General Interpretation

The sequence outlined above reveals the following general trends:

1. a north-to-south decrease in the importance of lateral meltwater erosion

Distal Glaciation

Figure 7.6 Low oblique aerial view of the Melville Moraine. The moraine has a gentle proximal and steep distal flank. It curves to the right into a valley where it has attached to it a gravel delta terrace marking relative sea level at time of formation, visible in the lower right corner with three snow banks on the delta riser. From the delta, the moraine continues left along the bottom edge of the photograph. Reproduced with the permission of Natural Resources Canada 2010, courtesy of the Geological Survey of Canada. (Photo 205I4IF by Lynda Dredge).

Figure 7.6 Low oblique aerial view of the Melville Moraine. The moraine has a gentle proximal and steep distal flank. It curves to the right into a valley where it has attached to it a gravel delta terrace marking relative sea level at time of formation, visible in the lower right corner with three snow banks on the delta riser. From the delta, the moraine continues left along the bottom edge of the photograph. Reproduced with the permission of Natural Resources Canada 2010, courtesy of the Geological Survey of Canada. (Photo 205I4IF by Lynda Dredge).

2. a north-to-south increase in the importance of subglacial meltwater action

3. a north-to-south increase in the development of streamlined subglacial bedforms, with drumlin fields first appearing in the zone of minor subglacial meltwater action and significant lateral meltwater action

4. a north-to-south increase in the amount of debris in the ice sheets (ignoring recognizable influences of bedrock resistance)

5. a maximal concentration of large ridged and hummocky moraines in the zone of well-developed streamlined bedforms, minor subglacial meltwater action, and significant lateral meltwater action.

We interpret these trends in terms of the basal-ice thermal conditions in the marginal and near-marginal zones. For the moment, we attempt to explain only the generalized pattern.

1. In regions dominated by lateral and proglacial meltwater channels, where ice-marginal accumulations are either absent or consist of ice-thrust material, the ice-marginal zone — and perhaps most of the retreating glacier — was cold-based. Warm-based ice was limited mainly to deep, fjordic valleys, the only sites of significant ice-marginal deposition (see Dyke, 1993, for detailed arguments on the thermal evolution of ice caps growing on permafrost). Although these ice caps were capable of scouring in their outer zones while at maximal extent, they became cold-based during the early stages of deglaciation, probably as a response to ice thinning, reduced strain heating, and concomitant geothermal heat dissipation. This favoured the development of lateral meltwater channels in such large numbers that they record recession at a very fine temporal resolution possibly of (annual?) melt-event scale (Fig. 7.7; see also Bednarski, 1998, 2002; O Cofaigh, 1998; England et al., 2000).

2. The bulky Laurentide ridged and hummocky moraine belts and associated lateral and proglacial channel systems also formed in cold-based marginal zones above contemporaneous sea level. These cold-based zones were narrow, no more than 10—40 km wide, because the moraine belts are flanked up-ice by well-developed streamlined bedforms that splay toward the moraines, which we conventionally interpret as having formed under warm-based ice. The abundant debris delivered to the ice-marginal zone was generated in the warm-based zone up-ice. This fact strongly differentiates this landsystem from its more northerly neighbour, which is nearly devoid of ice-marginal debris. The trivial development of subglacial meltwater features directly inboard of the moraines indicates that only small amounts of subglacial water were available. Surface meltwater on the warm-based ice behind the cold-based marginal fringe probably was unable to reach the bed due to the low temperatures of the upper ice layers, cold upper ice being an inevitable consequence of the low mean annual air temperatures of the region. Furthermore, thick ice would be expected in the distal warm-based zone because of compressive flow against the cold-based fringe. There are few tunnel channels or eskers extending through these end moraine belts, from which we infer that there were few breaks in the marginal fringe of cold-based ice. However, on a broader scale, wide marine channels interrupt the moraine belts. It is improbable that the cold-based marginal fringe extended across these channels, except where fronted by an ice shelf, and hence probable that subglacial meltwater drained almost entirely along these corridors. The distribution of eskers and subglacial channels (e.g. Sharpe, 1992; St Onge and McMartin, 1995) is in accord with this view.

