Ungava, Canada

Palaeogeography, Palaeoclimatology, Palaeoecology 193 (2003) 473^501 www.elsevier.com/locate/palaeo

Early Holocene glacial lakes and ice marginal retreat pattern in Labrador/Ungava, Canada Krister N. Jansson Department of Physical Geography and Quaternary Geology, Stockholm University, SE-106 91 Stockholm, Sweden Received 31 May 2002; accepted 24 January 2003

Abstract The existence of glacial lakes Naskaupi, McLean, Minto, Me¤le'zes, and Wapussakatoo during the deglaciation of Labrador/Ungava, Canada, was recognized at the end of 1950s. However, glacial lakes have seldom been included in regional reconstructions to constrain the outline of successive ice margins during the glacial retreat in Labrador/ Ungava. Reconstruction of the ice flow succession in Labrador/Ungava has often primarily focused on till lineation systems. These reconstructions often depict a late glacial ice dispersal center situated over central Labrador/Ungava. More recent studies on glacial geomorphology including meltwater features, however, suggest that north-central Labrador/Ungava exhibited cold-based conditions at least during the latest deglaciation. Cold-based conditions inhibit basal sliding and formation of landforms, except for meltwater traces such as meltwater channels, glacial lake shorelines and deltas. This situation implies that meltwater traces are the main source of information when reconstructing the spatial retreat pattern during a cold-based deglaciation. Evidence presented in this study, such as glacial lake shorelines, fossil deltas, and spillway and drainage channels in north-central Labrador/Ungava, indicates the existence of numerous previously unmapped glacial lakes. The mapped glacial lake features are synthesized to a reconstruction of several glacial lake stages which, in turn, are used to constrain the late glacial ice margin retreat pattern over the inferred cold-based areas of north-central Labrador/Ungava. A total of 26 glacial lakes (65 substages) existed during the deglaciation of Labrador/Ungava. These lakes were impounded along the southern margin of the shrinking ice sheet. The required damming ice margins indicate that the last ice remnant of the Laurentide Ice Sheet in Labrador/Ungava was situated over the southern Ungava Bay and the adjacent southern shore. < 2003 Elsevier Science B.V. All rights reserved. Keywords: glacial lake; shorelines; deglaciation; retreat pattern; Labrador/Ungava; Laurentide Ice Sheet

1. Introduction The purpose of this study is to reconstruct the glacial lake system of Labrador/Ungava during the last deglaciation and to constrain the late glacial ice marginal retreat pattern of north-central

E-mail address: [email protected] (K.N. Jansson).

Labrador/Ungava (Fig. 1). Recent studies show evidence that the Late-Wisconsinan ice marginal retreat over north-central Labrador/Ungava occurred under cold-based conditions (Kleman et al., 1994; Kerwin, 1996; Kleman and Ha«ttestrand, 1999; Clark et al., 2000; Jansson et al., 2002). Cold-based conditions inhibit basal sliding and formation of landforms, except for those created by meltwater such as channels, glacial lake

0031-0182 / 03 / $ ^ see front matter < 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0031-0182(03)00262-1

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Fig. 1. (A) Labrador/Ungava with hydrology referred to in text. Boxes indicate the outlines of Figs. 9^16. (B) Location map. Solid black lines indicate major watersheds and arrows direction of drainage. (C) The two major glacial landform systems of Labrador/Ungava. Arrows indicate generalized ice £ow direction from Prest et al. (1968) and gray indicates area inferred to have been covered by cold-based ice (Kleman and Ha«ttestrand, 1999). The broken line shows the zone where these two landform systems intersect (referred to as the horseshoe unconformity by Clark et al. (2000)). In contrast this zone is interpreted by e.g. Prest (1970), Boulton et al. (1985), and Dyke and Prest (1987) to re£ect a late glacial ice divide. (D) General pattern of glacial lakes in Labrador/Ungava, redrawn from Gray and Lauriol (1985).

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shorelines, and deltas (Sugden and John, 1976). When reconstructing a cold-based deglaciation, therefore, meltwater traces are the main available source of information (Sugden and John, 1976; Borgstro«m, 1989). Or, as stated by Ives (1960a,b): ‘‘understanding the evolution of former glacial lakes in Labrador/Ungava is a key element in reconstructing late Laurentide Ice Sheet recession pattern’’. To date, the existence of the following glacial lakes has been well-documented: McLean and Naskaupi in the east (Henderson, 1959; Ives, 1959b, 1960b; Matthew, 1961; Barnett, 1963, 1967; Harrison, 1963; Barnett and Peterson, 1964; Peterson, 1965; Prest et al., 1968; Ives et al., 1976; Clark and Fitzhugh, 1990), Wapussakatoo in the south (Gill et al., 1937; Henderson, 1959; Harrison, 1963), glacial lakes in the Schefferville area (Slipp, 1952 in Liverman and Vatcher, 1993; Usher, 1953 in Liverman and Vatcher, 1993; Henderson, 1959; Liverman and Vatcher, 1993), and Minto and Me¤le'zes in the west (Lauriol and Gray, 1983, 1987; Gray and Lauriol, 1985; Gray et al., 1993). In comparison, little is known about the evolution of the glacial lakes in central Labrador/Ungava and in the Torngat Mountains (Ives, 1957, 1960c; Andrews, 1961; Matthew, 1961). Shorelines that form parts of glacial lakes Caniapiscau and Chaigneau were identi¢ed by Hughes (1964), Gray et al. (1993) and Ives (1957). Information on the outline of these lakes, their various sub-stages, the orientation of the damming ice margin, and the pattern of drainage is scarce but of critical importance to an understanding of the late glacial environment and retreat pattern in Labrador/Ungava. Three patterns of ice sheet retreat in Labrador/ Ungava have been proposed: (1) a last ice remnant disintegrating over the Sche¡erville area, (2) a last ice dispersal center situated over the southern Ungava Bay and the adjacent southern shore, and (3) a late glacial U-shaped ice divide situated over central Labrador/Ungava. Some of the early studies depicted a last ice dispersal center over the Sche¡erville area, central Labrador/Ungava (Ives, 1959a, 1968; Kirby, 1961; Barnett, 1963; Harrison, 1963; Barnett and Peter-

