traces (typically drumlins and flutes) but lacking aligned meltwater traces. In some cases they can be interpreted as the sites of former ice streams; in other cases they may have formed by slow sheet-flow far inside the margin. If such a swarm is defined by glacial lineations lacking a later overprint, the termination of lineation creation was probably caused by change to a frozen bed. If a swarm is defined by a low-frequency but regional occurrence of older striations, no inferences can be made regarding basal temperature during subsequent events. On the spatial scale of individual roches moutonnées, lee-side protection and preservation is operational. Hence, old striae can be preserved, despite sustained wet-based ice flow from other directions.

3 Ice-stream swarms. These swarms represent events of enhanced ice flow, draining considerable amounts of ice. They typically have a strongly convergent head zone, probably reflecting the transition from sheet flow to stream flow. Ice-stream swarms associated with land-terminating flow often have a distinctive bottleneck pattern, with a divergent terminal zone, whereas water-terminating swarms lack the divergent-flow zone (Stokes & Clark, 1999). At present we use only one class for ice-stream landscapes, but as understanding progresses, it is likely that a more elaborate classification will have to be developed to adequately cover functionally different ice-stream types, as described above in the section on key considerations.

Landform systems may be formed during one single event or formed in a time-transgressive fashion. A mapped swarm is therefore the orthogonal projection of a system that may be sloping in the three-dimensional time-distance domain.

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