3. If there was a cold-based, marginal ice fringe in the next inner zone, dominated by

Figure 7.7 Lateral meltwater channels, northern Baffin Island. The channels formed along both sides of the valley that drains from southwest to northeast, but most profusely on the southeast-facing slope, where there are about 30 channels in the centre of the photograph. (NAPL AI6263-92.)

drumlins and eskers, it was much narrower and perhaps discontinuous. Here there was abundant basal meltwater under at least the outer tens of kilometres of the ice sheet. With the exception of the end moraines noted above, which probably indicate readvances (Falconer et al., 1965), only minor ice-marginal features occur in this zone. Continuous recession is thus inferred. 4. It has long been a mystery why Rogen moraine in North America is nearly limited to the final ice recession centres of Keewatin and Quebec-Labrador (e.g. Dyke and Prest, 1987; Aylsworth and Shilts, 1989). We suspect that the explanation of this distribution lies in changing basal thermal conditions similar to the proposition of Hattestrand (1997). Dyke et al. (1992) observed that Rogen moraines on Prince of Wales Island — one of the rare occurrences beyond the recession centres — are situated along the erosional contact around the head of a drumlin field. Up-flow from there an older drumlin field survived the younger flow phase under cold-based ice, unmodified by the younger flow. The implication of this relationship is that these Rogen moraines formed at the boundary between sliding and non-sliding (cold-based) ice. On a much larger scale, the Rogen moraine field of Quebec-Labrador is identically situated around the head of the Ungava Bay drumlin field (Prest et al., 1968). The contact between the Rogen moraines (older) and the drumlins (younger) is an erosional unconformity commonly referred to as the Labrador Ice Divide (Veillette et al., 1999). If the basal sliding/non-sliding boundary propagates up-ice during deglaciation, Rogen moraines will be preserved only near the final position of the boundary, which will normally be close to the final centres of ice recession. Those that formed earlier were probably reshaped into drumlins, a process that can be inferred from the common superposition of flutings and drumlins on Rogens and from the lateral transition from Rogens to drumlins where these features are arrayed in flow-aligned trains (Dyke and Dredge, 1989). Rogen moraines are bedforms that formed behind the margin. Some may nevertheless be difficult to distinguish from small recessional moraines. A useful distinguishing characteristic is that Rogens are apparently never kettled, whereas end moraines commonly are.

In summary, the salient and broadest characteristics of glacial landscape zonation, including the distribution and types of ice marginal landforms, can be understood in terms of the changing configuration of basal-ice thermal conditions between the last glacial maximum and deglaciation. There appear to have been:

a. extensive areas of receding cold-based ice wherein little debris had accumulated and where lateral meltwater channels formed profusely b. long, cold-based margins, along which broad morainal belts formed, backed by warmed-based ice, and c. broad, warm-based marginal zones flanking inboard cold-based central ice zones, along the contact of which Rogen moraines formed.

We interpret (a) above traditionally as a cold-based glacier landsystem. Similar interpretations employing similar principles are applied in Scandinavia (e.g. Kleman et al., 1999). We consider in greater detail the more complex ice-marginal landform assemblages. Assemblages that were necessarily formed in a permafrost environment may provide useful analogues for glacial landforms produced in deglacial permafrost environments further south. However, there is much divergence of interpretation of moraine-like features within the permafrost zone. Therefore, we first outline some general restrictions that must on principle apply to interpreting deglacial features formed in permafrost areas and then discuss the three major conceptual models that are currently applied to interpretations of moraine-like belts. These are:

• pseudo-moraines consisting of thermokarsted regional ground ice sheets formed by ice segregation

• hummocky moraine formed by regional stagnation of a broad ice-marginal zone, and

• moraines with cores of glacier ice formed along active ice margins.

The term 'segregation' here refers to the formation of massive ground ice bodies by the process of soil water migrating to, and freezing along, a freezing front.

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