the nature and age of the contact between the laurentide and cordilleran ice sheets in the western...

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The Nature and Age of the Contact between the Laurentide and Cordilleran Ice Sheets in theWestern Interior of North AmericaAuthor(s): B. O. K. ReevesSource: Arctic and Alpine Research, Vol. 5, No. 1 (Winter, 1973), pp. 1-16Published by: INSTAAR, University of ColoradoStable URL: http://www.jstor.org/stable/1550295 .

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Arctic and Alpine Research, Vol. 5, No. 1, 1973, pp. 1-16

THE NATURE AND AGE OF THE CONTACT BETWEEN THE LAURENTIDE AND CORDILLERAN ICE SHEETS IN THE

WESTERN INTERIOR OF NORTH AMERICA

B. O. K. REEVES

Department of Archaeology University of Calgary

Calgary, Alberta T2N 1N4

ABSTRACT

The presumed existence of a single mass of coalesced Cordilleran and Laurentide ice during most of late Wisconsin time is central to many archaeological hypotheses on the peopling of the New World. The area under concern is a 2,400-km belt of the Western Interior Plains and adjacent mountains, extending from the 49th parallel to the Arctic Ocean. Multiple Cordilleran glaciation occurred in the Rocky Mountain area during both late and early Wis- consin time. Radiocarbon dates indicate the mountain valleys were largely ice-free by 10,500 BP. Multiple Laurentide glaciation also

is well established, the last advances in southern Alberta (late Wisconsin) having terminated east of the mountain front. Southern Alberta and southwestern Saskatchewan were ice-free by ca. 15,000 BP. Incontravertible evidence for coalescence west of the late Wisconsin ice front comes only from the Athabasca Valley, where Roed found that the two glaciers coalesced and flowed southeast. This event occurred either in early Wisconsin or Illinoian time. Since then the western border of the plains of Alberta has remained ice-free.

INTRODUCTION

The presumed existence of a single mass of coalesced Laurentide and Cordilleran ice at certain episodes of the late Quaternary (Bryson et al., 1969; Prest, 1969) is often invoked as a factor in archaeological hypotheses on the peopling of and later population movements within the New World. It is the intent here to present a brief summary of the geological and geochronological data relative to this problem which suggest that an "Ice Free Corridor" has existed since early Wisconsin or Illinoian times, i.e., for more than 55,000 years.

The area under consideration (Figure 1) extends about 2,400 km, from the 49th parallel to the Arctic Ocean, and includes three major

physiographic units: (1) the relatively flat Interior Plains, (2) the Precambrian Shield, and on the west (3) the Eastern Cordillera. Both (1) and (2) were repeatedly inundated during the Quaternary by Laurentide ice sheets of varying dimensions. The Rocky and Mac- kenzie mountains also were repeatedly occupied by valley and piedmont glaciers. With the exception of the northern Rockies these glacial masses were isolated in the late Pleistocene from the Cordilleran ice sheet. In contrast to the Rockies, extensive valley areas in the Mackenzie Mountains were also occupied by Laurentide ice. In the Richardson and Franklin mountains, only Laurentide ice occupied the valleys. The

B. O. K. REEVES / 1



Foothills Belt, with the exception of the Rich- ardsons and Franklins, was occupied, not nec- essarily contemporaneously, by both Cordil- leran and Laurentide ice.

In the southern Interior Plains nunataks were rare, although in the adjacent foothills and mountains, large areas above and adjacent to valley and piedmont glaciers were ice-free during the latest Cordilleran advances. These unglaciated areas are much larger in the foothills and plateaus bordering the plains to the north. At approximately 60?N there is a major unglaciated mountain-plateau area. However, considerable quantities of meltwater from both the Laurentide and Cordilleran ice sheets discharged northward across parts of this area into the northern Yukon Basin.

Two major river systems, the Mackenzie and the Saskatchewan-Nelson, drain most of the area. These systems, which generally reflect the preglacial drainage patterns, underwent

many changes during the various deglaciation stages, most significant of which were the diver- sion of waters into the Missouri River system and the formation of large proglacial lakes which existed for protracted periods of time in some areas.

Research into the Pleistocene of this vast area has until recently concentrated only on the easily accessible areas of the Interior Plains and adjacent mountains. particularly the South Saskatchewan River Basin.

A review was made of all obtainable litera- ture (approximately 300 articles covering the period of 1875 to 1971) pertinent to the problem of determining the current concepts in the area and their relationship to the following variables: (1) the existence of multiple synchronous Laurentide and Cordilleran glaciation, (2) nature of the contact between the two ice sheets, and (3) dating of events.

CORDILLERAN ICE

Multiple tills and ice-frontal positions were identified at a number of locations along the mountain front. Also identified were earlier Pleistocene tills that were on old erosion surfaces in the Glacier National Park area (Alden, 1932; Horberg, 1952; Richmond, 1965).

MULTIPLE TILLS

Multiple till units, identified both within and outside the mountain front, may relate to successive late Pleistocene Cordilleran piedmont glaciations.

On the east slope of Glacier National Park, Calhoun (1906), Alden and Stebinger (1913), and Alden (1932) described stratigraphic sequences that record multiple Cordilleran glacial advances, commonly overlain by Laurentide erratics.