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son, 1964; Hughes, 1964; Peterson, 1965; Bryson et al., 1969; Ives et al., 1976). The recognition of glacial lakes McLean and Naskaupi, and the required outline of the ice margins damming these lakes, however, indicated an ice marginal retreat towards the northwest or west-northwest and a last ice dispersal center situated over the southern Ungava Bay and the adjacent southern shore (Ives, 1960a,b; Matthew, 1961; Allard et al., 1989; Clarha«ll and Jansson, submitted). Gray and Lauriol (1985) drew the same conclusion, based on the ice sheet extent and ice marginal positions required for the impounding of glacial lakes Nantais (northern Ungava Peninsula), Minto, Me¤le'zes, Cambrien, McLean, and Naskaupi (Fig. 1). These interpretations of a last ice remnant situated over the southern Ungava Bay and the adjacent southern shore did not gain general acceptance, however, and were replaced by the concept of a late glacial U-shaped ice dispersal center situated over central Labrador/Ungava (Fig. 1C). This concept is a recurring feature in most recent interpretations of the glacial geomorphological record in Labrador/Ungava (e.g. Prest, 1970; Boulton et al., 1985; Dyke and Prest, 1987; Gray et al., 1993). The background to this concept, which is based on interpretations of glacial lineations and striae, is that the glacial geomorphology in Labrador/Ungava is dominated by two major glacial landform systems. One is a radial pattern of till lineations, ribbed moraine ¢elds, and eskers that can be traced inwards from the peripheral eastern, southern, and western parts of Labrador/Ungava towards the northcentral part of the peninsula. The other landform system consists of a convergent pattern of till lineations (cf. Prest et al., 1968; Jansson et al., In press), and indicates northward ice £ow towards Ungava Bay (Ungava Bay landform swarm). The two landform systems overlap in a U-shaped zone (Klassen and Thompson, 1993; Veillette et al., 1999; Clark et al., 2000), which was interpreted to re£ect a late glacial ice dispersal center (Prest, 1970, 1984; Shilts, 1980; Dyke et al., 1982; Boulton et al., 1985; Dyke and Prest, 1987; Gray et al., 1993; Parent et al., 1995; Veillette et al., 1999).

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Fig. 2. Gray-scale contour map of the area of investigation (black box). Numbers are map index numbers of the Canadian Topographic System.

Recent reconstructions of the Labrador/Ungava ice recession pattern (Kleman et al., 1994; Clark et al., 2000; Jansson et al., 2002) and studies of the Ungava Bay landform swarm (Jansson and Kleman, 1999; Jansson et al., in press), however, show that the Ungava Bay landform swarm is overprinted by two lineation systems, one of which is of inferred Last Glacial Maximum age. Hence, Jansson et al. (2002) suggested that these lineation systems escaped glacial erosion through cold-based conditions during the deglaciation. The Ungava Bay landform swarm should, therefore, not be included in reconstructions of the

Late-Wisconsinan ice retreat pattern in Labrador/Ungava. This study aims to re¢ne the reconstruction of the Laurentide Ice Sheet retreat pattern over Labrador/Ungava, using traces of the glacial lake system and their required damming ice margin. This work embraces previously known glacial lakes and presents a reconstruction of numerous previously unknown glacial lakes in Labrador/Ungava. Here, the widespread occurrence of glacial lake traces (shorelines, deltas, spillways, and drainage channels) is analyzed in a digital elevation model (DEM) environment.

Fig. 3. Stereogram of glacial lake Pons features in the upstream western section of Rivie're Pons. The glacial lake shorelines were formed by di¡erent sub-stages of glacial lake Pons, which formed parts of glacial lake Caniapiscau stages 7, 8, and 9 (Cp-7 520 m a.s.l., Cp-8 490 m a.s.l., and Cp-9 460 m a.s.l.; Fig. 11). A points to glacio£uvial deposits and Bs point to glacial lake shorelines. Canadian Air Photo Library A14346-75, 76.

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Fig. 4. Stereogram of glacial lake Pons meltwater features in the middle reaches of Rivie're Pons. A points to clean-washed bedrock, B points to a fossil delta, and Cs point to meltwater channels (routes) indicating an eastward drainage. Canadian Air Photo Library A15634-194, 195.

2. Methods 2.1. Geomorphological mapping Black and white aerial photographs at scales of 1:30 000 and 1:60 000 were employed for map sheets 23F (53‡40P to 54‡00P), 23G, 23J, 23K, 23L (54‡35P to 55‡00P), 23M, 23N, 23O, 24B, and 24C (Fig. 2). Aerial photographs cover

6.8U6.8 km or 13.7U13.7 km, with a potential spatial resolution of approximately 1.5 and 3 m, for the respective scales. The aerial photographs were interpreted in a Zeiss Jena Interpretoscope and a Topcon stereoscope with 2^16U and 1.5 and 3U magni¢cation range, respectively. For map sheets 23E, 23F (53‡00P to 53‡40P), 23I, 23L (54‡00P to 54‡35P), 23P, 24A, and 24D (Fig. 2) a high-magni¢cation micro¢lm reader was em-

Fig. 5. Stereogram of channels southwest of the Rivie're Serigny/Rivie're Caniapiscau junction indicating an eastward drainage of meltwater. Arrows point to meltwater channels. Canadian Air Photo Library A11433-84, 85.

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Fig. 6. Stereogram of shorelines (arrowed) of the northeastern Caniapiscau Reservoir. The shorelines were formed during glacial lake Caniapiscau stage 3 (Cp-3 580 m a.s.l.; Fig. 10) at its northeastern extent. Canadian Air Photo Library A23592-224, 225.

ployed to study micro¢lm copies of aerial photographs at scales of 1:30 000 and 1:60 000 (cf. Jansson et al., 2002). The interpretations were transferred onto transparent overlays and scaled down to ¢t the Cana-

dian topographic maps at a scale of 1:250 000 (contour intervals at V30, 50, and 60 m). The scaled overlays were then redrawn on a manuscript map. To get a proper match between the location of mapped landforms in the aerial photo-

Fig. 7. Reconstruction of glacial lake Wapussakatoo. All panels show the GLOBE DEM covering the glacial lake Wapussakatoo area and black lines indicate shorelines identi¢ed by Harrison (1963). (A) The 590, 585, and 575 m a.s.l. contours were identi¢ed to be associated with zones where shorelines occur (black broken line shows 575 m a.s.l., gray broken line shows 585 m a.s.l., and gray solid line 590 m a.s.l.). (B) The outlines of the di¡erent lake levels of glacial lake Wapussakatoo are expressed by di¡erent gray-scale tone. (C) Possibly spillway routes (open arrows) and drainage routes (¢lled arrows) are situated in low points of the terrain, and they were successively exposed during ice marginal retreat. The ice margins (thick solid black line) were reconstructed to best ¢t the outline of glacial lake shorelines and to allow for the damming of the lakes. Related black arrows indicate the ice £ow direction associated with the marginal position.