In the Oldman River Basin, multiple Cor- dilleran tills, commonly separated by glaciolacustrine sediments or alluvium, with Lauren- tide tills interdigitated with or above them, were described from various sections (Dawson, 1881; Calhoun, 1906; Alden and Stebinger, 1913; Alden, 1932; Horberg, 1954; Stalker, 1962a; Wagner, 1966). Buried Cordilleran till is found as far east as Kipp, Alberta (Stalker, 1968a), indicating the existence at one time (Illinoian or earlier) of extensive Cordilleran piedmont lobes extending 80 km from the mountain front.

In the Bow Valley, Cordilleran till buried

under Laurentide deposits was recognized as far east as Calgary, about 130 km from the mountain front (Dawson, 1881; Dawson and McConnell, 1895; Coleman, 1909; Tharin, 1960; Stalker, 1962b), and in the Foothills west of Calgary, two buried mountain tills separated by outwash were identified (Nichols, 1931; Rutter and Wyder, 1969; N. W. Rutter, pers., comm., 1970).

Northward, in the Foothills of the Atha- basca Valley, Roed (1968) identified two Cordilleran tills separated by glaciofluvial materials. The last valley glacier in this area of the valley (know as the Obed) extended 80 km beyond the mountain front.

In the Peace River area, Rutter (Rutter, 1969, 1970; Reimchen and Rutter, 1972) ob- served multiple tills in front of and behind the mountain front. A similar relationship was ob- served in drill holes in the Fort Nelson area (Hage, 1944) and in the Mackenzie Mountains, again both in front of (Keele, 1910) and behind the mountain front (O. L. Hughes, pers. comm., 1970).

ICE-FRONTAL POSITIONS End moraines recording various ice-frontal

positions were described for a number of valleys. On the east and north slopes of Glacier National Park, Calhoun (1906), Alden (1932), and Richmond (1960, 1965) described succes-

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sive end moraine/ice-frontal positions, of both piedmont lobes and later valley glaciers. Simi- lar maximum-advance and recessional moraine/ ice-frontal positions were described for the Oldman River area (Dawson, 1875; Alden, 1932; Horberg, 1954; Stalker, 1959, 1960; Wagner, 1966; Reeves, 1972; Reeves and Dormaar, 1972) and for the Bow (Rutter, 1966), Athabasca (Roed, 1968), and Peace (N. W. Rutter, pers. comm., 1970) river valleys. Hughes (1970) also noted at least two Cordilleran end moraines in the Mackenzie Mountains that are far west of the maximum extent of the Laurentide ice.

DATING/CORRELATION PROBLEMS

Very few radiocarbon dates are available for the area. The only subtill dates are from the Peace River area (Lowdon et al., 1971). These indicate the earliest advance is early Wis- consin or pre-Wisconsin (N. W. Rutter, pers. comm., 1970). Dates from postglacial fluvial and cultural deposits in and adjacent to the mountain front indicate that major portions of the valleys were deglaciated by 10,500 years BP, i.e., Sun River Canyon, Montana [12,750 ? 350 BP (W-1644) (Ives et al., 1967, p. 517); 11,500 ?+ 300 BP (W-1753) (Masters et al., 1969, p. 214)]; Waterton Lakes National Park, Alberta [8,220 -+ 260 BP (GX-1435) (Reeves, 1972)]; Livingston Gap, Alberta [7,990 ?+ 150 BP (GSC-1158) (Reeves and Dormaar, 1972)]; Saskatchewan Crossing, Banff National Park [9,330 ? 170 BP (GSC- 332) (Westgate and Dreimanis, 1967)]; Ospika River, Finlay River; British Columbia [7,470 ? 140 BP (GSC-1069); 7,480 + 150 BP (GSC-1161) (Lowdon et al., 1971, p. 298)].

Dates from postglacial river alluvium terraces in the adjacent Foothills and Plains which may be generally correlated with late Wiscon- sin valley glaciations in the Rockies include 10,760 + 160 BP (GSC-612), 11,370 -- 170 BP (GSC-613), and 11,100 ?+ 160 BP (GSC- 989) (Stalker, 1968b) from Cochrane, Alberta, and 10,700 + 150 BP (J. A. Westgate, pers. comm., 1970) from the 18-m terrace of the North Saskatchewan River at Edmonton, Al- berta (Westgate, 1969). A date of more than 44,000 years BP (1-2259) (Buckley et al., 1968, p. 264) was obtained from the 30-m terrace of the Peace River, just east of the mountain front. The material dated was lignite (N. W. Rutter, pers. comm., 1970).

Radiocarbon-dated samples W-1644, 1753, GX-1435; GSC-332; and GSC-1161, 1069 are from postglacial fluvial and lacustrine deposits, which in some instances have been correlated with either late Wisconsin or early Holocene Valley ice-frontal positions. Two dates > 33,000 years BP (S-213) (McCallum and Wit- tenburg. 1968, p. 366) and 11,600 ? 500 BP (no number) were obtained from material in glacial deposits. The former from the Red Deer River, near Banff National Park, is suspect, it probably being lignite. The latter was obtained from a mammoth tusk found in a recessional moraine near Portage Mountain Dam, on the Peace River (Rutter, 1971).

While radiometric control is relatively good for the terminal late Wisconsin advances, the lack of such control for earlier stratigraphic and surficial deposits poses a major problem in their dating and correlation. Earlier workers, such as Alden and Calhoun, correlated all the glacial deposits with Wisconsin. Horberg (1954) divided the deposits into early and late Wis- consin. Dawson and McConnell (1895) and Stalker (1972a), in contrast, correlated portions of the multiple till stratigraphic sections with much earlier glacial stages.

Richmond (1960, 1965) correlates the deposits in Glacier National Park with the Pine- dale/Bull Lake and earlier glaciations in the central Rocky Mountains. Other workers, Wag- ner (1966) in the Oldman, Rutter (1972) in the Bow, and Roed (1968) in the Athabasca, correlated their sequences, primarily on a finger matching basis with Richmond's correlations for Glacier National Park.

The lack of radiometric control has resulted in some diverse correlations. Wagner, for ex- ample, considers the surficial and buried Cor- dilleran deposits to be Pinedale (Woodfordian) in age. Fifty kilometers away, in Glacier Na- tional Park, Richmond assigns time-stratigraphic equivalent surficial and buried drifts to Pinedale and Bull Lake. In contrast, Stalker, on the same sections as Wagner, suggests correlations with pre-Illinoian stages. Rutter and Roed correlate all surface deposits in their areas, but not all the buried deposits, with Pine- dale. However, with adequate geochronological control, correlation between the northern and central Rockies is largely unwarranted at this time. It is hazardous to assign all these deposits to the late Wisconsin.




SUMMARY It seems evident that at least two major Cor-

dilleran glaciations occurred in the northern Rocky Mountains in late Pleistocene time. The earliest of these is early Wisconsin (Bull Lake) and/or pre-Wisconsin in age and probably represents the maximum late Pleistocene piedmont glaciation. A later major glaciation con-

sisting of a maximum of four advances is represented by distinctive ice-frontal positions located in front of and behind the mountain front. These are late Wisconsin (Pinedale) and early Holocene in age. Radiocarbon dates indicate that the maior portions of the valleys were ice-free by 10,500 to 12,000 years BP.

LAURENTIDE ICE

AREAS NORTH OF THE SOUTH SASKATCHEWAN RIVER

Evidence of multiple glaciation was found throughout the area. At least two major Laur- entide advances reached and penetrated the Richardson Mountains into the mouths of the eastern valleys of the Mackenzie Mountains (O. L. Hughes, pers. comm., 1970). Radiocarbon dates indicate the advances are late Wisconsin (Woodfordian) and early Wisconsin (Altonian) or pre-Wisconsin in age (GSC-181, 199, 204) (Dyck et al., 1965, p. 38). Evidence for a mid- Wisconsin (Farmdalian) Laurentide advance has not been found in this area. Rutter and Minning (1972, p. 178) noted that two Lauren- tide advances reached the mountain front in the southern portion of the Mackenzie Valley. At Fisherman Lake (Millar, 1968) two stages of Glacial Lake Liard separated by an erosional interval are underlain by an eroded till surface dated at 31,700 5 0 BP (1-3187).

In the Fort St. John area Mathews (1963) identified two Laurentide tills in section. Hage (1944) recorded multiple tills in drill holes at Fort Nelson and Reimchen and Rutter (1972) have evidence for two Laurentide advances in the Dawson Creek area. In the eastern Peace River area Henderson (1959) differentiated four till units, the latest of which correlated with a major readvance into the area. At Wa- tino, Westgate et al. (1971) obtained a series of infinite to finite radiocarbon dates from fos- siliferous sands underlying glacial lacustrine materials, and west of Henderson's mapped area, Jones (1961) found a single till in the Beaver Lodge area.

In the Athabasca Valley, Roed (1968) and St. Onge (1969) recognized two Laurentide tills. Intertill radiocarbon dates of greater than 40,000 years BP (GSC-1019) and 52,200 + 1760 BP (GSC-1019-2) (St. Onge, 1969, p. 217; Lowdon et al., 1971, p. 291) were obtained. Also in the same vicinity St. Onge (1968, p. 197; Lowdon et al., 1971, p. 291) obtained

dates of 10,900 ? 60 BP (GSC-859) and 12,400 ? 600 BP (GSC-903) from silts located higher in the stratigraphic sequence, and overlain by a till-like material. If the latter is till, the dates are unacceptable, for other radiocarbon determinations, e.g., 11,400 + 900 BP (GSC-1049) (Lowdon et al., 1971, p. 289), indicate that this area would have been deglaciated by this time.

Along the North Saskatchewan River (in the Edmonton area) multiple till sections were recognized since 1934. These were discussed recently by Westgate (1969).

Multiple tills were also recognized on the Red Deer River (Tyrrell, 1885; Craig, 1956; Stalker, 1960) and on the Bow, in the Calgary area (Dawson, 1881; Coleman, 1909; Nichols, 1931; Stalker, 1960; Tharin, 1960; Morgan, 1966; Rutter and Wyder, 1969).