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rounding till-covered landscape. Moreover, glacio£uvial accumulations often reveal erosional scars and sharp boundaries to the surrounding till-covered landscape. The glacio£uvial accumulations often occur in association with eskers and valley trains. Bedrock is distinguished from glacio£uvial accumulations by the presence of fracture patterns and other bedrock structures. Fossil deltas (Fig. 4) are situated above present lake levels and are interpreted using similar criteria as the glacio£uvial accumulations. The most diagnostic criteria are the £at top surface and the steep delta front.

Fig. 8. The probable area covered by glacial lake Wapussakatoo stage 1 could have been as limited as the dark gray area (preferred here) or as large as the dark and light gray areas combined. The arrowheaded broken line shows the range of possible ice marginal positions. The dark gray area indicates the preferred outline of glacial lake Wapussakatoo stage 1, with accompanying ice margin (solid black line) and ice £ow direction (¢lled arrow). The ice margin with accompanying ice £ow direction is reconstructed to best ¢t the distribution of shorelines and deltas (A) and drainage routes (B). The ice margin is also adjusted to ¢t the distribution of eskers (gray solid line). Esker distribution is from Prest et al. (1968).

graphs and the symbol on the map, at least three contemporary lake outlines were used for reference. Special attention was paid to glacial lake shorelines, for which the position of the nearest lake was used to place the symbol on the map. Previously published data on glacial lake shorelines and deltas of glacial lakes Wapussakatoo, Naskaupi, McLean, Minto, and Me¤le'zes were included in the analysis (Harrison, 1963; Barnett, 1967; Lauriol and Gray, 1983, 1987). 2.1.1. Glacio£uvial accumulations and fossil deltas The recognition of glacio£uvial accumulations was normally based on their gray tone and surface texture (Fig. 3); typically the lack of boulders at the surface contrasts strongly with the sur-

2.1.2. Meltwater channels The focus has been to map meltwater channels associated with glacial lakes, such as spillway and drainage channels. To separate spillway channels from other channels is not always straightforward. Spillway channels, however, occur in association with cols, at the lowest point of water divides, and are often associated with cleanwashed bedrock zones (Figs. 4 and 5). A complication is that these cols, exposed during the ice marginal retreat, may have been ¢rst used to drain a previous glacial lake stage. Hence, glacial lake drainage channels formed either in cols or on valley slopes parallel to the ice margin. Drainage channels formed parallel to the ice margin are sometimes di⁄cult to distinguish from regular lateral meltwater channels. Typically, drainage channels appear larger and are often associated with areas of washed bedrock. A reconstruction of the outline of a glacial lake and its lowest points around the perimeter must be carefully examined to understand the classi¢cation of di¡erent meltwater channels. 2.1.3. Glacial lake shorelines A zone of wave-washed till represents the most common type of glacial lake shorelines. These zones are clearly recognized in the aerial photographs as light gray bands of washed boulders (Figs. 3 and 6). Shoreline terraces exist but they are less common. Both these features were mapped as glacial lake shorelines. Glacial lake shorelines often occur in stepped sequences indicating that the glacial lake existed at several dif-

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ferent levels. Glacial lake shorelines are relatively easy to distinguish from bedrock structures, because the latter typically have a more extensive spatial distribution than glacial lake shorelines, and seldom are strictly horizontal. There is reason to believe that shorelines are underrepresented in the aerial photo interpretation. In forested terrain many shorelines are revealed by the fact that tree cover is thin or absent on cobble- or stone-covered shorelines. Where shorelines are narrow and less well developed, no gap in the tree cover will be present. The consequence is that many shoreline systems mapped as fragmentary may in fact be more continuous than the mapping indicates. 2.2. Reconstruction of the glacial lakes The glacial geomorphological data were transposed on a digital terrain model produced from the Global Land One-km Base Elevation (GLOBE). GLOBE comprises a 30Q latitude^longitude array (Hastings et al., 1999). At the latitude of the investigation area the grid spacing is approximately 925 m (in latitude) by 510 m (in longitude). The ¢rst stage in the analysis was to identify levels in the terrain containing a high concentration of glacial lake shorelines and deltas within a particular basin (Fig. 7). These levels were isolated in the terrain model and the drainage basins de¢ned for each level were handled as a potential glacial lake outline. The reconstructed glacial lake outlines were then adjusted to ¢t the spatial distribution of glacial lake shorelines, deltas, and spillway and drainage channels. The ice margin corresponding to each glacial lake level was positioned to best ¢t the above-mentioned geomor-

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phological traces. For glacial lakes for which the available data did not allow the marginal position to be determined accurately, the damming ice margin was adjusted to best ¢t adjacent ice marginal positions and the direction of eskers (Fig. 8). For a reconstruction of the outline of glacial lakes, the quality and density of topographical input is critical. The topographical data sources were the GLOBE DEM and Canadian topographical maps with contour intervals of 100 ft, 200 ft or 50 m. In the area southwest of Sche¡erville these data sets were of insu⁄cient resolution to allow a detailed reconstruction of the glacial lakes. A comparison with previous reconstructions (e.g. glacial lake Naskaupi) indicates that similar results are derived for high-relief areas. Di⁄culties in depicting meltwater drainage and spillway routes, however, exist for the low-relief areas in the central part of Labrador/Ungava, because the relatively low resolution of the DEM prevents an accurate determination of their location.

3. Results Central Labrador/Ungava, which is part of the Canadian Shield, is relatively £at with isolated hills. This topography makes the reconstruction of glacial lake outlines considerably more di⁄cult than in high-relief terrain with well-de¢ned valley systems. In some of the maps (Figs. 9^16), shorelines fall outside the reconstructed glacial lake outlines. The locations of these shorelines are correct, and the apparent mismatch is caused by the inability to resolve the details of the glacial lake outlines in the DEM. Glacial lake traces in Figs. 9^16 that are situated within reconstructed glacial

Fig. 9. The reconstructed glacial lake systems of Labrador/Ungava (cf. Table 1). The indicated landforms are extracted from the ‘Glacial map of north-central Labrador/Ungava, Canada’ (Jansson, 2002). The contour maps are constructed from the GLOBE DEM. Sketches at the bottom of each panel show the outline of every glacial lake sub-stage, the damming ice margin, and the associated ice £ow direction. (A) Glacial lake Wapussakatoo de¢ned by shorelines from Harrison (1963). No spillway or drainage channels were identi¢ed. Drainage routes are, however, suggested in Table 1. For location see Fig. 1. (B) Glacial lake Boilay is de¢ned by a small number of shorelines and a delta. Drainage was towards the west (see Table 1). (C) Glacial lakes Attikamagen, Petitsikapau, Astray, Marble, and Sims. All these lakes existed in a low-relief terrain where the drainage basins were di⁄cult to identify. Their existence is, however, de¢ned by a large number of shorelines. The resolution of the topographical data does not allow a detailed reconstruction of the drainage routes.