SOUTH SASKATCHEWAN RIVER The most detailed geologic studies were

done in the South Saskatchewan River Basin, and before discussing the stratigraphic sections it is relevant to first mention the surficial drifts and ice-frontal positions in this area.

In Montana, Alden (1932) and Lemke et al. (1965) identified two major ice-frontal positions.

In southern Alberta various workers (Hor- berg, 1952, 1954; Stalker, 1959, 1962b; West- gate, 1965; Wagner, 1966) differentiated a number of surficial drift sheets and ice-frontal positions. While some disagreement exists, at least three major surficial units and ice-frontal positions are agreed upon. The oldest consists of a modified drift occuring at high elevations on the Milk River Ridge and Cypress Hills. Below this lies well-defined ice marginal features related to a later advance which deposited the major surficial drift sheet in the area. The youngest advance into southwestern Alberta, the Lethbridge end moraine (Bretz, 1943; Hor- berg, 1952: Westgate, 1965; Wagner, 1966),

B. 0. K. REEVES / 5



is correlative with a distinctive till unit. flow direction, and fresh surficial features (Position A, Figure 2).

In southeastern Alberta, Westgate (1965) distinguished five surficial drift sheets which he correlates with ice-frontal positions in southwestern Alberta, Montana, and Saskatchewan.

Christiansen (1965) defined a number of ice-frontal positions which relate to distinct advances, minor readvances or still-stands of the late Wisconsin ice in Saskatchewan and correlated them to subsurface stratigraphic units.

A stratigraphic transect from Prince Albert, Saskatchewan, to Medicine Hat, Alberta, was run by Christiansen (1968a, 1968b, 1969). The Medicine Hat area was also studied by Westgate (1965), Berg (1969), and Stalker (1969). Intertill radiocarbon dates from west central Saskatchewan range from finite to infinite in age, indicating a major nonglacial interval, representing the mid-Wisconsin, separates the last two ice advances in the area (Christiansen, 1968b). The latest advance to reach western Saskatchewan dates ca. 20.000 to 18,000 radiocarbon years BP. Radiometric dating of the Medicine Hat sections has yet to be satisfac- torily resolved. Stalker (1969) places the latest advance in the area as possibly late Wis- consin in age. Intertill dates from below this advance yield infinite determinations (> 38,000 BP [GSC-1044]). However, at another section a series of finite age determinations were obtained from intertill fluvial deposits below two tills (25,000 ? 800 BP [GSC-1370] Lowdon et al., 1971, p. 288; 24,490 ? 200 BP [GSC- 205] Dyck et al., 1965; and 28,630 ? 800 BP [GSC-578] Lowdon et al., 1967).

Stratigraphic sections exposed on the banks of the Oldman River, from east of Taber, Al- berta, to west of Lethbridge, Alberta, were studied by a number of workers (Horberg, 1952; Stalker, 1962b; Vernon, 1962; Wagner, 1966). Radiocarbon determinations from below the latest till units, which represent the last ice advance in the area, all yield infinite dates, the oldest being greater than 49,000 years BP (GSC-1233) (Lowdon et al., 1971, p. 289) at Taber Bluff.

These stratigraphic sections have not yet been correlated with those at Medicine Hat, and one should not assume that the latest till unit in the Lethbridge area at the Lethbridge end moraine, is equivalent in time to the latest at Medicine Hat. However, if Christiansen's cor-

relations of the Medicine Hat section and dated sections to the northeast are accepted, the latest till and the Lethbridge end moraine can date no later than 18,000 to 20.000 BP. Since the latter is a major readvance in the area, all surficial and buried units of glacial deposits and ice-frontal positions to the west are therefore early Wisconsin and/or Illinoian in age. (The mid-Wisconsin is represented in section by erosional surfaces and nonglacial deposits.)

An interesting radiocarbon date was obtained some years ago from wood in a section a few miles west of the Lethbridge end moraine near Monarch, Alberta. The sample, yielding an age greater than 26,000 years (L-221C) (Stalker, 1960) and also greater than 54,400 years (GSC-237), was collected from fluvial deposits. This section (among others) was interpreted differently by various workers. Stalker (1960), although he gives no detailed section description, says that the date came from intertill fluvial deposits. However, if as he says this date came from above Horberg's lower till (Horberg, 1952) there is no later till in the section according to Horberg (1952) and Wag- ner (1966). Vernon (1962), who has done the most detailed study of the tills of the area, de- scribes the section without mentioning the date as one with no till unit above the original location of the sample described by Stalker and Horberg. (Wagner misassociated the date with intertill fluvial deposits from the Kipp section [located 4 km southeast of the Monarch section] which also has an infinite date of greater than 35,000 years [L-433B, Stalker, 1962a].) Clear- ly then, if the associated sediments are postglacial in age, this would support the contention that the area west of the Lethbridge end moraine was not covered by late Wisconsin ice.