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Fig. 11. Glacial lakes Caniapiscau (stages 7^10) and Costebelle. Glacial lakes Chaigneau, Goodwood, Swampy, Se¤rigny, and Pons form parts of glacial lake Caniapiscau. In some areas there is only a small number of glacial lake shorelines and deltas and the glacial lake extent is largely based on the drainage basin (broken blue line). Legend from Fig. 9. For location see Fig. 1.

lake basins are often associated with islands too small to be resolved by the terrain model. The problems arising from the coarseness of the DEM in relation to the precise location of mapped glaciolacustrine features also hamper a correlation of interpreted drainage channels with

inferred drainage events of glacial lakes. The general drainage pattern, however, appears to have been towards east or east-northeast, along the ice margin. The reconstruction of the outline of glacial lakes Costebelle (Ce-2 580, Fig. 11), Naskaupi

Fig. 10. Glacial lakes Caniapiscau (stages 1^6), McPhadyen, Delornieu, Chaigneau, Junot, Druillettes, Goodwood, and Chastenay. Glacial lake Swampy forms part of glacial lake Caniapiscau stage 6. In some areas there is only a small number of glacial lake shorelines and deltas and the glacial lake extent is largely based on the drainage basin (broken blue line). Legend from Fig. 9. For location see Fig. 1.

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(N-1 520, N-2 450, N-6 330, and N-7 310, Figs. 12 and 14), Delay (D-1 450, D-2 420, and D-4 330, Fig. 13), Caniapiscau (Cp-12 330 and Cp-13 310, Fig. 14), and Cambrien (Cb-1 290 and Cb-2 220, Fig. 15) and the outline of associated damming ice margin is partly tentative because of the lack or sparseness of glacial lake shorelines and deltas in some areas of the drainage basins. The outline of the damming margin for these lakes is reconstructed on the basis of the continuity of the direction of the ice margin retreat inferred from surrounding glacial lakes, the direction of eskers, and the outline of the drainage basin. It is, therefore, possible that the size of these lakes is overestimated (Figs. 11^15). 3.1. Glacial lakes The reconstruction of the glacial lakes is presented in Table 1 and Figs. 9^16. Data on glacial lake features such as the occurrence of shorelines, deltas, spillway and drainage channels, and accumulations of glacio£uvial sediment are extracted from the ‘Glacial map of north-central Labrador/ Ungava, Canada’ (Jansson, 2002). The reconstructed glacial lakes in Figs. 9^16 are presented in their topographical context to give an impression of the outline of the drainage basin except for Fig. 9C (glacial lakes Astray, Attikamagen, Petitsikapau, Marble, and Sims) for which topographical data only allow an overview of the drainage basins. The existence of these lakes is, however, veri¢ed by the existence of glacial lake shorelines identi¢ed in this study and by Slipp (1952) in Liverman and Vatcher (1993), Usher (1953) in Liverman and Vatcher (1993), Henderson (1959), and Liverman and Vatcher (1993). Figs. 9^16 include all lake-related landforms within the respective map area and not only those landforms that are associated with the glacial lake(s) shown in each panel. Collectively, Figs. 9^16 show the complete re-

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constructed record of glacial lakes in the investigated area. The selection of lakes included in each of the panels is based on graphical clarity, i.e. not all lakes within a given geographical area are included in each ¢gure. A summary of the glacial lake systems is presented in Fig. 17. For details on glacial lake substages, spillway and drainage routes, destination of drained meltwater, and references to information from published literature, see Table 1. The number of lakes, however, depends on how the lakes are named. I chose to re-name those glacial lakes for which this study shows that signi¢cant changes in outline or drainage routes occurred (e.g. the transition between glacial lakes Caniapiscau and Cambrien). The glacial lakes Sims, McPhadyen, Druillettes, Goodwood, and Minto apparently only existed at one level or were part of a larger lake. Large glacial lakes such as McLean, Caniapiscau, Naskaupi, Cambrien, Chaigneau, and Me¤le'zes experienced several sub-stages at di¡erent elevations. These lakes ¢lled larger basins, allowing them to exist for longer periods of time than the small lakes. The central lakes (e.g. glacial lake Chaigneau, Caniapiscau stages 7^10) generally drained eastwards into glacial lakes McLean and Naskaupi which, in turn, established spillways through cols in the Torngat Mountain watershed to the east. Early stages of the glacial lakes (e.g. glacial lakes Caniapiscau stages 1^6 and Delay) drained westwards to Hudson Bay. During the last stages of the deglaciation glacial lakes drained into Ungava Bay (glacial lakes Minto, Me¤le'zes, and Cambrien ; Table 1). During the aerial photograph interpretation some anomalous glacial lake features were identi¢ed. These did not easily ¢t into the development of the glacial lakes as shown in Fig. 17. These features, predominantly shorelines, are often situated at high-elevation areas where no drainage basins could be identi¢ed in GLOBE. Also, chan-

Fig. 12. Glacial lakes Naskaupi (stages 1^3), McLean (stages 0^6), and La Touche. In some areas there is only a small number of glacial lake shorelines and deltas and the glacial lake extent is largely based on the drainage basin (broken blue line). Legend from Fig. 9. For location see Fig. 1.

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Fig. 13. Glacial lakes Gayot, Delay, and Loudin. In some areas there is only a small number of glacial lake shorelines and deltas and the glacial lake extent is largely based on the drainage basin (broken blue line). Legend from Fig. 9. For location see Fig. 1.

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nels formed by meltwater drainage towards the north occur predominantly north of Sche¡erville (cf. Ives, 1959a, 1960b).