This statement is also supported by Bik's (1969) work in southeastern Alberta. Bik obtained a radiocarbon date of 34,900 2000 BP (1-1878) from charcoal in colluvium above till, southeast of Medicine Hat. This till is considered by Bik to belong to the Pakowki drift sheet (Westgate, 1965, 1968) which is older than the Etzikom drift, correlated by Westgate (1968) with the Lethbridge end moraine. The Pakowki drift correlates with the deposits west of the Lethbridge end moraine in southwestern Alberta (Westgate, 1968). The data suggests that these deposits are early Wisconsin and/or Illinoian in age.

Bik also obtained a date of 20,600 ? 410 BP (1-2607) from charcoal in the first terrace of




-

$\ L :-;

000 Valderian ca. 11,000 B.P.

Woodfordian ca. 20,000 B.P. .ALTERNATE POSITONS)

South Saskatchewan Basin

*** B . eeec

Central/Northern Alberta AAAAO

Elsewhere

--Early Wisconsin OR Illinoian >55,000 B.R

100 0 100 200 300 K,lon. etres

FIGURE 2. Late Pleistocene Cordilleran and Laurentide ice frontal positions (partially after Prest, 1969).

B. O. K. REEVES / 7



Manyberries Creek. The terrace, developed on Pakowki till, is located about 48 km south of the previous section, on the Missouri (Milk River) drainage. This date supports the conclusion suggested by Bik (1969, p. 121) for the age of the Pakowki drift. It should be noted that Westgate (1968) considered this drift to be late Wisconsin in age, on the basis of his correlation with dated sections in Saskatchewan. and an intertill date from Medicine Hat (24 290 ? 200 BP) (GSC-205) (Westgate, 1968). The latter date is no longer acceptable. given that dates from higher in the stratigraphic section were recently determined greater than 38.000 years (GSC-1044) (Stalker, 1969; Lowdon et al., 1971, p. 288).

The Manyberries terrace is correlated by Bik with the highest stream terraces on creeks in the Saskatchewan Basin located on the north flank of the Cypress Hills. These stream terraces, developed on Etzikom drift, were trans- gressed by proglacial lacustrine sediments representing a post-Etzikom advance somewhere to the north. Clearly then, the Etzikom drift, and therefore the Lethbridge end moraine, were deposited no later than 20,000 years ago. Since an erosional interval occurred after the deposition of the Etzikom drift and prior to trans- gression by proglacial sediments, a major period of degradation (probably the mid-Wis- consin) may have occurred, and the drift itself may be early Wisconsin or Illinoian in age.

The data suggests that significant ice-frontal positions representing the late Wisconsin and possibly earlier advances, lie behind the Leth- bridge end moraine, such as Westgate's (1965, 1968) Oldman surficial drift unit (Position B, Figure 2). It is possible that the Oldman Ad- vance correlates with Christiansen's late Wis- consin advance, ca. 20,000 to 18,000 BP. How- ever, this may not be the case.

David (1964) studied the terraces of the Red Deer-South Saskatchewan River from Empress, Alberta, eastward to 108?W defining a series of eight terraces. The highest three, found only in the western part of the area, correlate with levels of proglacial lakes. The next three correlate with Q'Appelle Valley terraces and with the oldest part of the Q'Appelle Val- ley floor alluvial fill. These were formed during the time the South Saskatchewan River drained through the Q'Appelle Valley outlet. David's lowest two terraces, the High and Low Alluvial terraces, formed after the drainage of the South

Saskatchewan was diverted northward at the Elbow. The alluvial fill in the valley floor is approximately 30 m thick and David states that prior to formation of the High and Low Alluvial terraces, and after the northward shift- ing of the river, the channel downcut 30 m below the present level and subsequently agraded to at least the level of the High Alluvial terrace. The latest two terraces are cut into this earlier alluvium.

Dates of 14.200 ? 1,120 BP (GSC-1199) (Stalker, 1970) and 20,400 years ago (GSC- 1387) (A. M. Stalker, pers. comm., 1971) were obtained from the 12-m terrace of the Red Deer River at Empress, 60 m below the prairie level. From David's data (1964, Figure 12, Table 6). the dated samples come from his Low Alluvial terrace. These dated samples came from a coarse aggradiational fill; whether this fill relates to David's major alluviation mentioned above or some subsequent aggradi- ation period is not known. However, if this area was glaciated by Christiansen's late Wis- consin advance, ca. 20,000 to 18,000 BP, in- dicated by his stratigraphic correlations, could the events represented in the alluvial sequence have taken place in an interval of about 4,000 to 6,000 years? If all these events did not occur in this interval, and Christiansen's correlations are incorrect, then the late Wisconsin ice front may lie to the east/northeast (Position C, Figure 2). In any event, the data indicate that deglaciation of most of the western plains in southern Alberta and west-central Saskatche- wan had occurred by at least 15,000 years ago.

CORRELATIONS WITH NORTHERN AREAS As will be discussed in the following section,

the evidence indicates that the last Laurentide ice reaching the foothills in the Athabasca area carried down the Foothills Erratic Train into southwestern Alberta (west of the Lethbridge end moraine) and entered the Athabasca area from the northeast. This suggests that the ice had not previously been deflected northward. Perhaps the maximum advance of late Wiscon- sin ice in the Peace River area was synchronous with Henderson's (1959) late advance (Posi- tion 1, Figure 2), an advance which, depending on the age of the Lethbridge end moraine and Oldman advance, may be correlated with any of the positions, A, B, or C. Northward, detailed data are lacking, but one may assume the late Wisconsin ice-front trended northwesterly




towards Fisherman Lake and the Mackenzie Mountains. As noted earlier, it is represented at the former locale by glaciolacustrine sediments.