4. Interpretation of ice marginal retreat pattern The ice marginal retreat during the last deglaciation in Labrador/Ungava caused damming of signi¢cant bodies of water in river valleys that normally drain into Ungava Bay. The damming of the glacial lakes started when the ice margin retreated over the watersheds separating drainage towards the Hudson Bay/Ungava Bay, St. Lawrence/Ungava Bay, and Labrador Sea/Ungava Bay. The deglaciation of Labrador/Ungava leads to the consecutive formation and lowering of 26 glacial lakes (65 sub-stages; Fig. 17). The reconstructed ice retreat pattern is based on the reconstructed glacial lake sequence and the general direction of the eskers (Fig. 17). Every marginal position responsible for the impounding of a glacial lake, detected in this study, forms a part of the reconstruction. The results of this reconstruction indicate that the late glacial ice marginal retreat was towards the southwestern part of Ungava Bay (Fig. 18), and that the derived ice retreat pattern was relatively symmetric. The ¢rst lakes to form, according to this reconstruction, were glacial lake Naskaupi (Clark and Fitzhugh, 1990) at the eastern margin and glacial lake Wapussakatoo at the southern margin. Shortly thereafter glacial lake Minto started to form (Lauriol and Gray, 1983). Glacial lakes Me¤le'zes and Cambrien were the last major lakes to exist in Labrador/Ungava and they drained during the ¢nal deglaciation into the western and the eastern part of Ungava Bay, respectively. According to this reconstruction, the impounding of the glacial lakes occurred continuously during the deglacial ice marginal retreat.

5. Discussion The remaining uncertainties of this reconstruction are mainly caused by the limitations imposed by the relatively coarse resolution of the GLOBE

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DEM, the precision in shoreline and delta elevation determination, and the errors potentially related to di¡erential glacio-isostatic recovery. A reconstruction based on a topographical data set with a higher resolution would have reduced the number of shorelines that fall outside the current reconstructed glacial lake outlines. However, such a reconstruction would probably yield an even more complex pattern of successive glacial lakes. A postulated deglaciation pattern reconstructed with a higher-resolution elevation data set would most certainly yield irregular ice margins with ice tongues extending into the valleys. Reconstructions of the outline of glacial lakes (Pons and Caniapiscau) in the low-relief area in the interior of the peninsula by using the GLOBE DEM has been compared to reconstructions based on data from topographic maps with a contour interval of 30 m (Fig. 19). The result shows that small discrepancies in outlines of the glacial lakes and the damming ice margin occur between the two data sets. However, together with the outline of major drainage basins (Fig. 1B) the data sets clearly indicate that the glacial lakes were dammed by an ice margin that retreated northward. The bene¢t of the GLOBE data set was that its resolution allowed glacial lake and ice sheet reconstructions for large areas without the generation of an unwieldy data set. Glacial lake shorelines and deltas are assigned an elevation estimate using its precise position within the GLOBE DEM. The ice dispersal center of the Labrador/Ungava dome shifted considerably during the Late Wisconsinan (Boulton and Clark, 1990a,b; Jansson et al., 2002). Measurements of shoreline tilts from the Torngat Mountains, the Naskaupi area, and the Labrador City area (LRken, 1962; Andrews, 1963; Harrison, 1963; Barnett and Peterson, 1964) indicate a glacio-isostatic recovery pattern with a zone of maximum postglacial uplift in central Labrador/Ungava (Andrews and Barnett, 1972). Results from the George River estuary (Fig. 1), on the other hand, indicate a uniform isostatic recovery pattern (Allard et al., 1989). The measured tilt of shorelines of glacial lake Naskaupi stage 2 (Barnett and Peterson, 1964) indicates that mapped shorelines should be corrected for a non-uniform glacio-isostatic recovery

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when reconstructing precise outlines of glacial lakes. However, the sparseness of well-dated shoreline tilt data and the di⁄culties in correlating fragmentary shoreline systems in the study area at present render such an attempt impractical. 5.1. The glacial lakes An important question is whether the mapped shorelines re£ect large open glacial lakes or marginal systems of small lakes adjacent to residual ice masses in valley bottoms (e.g. Lundqvist, 1972). It is possible that the glacial lakes shown in Fig. 17A re£ect a range from fully open lakes to systems of marginal lakes or less well-ordered lake system in dead-ice-¢lled basins. However, shoreline features formed by glacial lakes Naskaupi, McLean, and Minto are widely distributed along the lake perimeter with elevation consistency for the di¡erent sub-stages (Barnett and Peterson, 1964; Gray and Lauriol, 1985), which indicates a large and continuous water table. This was probably the situation for smaller lakes, such as glacial lakes Delay, Wapussakatoo, and Pons, which were dammed in well-de¢ned basins showing a similar distribution of shorelines as e.g. glacial lake Naskaupi. The nature of early and late stages of glacial lake Caniapiscau, as well as e.g. glacial lake Cambrien, is more problematic to interpret. Fig. 19 shows shorelines and deltas, interpreted to re£ect the elevation/outline of glacial lake Caniapiscau sub-stage 9 (Cp-9 460), plotted against the 460 m a.s.l. contour line extracted from the topographic map (contour interval of V30 m). The distribution and elevation consistency of fossil shorelines and deltas along the lake perimeter and the existence of shorelines on islands indicate that glacial lakes in low-relief areas in the central part of the peninsula existed as open lakes (Fig. 19). The existence of marginal lakes could not be excluded, although data presented in this study support the interpretation

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that open lakes were the most common type of glacial lakes in Labrador/Ungava. A comparison between earlier reconstructions of glacial lake Naskaupi stage 2 (Barnett and Peterson, 1964), glacial lake McLean stages 2 and 3 (Barnett, 1967), and this reconstruction shows broadly similar results. The spillway routes of Naskaupi stage 2 into Kogaluk and Kanairitok Rivers (Ives, 1960b; Barnett and Peterson, 1964) and the ‘inferred’ spillway routes from McLean stages 2 and 3 across the Whale River/George River divide ¢t well with the results of this study (Table 1 and Fig. 1). Earlier reconstructions of glacial lakes McLean and Naskaupi depict a damming ice margin situated west or west-northwest of the lakes, which is in agreement with this study (Figs. 12, 14, and 17). Naskaupi stage 2 was inferred to also have required a southerly damming ice margin, which hindered drainage, over the col (475 m a.s.l.) towards the south. This reconstruction, however, depicts the existence of glacial Naskaupi stage 2 at a level of approximately 450 m a.s.l., and does not recognize the need for a southerly damming ice margin. Glacial lakes Naskaupi and McLean were situated in well-de¢ned basins, which yield a relatively uncomplicated reconstruction, in contrast to the situation in central Labrador/Ungava. Here, in the interior of Labrador/ Ungava, shorelines were identi¢ed by Hughes (1964) and Klassen et al. (1992). These, however, were never used for regional reconstructions of the glacial lake system, except for schematic sketches presented (Fig. 1C) by Gray and Lauriol (1985) and Gray et al. (1993). Angular discrepancy occurs between ice £ow directions inferred from eskers and from reconstructed ice marginal positions for high levels of glacial lake Caniapiscau and glacial lakes Delornieu and Boilay (Fig. 17). The reason for this is not fully understood. However, a southwest^ northeast-trending ice margin, aligned with the eskers, was most probably required for the impounding of the glacial lakes at the southern

Fig. 14. Glacial lakes Caniapiscau (stages 11^13), McLean (stage 7), and Naskaupi (stages 4^7). In some areas there is only a small number of glacial lake shorelines and deltas and the glacial lake extent is largely based on the drainage basin (broken blue line). Legend from Fig. 9. For location see Fig. 1.

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Fig. 15. Glacial lake Cambrien. Glacial lake Nachicapau forms part of eastern glacial lake Cambrien. The reconstructed outline of the western part of glacial lake Cambrien (sub-stages Cb-1 and Cb-2) falls outside the mapped area and is to a large extent based on the outline of the drainage basin (broken blue line). Legend from Fig. 9. For location see Fig. 1.

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Fig. 16. Glacial lakes Me¤le'zes and Minto de¢ned by shorelines and deltas by Lauriol and Gray (1983, 1987), Gray and Lauriol (1985), and Gray et al. (1993). Legend from Fig. 9. For location see Fig. 1.

and southwestern part of the study area. A strict correlation of the reconstructed ice margin perpendicular to the direction of the eskers would, in this area, result in open drainage routes towards the west which, in turn, would have prevented the formation of high levels of glacial lake Caniapiscau and glacial lakes Delornieu and Boilay (Fig. 17A). The discrepancy in direction between the eskers and the reconstructed ice margin

required for the damming of high levels of glacial lake Caniapiscau and glacial lakes Delornieu and Boilay can be explained by: (1) the fact that the eskers, at least in the southern part of the study area, were formed well inside the ice margin ; (2) a rather irregular outline of the ice sheet remnant, involving a south^north-trending ‘snout’ at the southern margin. A reconstruction of the ice marginal recession pattern including such a snout ¢ts

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Table 1 The glacial lakes of Labrador/Ungava

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Table 1 (Continued).

The division into sub-stages is based on the elevation of shorelines and deltas. Sub-stages of glacial lakes Naskaupi and McLean di¡er from earlier reconstructions (Ives, 1960b; Matthew, 1961; Barnett, 1963, 1967; Barnett and Peterson, 1964; Peterson, 1965). Probable spillways and drainage routes were identi¢ed on topographical maps at a scale of 1:250 000. Numbers refer to glacial lake numbers in Fig. 17. *N.D., no data.

reasonably well with the pattern of eskers and the outline of the glacial lakes (Fig. 17B); (3) nonuniform glacio-isostatic recovery may have caused an erroneous interpretation of western (damming) points and cols for the problematic southwestern lake basins. It is possible that cols active for high levels of glacial lake Caniapiscau and glacial lakes Delornieu and Boilay were north of the present-day cols used in the reconstruction shown in Fig. 17A. An alternative reconstruction including an ice marginal retreat pattern probably compatible with deglacial cols situated north of the presentday cols and also adjusted to reasonably ¢t the esker pattern is shown in Fig. 17C.

5.2. Chronology of the ice margin retreat and the glacial lakes 5.2.1. Coastal areas The chronological control of the ice margin retreat in Labrador/Ungava is weak, except for coastal areas. The peripheral southwestern parts of the study area were most probably ice-free at 8.2^8.0 kyr B.P., close in time to the formation of the The Sakami moraine (Fig. 18; Hillaire-Mercel, 1976; Hardy, 1977). These dates together with 14 C dates from the Labrador coast (LRken, 1962; Lowdon and Blake, 1980; Clark, 1988; Clark et al., 1989; Awadallah and Batterson, 1990; Clark and Fitzhugh, 1990) and the pattern of end mor-

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aines, till lineations, and eskers (Prest et al., 1968; Vincent, 1989) indicate an outline of the Laurentide Ice Sheet at 8.0 kyr B.P. parallel to the Sakami moraine southeast of James Bay, and standing in the Nain area at the Labrador coast. The Nain area is inferred to have become ice-free at 7.9^7.6 kyr B.P. (Clark and Fitzhugh, 1990). Based on extrapolation of 14 C-dated emergence curves, Clark and Fitzhugh (1990) conclude that the initiation of glacial lake Naskaupi occurred at 7.5 kyr B.P. This date is supported by minimum ages of the deglaciation in the Hopedale area (Awadallah and Batterson, 1990). I suggest this to be coincident with the commencement of glacial lakes in Labrador/Ungava. Fillon and Harmes (1982) identi¢ed large quantities of suspended sediments deposited by turbid meltwater injections between 8.4 kyr B.P. and 6.1 kyr B.P. in Saglek Fiord (Fig. 1), which supports drainage of the Labrador/Ungava glacial lakes over the Torngat Mountains watershed into the Labrador Sea (Fig. 1B). The termination of this glacial lake period probably coincides with the ¢nal deglaciation of the southwestern Ungava Bay lowlands at 6.2^6.0 kyr B.P. (Gray and Lauriol, 1985; Lauriol and Gray, 1987, 1997). The chronological control between 7.5 and 6.0 kyr B.P. is based on 14 C dates of the formation of glacial lake Minto at 7.0 kyr B.P. and of the ¢nal drainage of glacial lake Naskaupi at 6.5 kyr B.P. (Lauriol and Gray, 1983; Clark and Fitzhugh, 1990). These dates allow some temporal control on the timing of the ice recession of the western and eastern portions

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of this section of the Laurentide Ice Sheet (Fig. 18). The abrupt end of deposition of suspended material by freshwater in Saglek Fiord at 6.2^6.0 kyr B.P. (Fillon and Harmes, 1982), in eastern Ungava Bay at 6.2 kyr B.P. (Andrews et al., 1995), and in the Karlsefni Trough after 6 kyr B.P. (Hall et al., 1999) may indicate the end of the glacial lake era. Late deglacial glacial lake drainage into Ungava Bay may be related to a 6 80 m thick sediment sequence in the marginal channel of the southern Ungava Bay (MacLean et al., 1991, 2001; Andrews et al., 1995). The suggested reconstruction (Fig. 18) ¢ts reasonably well with data indicating that Akpatok Island in the northwestern part of Ungava Bay, the Ungava Bay platform, and northern Ungava Peninsula became ice-free at approximately 8.0 kyr B.P. (Jennings et al., 2001; Gray, 2001). 5.2.2. Interior Labrador/Ungava The suggested retreat pattern is in reasonable agreement with 14 C dates in the peripheral areas. The situation in the interior of Labrador/Ungava is di¡erent with nearly all dates in the region of Caniapiscau reservoir and southern Ungava Bay indicating minimum deglacial ages between 7.0 and 6.0 kyr B.P. (Gray et al., 1980; Richard et al., 1982; Blake, 1982; Lauriol, 1982; King, 1985; Lauriol and Gray, 1987, 1997; Vincent, 1989; Gray, 2001). The 14 C data in this region show no clear age gradient and therefore the constraint of any reconstructed ice marginal retreat pattern is weak.