Alternatively, if Henderson's latest advance is not late Wisconsin. the ice-front mav have lain considerably further northeast (Position 2, Figure 2) and was responsible for the impound- ing of the large proglacial lakes in northern Al- berta (Taylor, 1960). Regardless of which position may be correct (if either), nostglacial dates of 13,580 ? 260 BP (GSC-698), and 13,510 -- 230 BP (GSC-694) (Lowdon and Blake, 1968, p. 222) from shells in glaciolacustrine sediments in the Swan Hills area northwest of Edmonton indicate that this area was ice-free by that time. To the northeast, the earliest postglacial date available, 11,400 ? 900 years BP (GSC-1049), is from gytta in basal lake deposits in a small lake east of the town of Atha- basca (see also GSC-1093, GSC-1053, Lowdon et al., 1971, pp. 289-290; and GSC -1205, St. Onge, 1970, p. 184). Further north, near Dawson Creek, British Columbia, a date of 9,960 ? 170 BP (GSC-1548) (N. W. Rutter, pers. comm., 1971; Reimchen and Rutter 1972; p. 177) was obtained from snails collected in a silt mound, located below the third latest proglacial lake level. This date was ori- ginally erroneously calculated at 16,300 + 180 years BP. In the western Peace River area, Reimchen and Rutter (1972; p. 176) established a sequence of seven proglacial lakes, the highest (833 m, 2,750 ft) of which correlates with beach deposits on the Portage Mountain recessional moraine (Rutter, 1971, p. 178), indicating that this high level lake existed after deposition of the moraine (Mathews, 1972; Rutter, 1971), radiocarbon dated at 11,600 years BP. If this high-level lake is related to the ca. 830-m (2,700-ft) lake stage identified by Jones (1961) in the adjacent Beaverlodge area, and to Glacial Lake Rycroft (St. Onge, 1966; Henderson, 1959) date at ca. 13,500 years BP (GSC-698, GSC-694) (Lowdon and Blake, 1968, p. 222) then the Portage Moun- tain date is unacceptable.

Regardless of the dating problem it is evident that proglacial lakes of various sizes existed for a protracted period of time in the Peace River area. The exact dating of these and other proglacial lakes must await further radiocarbon determinations, since there is at least a

1,500-year discrepancy between the dates obtained on shell and other materials. For ex- ample, compare 10 900 ? 160 BP (GSC-859) (shell) and 12.400 - 600 BP (GSC-903) (wood) (St. Onge. 1968, p. 187; Lowdon et al., 1971. p. 291) from the same stratigraphic unit.

While one hesitates to use questionable radiocarbon dates. there are four dates support- ing Postition 1 and/or 2 in the central/northern Alberta area. A date of 21.700 ? 840 BP (GSC-1129) (Lowden et al., 1971, p. 290) was obtained from humic acids in a buried Ah soil horizon, from the 30-m terrace on the North Saskatchewan River west of Edmonton near Duffield, Alberta (Pawluk and Dumanski, 1969). Pawluk (Lowdon et al., 1971, p. 290) considers this date unacceptable, and suggests that the sample was contaminated by lignite, although evidence of it could not be found in the sample. He also rejects the date on the basis of a date of 8,320 ? 150 BP (GSC-767) (Low- don and Blake, 1968, p. 221) from a nearby section. Since this latter date is from floodplain deposits of the 9-m terrace on the North Sas- katchewan River, the writer fails to see how a comparison can be made. From 25 km north of Calgary, three samples of peat interbedded with fluvial deposits yielded dates of greater than 35,000 years (S-204), 33,500 + 200 BP (S-205), and 26,700 + 1,400 BP (S-206) (Mc- Callum and Wittenberg, 1968, p. 365). While these dates support the contention of this paper that on the basis of acceptable geological and geochronological date the late Wisconsin ice did not reach the foothills/mountain front, they cannot be accepted at this time.

SUMMARY The evidence discussed indicates that mul-

tiple Laurentide glaciation occurred throughout the western interior, and the available radiocarbon determinations suggest that the earliest advances occurred during the early Wisconsin/ Illinoian (and probably also earlier). Ice- frontal positions in southern Alberta, correlat- able with the latest till units in that area, indicate that late Wisconsin ice did not penetrate the western portion of the area, and may not have reached southern Alberta. The data from central/northern Alberta, while very incom- plete, strongly suggest that the late Wisconsin ice lay somewhere northeast of the foothills/ mountain front.

B. 0. K. REEVES / 9



EVIDENCE OF COALESCENCE OF CORDILLERAN AND LAURENTIDE ICE

Relationships between the two ice sheets were studied in the Upper Oldman River Basin, the Bow River area and the Athabasca Valley area. Only in the latter area has indisputable field evidence for coalescence of the two ice sheets been found (Roed, 1968).