Fig. 17. (A) The glacial lakes formed during the last deglaciation in Labrador/Ungava. Di¡erent stages within each glacial lake system are indicated by similarly graded color. Each glacial lake stage is accompanied by a damming ice margin (in blue) constructed to yield a best ¢t with drainage traces, shorelines, deltas, and the outline of the drainage basin. Eskers are presented as red lines, from Prest et al. (1968). Numbers refer to glacial lake systems. 1 Wapussakatoo, 2 Caniapiscau stages 1^6, 3 Delornieu, 4 Attikamagen, 5 Petitsikapau, 6 Astray, 7 Marble, 8 Sims, 9 Druillettes, 10 Chaigneau, 11 Goodwood, 12 Chastenay, 13 Junot, 14 McPhadyen, 15 Costebelle, 16 Boilay, 17 Caniapiscau stages 7^10, 18 Gayot, 19 Delay, 20 Naskaupi, 21 McLean, 22 Cambrien, 23 Caniapiscau 11^13, 24 Me¤le'zes, 25 Minto, and 28 La Touche (see Table 1). Post-glacial marine limit (gray) from Vincent (1989). Black box refers to the extent of panels B and C. (B,C) Alternative reconstructions of the ice margin retreat pattern in the southern part of the investigated area. Blue lines indicate marginal positions during the last deglaciation towards north. (B) The reconstruction is based on the outline of high levels of glacial lake Caniapiscau and glacial lakes Delornieu and Boilay and the direction of eskers in the area. (C) The reconstruction is based on the direction of eskers in the area. This is a possible scenario if non-uniform glacio-isostatic recovery has caused changes in the location of cols and western damming points for the southwestern lakes.

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Labrador Sea

Labrador City

Sakami moraine

0

200 km

Fig. 18. The late glacial retreat pattern in Labrador/Ungava reconstructed from the glacial lake system, damming ice margin outlines, and esker distribution. The ¢gure shows time slices of ice sheet retreat and glacial lake extent (dark gray areas) and should be read from the top left corner to the bottom right corner. The age assignments are based on the suggested age of the initiation of glacial lake Naskaupi at 7.5 kyr B.P. by Clark and Fitzhugh (1990), the suggested initiation of glacial lake Minto at 7.0 kyr B.P. by Lauriol and Gray (1983), the suggested timing of the ¢nal drainage of glacial lake Naskaupi at 6.5 kyr B.P. (Clark and Fitzhugh, 1990), and the timing of the ¢nal deglaciation of southwestern Ungava Bay at 6.0 kyr B.P. (Gray and Lauriol, 1985; Lauriol and Gray, 1987, 1997). Heavy black lines show prominent end moraines (Vincent, 1989) and eskers are marked with thin black lines (Prest et al., 1968). The eskers north of the horseshoe unconformity (Figs. 1C and 17) are somewhat problematic to ¢t into the suggested ice marginal retreat pattern south of Ungava Bay, because tributaries indicate formation during a southward retreating ice margin.

5.3. The retreat pattern As shown here, glacial lakes in Labrador/Ungava existed because of a damming ice margin

located to the north. The ice margin retreat, during the deglaciation, towards the southern Ungava Bay lowland allows for the consecutive formation and lowering of 26 glacial lakes (65 sub-

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Fig. 19. A comparison between the 460 m a.s.l. contour line of the GLOBE data set (gray) and data extracted from the topographic map with a contour interval of V30 m (black line). Both data sets show similar outlines for glacial lake Caniapiscau sub-stage 9, which indicates the accuracy of the position of the reconstructed damming ice margin. The GLOBE DEM did not resolve the Pons River basin. Fossil shorelines (arrows) and deltas (¢lled circles) are distributed around the lake perimeter and on islands at 460 m a.s.l., which indicates that Caniapiscau 9 existed as an open lake.

stages, Fig. 17). Glacial lakes Naskaupi and McLean were incorporated in earlier regional reconstructions of the ice marginal retreat pattern (Prest, 1970; Dyke and Prest, 1987; Vincent, 1989; Veillette et al., 1999). These reconstructions depict a late glacial U-shaped ice divide situated over central Labrador/Ungava. Hence, a U-shaped ice divide fails to explain the existence of the numerous glacial lakes to the west and south of Ungava Bay (Figs. 1C and 17) during the deglaciation (Hughes, 1964; Taylor, 1982; Lauriol and Gray, 1983, 1987; Allard and Seguin, 1985; Gray and Lauriol, 1985; Gray et al., 1993; Clark et al., 2000; Jansson et al., 2002; Jansson, 2002 ; this study). An ice marginal retreat towards a central Labrador/Ungava U-shaped ice divide (Fig. 1C) would imply that the lakes formed underneath the shrinking Laurentide Ice Sheet. Gray and Lauriol (1985) suggested, based on a simpli¢ed picture of the glacial lake outline (Fig. 1C), that the last ice remnant was situated over southern Ungava Bay. Clark et al. (2000) suggested a slightly di¡erent scenario, where the ice sheet during the marginal retreat towards the southern Ungava Bay lowlands separated into several residual ice caps. Clark et al. (2000) advanced this suggestion because of the presence of valley trains of glacio£uvial deposits formed by northward-£owing meltwater during late deglacial

time. It is possible that these features re£ect a late separation of the Laurentide Ice Sheet into several residual ice caps as Clark et al. (2000) suggested. However, it could not be excluded that these features were formed by meltwater from glacial lake drainages towards southern Ungava Bay, either along the ice margin or subglacially late during the deglaciation. A complicating feature is the presence of meltwater features in the Sche¡erville area indicating a southward retreat of the ice margin. Other landforms consistent with this retreat pattern exist, such as the ribbed moraine ¢elds west of Lac Privert. These ribbed moraine ¢elds could re£ect either formation during ice margin retreat or a fracturing of the existing drift cover during the formation of the older Ungava Bay landform swarm. Ha«ttestrand (1997), Ha«ttestrand and Kleman (1999), and Kleman and Ha«ttestrand (1999) suggested that ribbed moraine forms by brittle fracturing of frozen subglacial sediments, in response to basal stresses induced at the boundary between proximal frozen- and distal thawed-bed areas. A patchy subglacial temperature distribution was also inferred for the formation of horned crag-and-tails, which occur exclusively in the Ungava Bay landform swarm (Jansson and Kleman, 1999). Hence, it is possible that the ribbed moraine ¢elds west of Lac Privert were formed at