In the Oldman River area, Cordilleran till is always stratigraphically below a Laurentide till. In places, glaciolacustrine sediments or outwash separate the two. The glaciolacustrine sediments along the mountain front contain Laurentide erratics indicating that large proglacial lakes were impounded in front of the Laurentide ice sheet when at its maximum extent. Workers such as Alden (1932), Calhoun (1906), and Horberg (1954) concluded, primarily from this evidence, that the two ice advances were not synchronous and did not coalesce. They interpreted the evidence of mixed Cordilleran-Laurentide till, first noted in the vicinity of the 49th parallel by Dawson in 1874, to represent the reworking of Cordilleran glacial deposits by the Laurentide ice as it advanced up the regional slope. While they considered the respective glacial advances to be nonsynchronous, they found no evidence to indicate any significant difference in age between the two. Although Stalker has published no detailed studies of this particular problem, it would seem that he would (1956, 1960) concur with their conclusions.

In opposition to this viewpoint, Richmond (1960, 1965) and Wagner (1966) conclude that the advances of the two ice sheets were synchronous and at least partially coalescent in the area. Wagner, in a study of the surficial stratigraphy, including reinterpretation of many of Stalker's and Horberg's sections, concludes that coalescence probably occurred along some segments of the border between the two ice sheets in the area. This evidence consists of interdigitated surficial drifts and stratigraphic sections, transitional moraines, mixed till and the lack of significant differences in expression of weathering. Richmond (1965) published no detailed studies, but apparently based his inter- pretation of coalescence on evidence similar to Wagner's.

While not denying that coalescence may have occurred in some area, this writer would suggest the following sequence of events: At some time after maximum growth of the Cordil- leran piedmont lobes, Laurentide ice entered

the area, overrode the Cordilleran deposits and advanced to its maximum position in the foothills where it built terminal moraines; it impounded large proglacial lakes along the mountain front which deposited glaciolacustrine sediments on the deglaciated surfaces of the Cordilleran valley piedmonts and foothill moraines, and ice rafted erratics along the mountain front. At this time Cordilleran ice probably stood at active or stagnating termini, located at or behind the mountain front. This sequence of events probably occurred at least twice in the Upper Oldman River Basin. While the two ice masses are nonsynchronous, in the sense that they did not coalesce or reach their maximum extent at the same time, the absence of weathering zones in section, or pronounced differences in soil development or relief indicate that they are stage synchronous (Dawson, 1875).

A feature of major significance to the study of the Laurentide and Cordilleran coalescence in the western plains-foothills is the Foothills Erratics Train (Stalker, 1956). First noted by Hector in 1858, the erratics were traced by Stalker from the Athabasca Valley area to the 49th parallel. The train is composed of quartz- itic erratics with feldspar derived from the Rocky Mountain region of Jasper National Park (Mountjoy, 1958; Roed et al., 1967). These erratics were transported southeast by the Laurentide ice (Roed, 1968) which coalesced with Cordilleran ice in the Athabasca Valley area.

In the Bow Valley-Calgary area, first obser- vations of the relationship between the Cordil- leran and Laurentide ice sheets were made by Dawson (1881). Since then, studies were made by a number of workers (Tyrrell, 1890; Cole- man, 1909; Nichols, 1931; Tharin, 1960; Morgan, 1966). Only Morgan considered that the mixed tills associated with the Foothills Er- ratics Train were indicative of the coalescence of ice sheets in the Calgary area. Most workers consider that the Bow Valley piedmont lobe lay a few miles west at this time, and that mixed till represents (Tharin, 1960) the addition of Cordilleran ice to the Laurentide as a result of coalescence further north. This phenomenon is explained, however, by the incorporation of Cordilleran till by the advancing Laurentide glacier.

In the Athabasca Valley area Roed (1968) made a definitive, detailed study of field rela-




tionships between a Cordilleran piedmont lobe, referred to as the Marlboro glacier, and the latest Laurentide ice advance, referred to as the Edson glacier. This study provides un- equivocal evidence for the coalescence of the two ice sheets in the area.

Ice directional indicators associated with both the Cordilleran and Laurentide glaciers indicate deflection of their respective flow direc- tions resulting from contact of the two lobes (Figure 3). The coalesced areas of the two glaciers flowed southeast and little mixing of sediments occurred in most of the contact area.

In contrast to the valley glaciers of the south, the Marlboro and other glaciers of the north, i.e., Peace and Liard, were augmented by ice from the Cordilleran ice sheet to the west (Roed et al., 1967). This additional ice had an important effect on the areal extent and tem- poral duration of the Marlboro Glacier and probably explains the firm evidence of coalescence at this location.

The deposition of the Foothills Erratics Train probably dates from this event. Roed (1968) agrees, but notes that the erratics could have been brought out from the mountains by an earlier advance and coalescence, for which there is some evidence, or carried down from another source area, perhaps the Smokey River Valley.

Few data are available on the relationship of the Cordilleran and Laurentide lobes in the northern Alberta-British Columbia area. Hage (1945) notes that Cordillran till underlies Laurentide till in the Fort Nelson area. Ice- directional indicators in the region suggest that at least one Laurentide advance flowed southeast along the foothills and mountain front. This advance probably was earlier than the one which coalesced with the Cordilleran ice at the Athabasca Valley, since ice-directional indicators on the latter suggest that it had not been previously deflected further north. Reinforcing this conclusion is evidence from the Peace River area (Henderson, 1959; Jones, 1961) where the tills trend in a southwestern direction. Cordilleran erratics were not identified by either worker in the area.

In the western Peace River area, Mathews (1972, p. 169) notes that Precambrian erratics

decrease and Cordilleran erratics increase west- ward in the surface Laurentide till. He suggests that this may result from coalescence of the two ice sheets. (However, as noted above, this phenomenon can be accounted for by other mechanisms.) Reimchen and Rutter (1972, p. 176) believe that this zone of mixed till represents an overlap of the maximum extents of the latest Cordilleran and Laurentide advances. They interpret Laurentide drumlins truncated by Cordilleran drumlins as indicating that the maximum Cordilleran advance occurred after the last Laurentide advance in this area.

Large tracts of unglaciated land occur north of the Peace River. Along the east flank in the Mackenzie Mountains, the Laurentide ice overrode an earlier Cordilleran piedmont lobe (O. L. Hughes, pers. comm., 1971) and penetrated far into the mountain valleys, forming large proglacial lakes which discharged through meltwater channels behind the front ranges, both northward into the Arctic Red River and west- ward across the Eagle Plain to the Northern Yukon area, forming at one point a meltwater channel 120 m deep and 2 km wide. The vast discharge from the Laurentide ice sheet was augmented by meltwater from the Cordilleran glaciers in the area. In the Mackenzie Moun- tains, however, only small valley glaciers existed, far removed from the Laurentide front at this time.

SUMMARY The above data suggest that the last co-

alescence of the Laurentide and Cordilleran ice sheets occurred only at Athabasca Valley. The approximate Laurentide frontal position (Fig- ure 2) at this time is marked by the Foothills Erratics Train. Adjacent areas along the nioun- tain/foothills front were partly free and partly occupied by proglacial lakes and channels and stagnating Cordilleran valley piedmont lobes. With the exception of the coalescent Marlboro/ Edson glaciers, at the Athabasca, no major glacial barrier restricted the movement of mobile biotic populations at this time. The duration of the coalescent interval is not known, but one to two thousand years would not be un- reasonable.

DATING AND CORRELATION

Postglacial dates from the Interior Plains south of 53?N indicate the area west became

ice-free at least 15,000 years ago. Consequent- ly, dating of the coalescence must rely on the

B. O. K. REEVES / 11



o 5 10 20Km. .....

I I Laurentide (Edson) Glacier Cordilleran (Marlboro) Glacier

+- Drumlins ice flow direction Ice flow direction inferred from grooves and lobate end moraine

- Inferred ice flow lines

Approximate surface contact zone of "" Continental and Cordilleran till

. ? Edmonton \ 0

\ r

I '

I Calgary

I^ M -- IM-^ --


FIGURE 3. Inferred ice flows of Cordilleran (Marlboro) and Laurentide (Edson) glaciers (After Roed, 1968; Fig. 16).



correlation and dating of the Laurentide advances to the east. From data discussed in this paper, two alternatives may be proposed:

(1) The last coalescence represents the maximum advance of the late Wisconsin ice into the western interior, ca. 18,000 to 20,000 BP. Such dating negates evidence previously discussed of the existence of significant ice- frontal positions to the east representing major Laurentide advances in late Wisconsin or earlier time.

(2) The last coalescence represents a maximum advance of early Wisconsin or Illinoian ice into the area more than 55,000 radiocarbon years ago. Significant ice-frontal positions re-

presenting late Wisconsin and probably earlier substages are present on the plains to the east, e.g., the Lethbridge end moraine. A major nonglacial interval separates these two advances, in which the whole of the interior plains was ice- free until ca. 20.000 years BP. In the North- west Territories, the last Wisconsin advance of the Laurentide ice reached the mountain front, thereby sealing off any biotic movements along the Mackenzie Plains.

In either correlation, most of the western interior plains was ice-free by 15,000 years BP, by which time a viable biomass (Stalker, 1970, 1971) was present on the southern Alberta plains.

CONCLUSION

If we must look for environmental mechanisms which control the movement of human populations to and within the New World, this writer suggests that the ice sheets should no longer be invoked as the key factor. They did not, in late Pleistocene times, coalesce to form a 2,400-km barrier of solid ice. At worst, within the last 100,000 years or so, this so-called barrier probably consisted of the coalescence

during a few thousand years of a few mountain piedmont lobes with the Laurentide ice sheet for relatively short distances. Large ice-free areas continued throughout the late Quaternary that presented no physiographic barrier to human movement. Whether the environment was suitable for support of viable biotic populations during the glacial maxima is another mat- ter entirely.

ACKNOWLEDGMENTS

I acknowledge with thanks the assistance, helpful criticism, opinions and access to unpub- lished data provided by Drs. Nat Rutter, Jack Wyder, Rudy Klassen, Owen Hughes, and Archie Stalker of the Geological Survey of Canada; Dr. John Westgate, University of Al-

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the nature and age of the contact between the laurentide and cordilleran ice sheets in the western...

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