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the time of Ungava Bay landform swarm creation, which is of suggested Last Glacial Maximum or older age (Jansson et al., 2002). Lateral meltwater channels indicating an ice margin retreat towards the north, south of Schefferville, and indicating an ice margin retreat towards the south, north of Sche¡erville are also in apparent con£ict with the reconstructed northward ice retreat towards the southern Ungava Bay lowlands (Ives, 1959a,b; Jansson, 2002). Ives (1959a) observed large erratic boulders that blocked lateral channels in the Helluva Lake area (Fig. 1A), and he concluded that they were emplaced after the formation of the channels. However, because of the presence of erratics amongst these boulders Ives (1959a) changed his interpretation and suggested that the channels and the boulders originated from sublateral or subglacial drainage. Because the channels are at odds with the reconstruction presented here, it is conceivable that these drainage channels formed during an earlier deglaciation and that the erratics were emplaced during the last deglaciation. Relict meltwater channels are a common feature in areas which experienced cold-based conditions during prolonged periods of time in, for example, Scandinavia (e.g. Rodhe, 1988; Kleman et al., 1992). The interpretation that the lateral meltwater channels north of Sche¡erville are relict agrees with the interpretation of cold-based conditions north of the horseshoe unconformity region (Kleman et al., 1994; Clark et al., 2000; Jansson et al., 2002). One of the notable features consistent with this interpretation is the contrasting density of eskers on either side of the unconformity (Fig. 17). The subglacial conditions during the ¢nal disintegration in the southern Ungava Bay lowlands and the adjacent southern shore remain elusive. The occurrence of eskers immediately south of the present Ungava Bay shoreline (Prest et al., 1968) may indicate that a transition to warmbased conditions occurred prior to ¢nal deglaciation. The origin of those eskers is still unclear but their tributaries seem to indicate formation during an ice retreat towards the south. The outline of numerous glacial lakes, on the other hand, indicates an ice marginal retreat towards the southern

Ungava Bay. Until the origin of these eskers is fully understood the ice margin retreat pattern in the southern Ungava Bay region will remain puzzling. 5.4. The northern margin During late glacial time, ice from the Labrador dome is inferred to have crossed the previously deglaciated Hudson Strait and continued onto southern Ba⁄n Island (Andrews et al., 1995). This event, the Noblet Inlet advance, occurred at 8.9^8.4 kyr B.P. (Miller et al., 1988; Manley, 1995; Jennings et al., 1998). At the time of the Noblet Inlet advance, the ice sheet over Labrador/ Ungava reached the coastline of Hudson Bay in the west and southwest (Vincent, 1989), northern Ungava Peninsula and the outer Ungava Bay in the northwest (Gray, 2001), and close to the Labrador Sea in the east (LRken, 1962; Clark and Fitzhugh, 1990). The present study lacks data from the northern margin, but considering the size and con¢guration of the northeastern sector of the Laurentide Ice Sheet shortly after the time for the Noblet Inlet advance (Fig. 18), the northern margin could well have reached the southern Ba⁄n Island. Hence, the result in this study seems not to be in con£ict with the postulated Noblet Inlet advance. However, the relationship between any late glacial advance onto Ba⁄n Island and the Ungava Bay landform swarm is more ambiguous (Jansson et al., In press). The ice sheet dynamics and ice margin retreat pattern in the Hudson Strait region after the Noblet Inlet advance is poorly resolved, but the results from the present study support the theory of a relatively late deglaciation of the Ungava Bay platform (MacLean et al., 2001) and southern Ungava Bay (Gray, 2001).

6. Conclusions Glacial lake traces such as shorelines, deltas, and meltwater channels are important tools, and in some cases the only tools, for reconstructing the ice marginal retreat pattern of a cold-based ice sheet. In Labrador/Ungava widespread traces of glacial lakes were formed during the last degla-

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ciation. When the ice sheet margin retreated towards the north, lower terrain was exposed successively. This led to the consecutive formation and lowering of 26 glacial lakes (65 sub-stages) that were impounded by the shrinking ice sheet. The best-known examples are glacial lakes Naskaupi and McLean. Spillways of these lakes breached the Torngat Mountains watershed, except for late stages of glacial lake Naskaupi, which drained northward into the eastern Ungava Bay. Glacial lakes Minto and Me¤le'zes drained into the western part of Ungava Bay. Glacial lakes Caniapiscau and Cambrien formed over the central Labrador/Ungava plateau. Combined, these two lakes existed at 15 or more consecutive levels between 609 m a.s.l. and 220 m a.s.l. Spillways and drainage channels reveal that initial meltwater escape was westwards towards Hudson Bay. Later, easterly drainage routes were opened into glacial lakes Naskaupi and McLean. During ¢nal deglaciation, glacial lake Cambrien drained into the eastern Ungava Bay. For these glacial lakes to exist, a damming ice margin must have been located to the north. Hence, the last ice remnant of the Laurentide Ice Sheet in Labrador/Ungava must have been situated over the southern Ungava Bay lowland, a situation incompatible with leading reconstructions of an ice sheet remnant with a U-shaped ice divide situated over central Labrador/Ungava (Fig. 1C).

Acknowledgements This study was funded by Swedish Natural Science Research Council grants to Johan Kleman and grants from the Swedish Society for Anthropology and Geography, Margit Ahltin’s fund of the Royal Swedish Academy of Sciences, Carl Mannerfelt’s fund, Axel Lagrelius fund, and Hans W:son Ahlman’s fund to K.J. These institutions are acknowledged for their ¢nancial support. Johan Kleman, Arjen Stroeven, Ingmar Borgstro«m, and Clas Ha«ttestrand, Department of Physical Geography and Quaternary Geology, Stockholm University, o¡ered constructive criticism and discussion during the project. I am

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also grateful to referee Chris Clark and one anonymous referee for their helpful comments that improved this paper.

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