A Glacial-Process Model: the Role of Spatial and Temporal Variations in Glacier Thermal Regime

A glacial-process model: The role of spatial and temporal variations in glacier thermal regime

H. D. MOOERS Department of Geology and Geophysics, 108 Pillsbury Hall and Limnological Research Center, 220 Pillsbury Hall University of Minnesota, Minneapolis, Minnesota 55455

ABSTRACT ucts of glaciation also provide important infor- there are several distinct thermal environments, mation on the physical nature and dynamics of each of which has a dominant set of erosional or Glacial landform assemblages associated the ice. Moran (1971), Clayton and Moran depositional processes. The position and charac- with the retreat of the Rainy and Superior (1974, 1977), and Moran and others (1980) teristics of these zones also vary with time during lobes of the Laurentide Ice Sheet in Minne- used field relationships to reconstruct the glacio- glacial advance and retreat. sota provide a record of spatial and temporal logical conditions necessary for the formation of The glacial sediments and landforms in Min- variations in glaciological processes. The large-scale glaciotectonic features, and they pro- nesota associated with the late Wisconsin St. two lobes advanced contemporaneously to vided a major advance in the understanding of Croix Phase provide a detailed record of the their maximum at the St. Croix moraine. The ice-marginal glacial processes. Mathews (1974) maximum and recessional events of the Superior portion of the moraine formed by the Rainy reconstructed longitudinal profiles of the Lau- and Rainy lobes. During recession, each major lobe is characterized by large frontal outwash rentide Ice Sheet and changed the way subse- lobe broke up into multiple, smaller sublobes, plains, broad ice-stagnation topography, gla- quent researchers have viewed ice lobes along its dynamically independent from one another. The cial thrust systems, and an absence of stream- southern margin (for example, Clayton and oth- well-defined recessional events and their asso- lined landforms and features produced by ers, 1985; Beget, 1986; Hughes, 1987a; An- ciated suites of landforms are well suited to stud- subglacial drainage. Recessional moraines are drews, 1987). Shreve (1985a, 1985b) used ies of the spatial and temporal variations in similar to the terminal moraine, but large- characteristics of eskers along with a simple in- glacial processes. The landform assemblages, scale glacial thrust systems are even more novative numerical technique to determine the along with paleoecological data, were used to common. In contrast, the part of the moraine glacial characteristics responsible for esker for- constrain numerical models of the thermal char- formed by the Superior lobe is composed of mation and to reconstruct longitudinal profiles acter and geometry of the Rainy and Superior closely spaced recessional ridges with little of portions of the Laurentide Ice Sheet in Maine. lobes of the Laurentide Ice Sheet in Minnesota ice-stagnation topography. Recessional ice Most of the aforementioned studies concern (Mooers, 1988, 1989a, 1989b). These recon- margins are marked by the termination of the nature of the ice sheet in one particular area structions are summarized below and are used to eskers and tunnel valleys into subaerially and at one point in time. Mickelson and others develop a conceptual model of the changes in deposited outwash fans. Drumlins are abun- (1983, 1986), however, discussed the impor- glacial processes and landform development dant, and sugblacial drainage features are tance of the spatial distribution of glacial land- during ice recession. well developed, as indicated by the numerous forms and asserted that temporal considerations tunnel-valley and esker systems. The thermal are equally important in interpreting physical THE STUDY AREA regime of the Rainy lobe changed little during characteristics of the ice. The value of such tem- retreat. Landforms indicate that a frozen toe poral reconstructions was convincingly demon- During the St. Croix Phase, the Superior lobe existed at the terminal and recessional posi- strated by Bjorck and Moller (1987) in central advanced to the southwest along the axis of the tions, and that conditions were too cold to Sweden, where glacial sediments and landforms Lake Superior basin, while the penecontempo- allow surface meltwater to penetrate to the were used to reconstruct glaciological character- raneous Rainy lobe traversed relatively higher glacier bed. The Superior lobe, on the other istics of the Scandinavian Ice Sheet during ice terrain in northeastern Minnesota (Fig. 1). The hand, apparently underwent an evolution recession. deposits of these lobes now cover much of cen- from a subpolar glacier at its maximum to a The basal and ice-marginal thermal regimes tral Minnesota, and the terminus of this advance temperate glacier during ice recession. Engla- of ice sheets vary spatially, as do the associated is marked by the prominent St. Croix moraine. cial and subglacial drainage systems became erosional and depositional processes (Weert- This large morainic system extends from Leech better developed during retreat, and drumlin man, 1961; Boulton, 1972; Paterson, 1981, Lake in north-central Minnesota southward to formation was favored during recessional p. 185; Eyles and Menzies, 1983, p. 19-23; an area southwest of St. Cloud (Fig. 1). South of phases. Clarke and others, 1984; Drewry, 1986, this point, the moraine is mantled by younger p. 17-18). Climatic change results in a temporal drift, but it can be traced to the southeast as a INTRODUCTION thermal evolution of ice sheets and further spa- prominent topographic feature cored with the tial variation in glaciological processes (Nye, distinctive St. Croix-Phase drift (Wright, 1972). Glacial sediments and landforms provide a 1960, 1963a, 1963b, 1965; Mickelson and oth- The moraine emerges from beneath the younger valuable record from which to study the glacial ers, 1983; Hughes, 1987b; Mooers, 1988, drift in the vicinity of Minneapolis and abruptly history of the Laurentide Ice Sheet. These prod- 1989a, 1989b); therefore, beneath an ice sheet turns to the northeast into Wisconsin. The part

Geological Society of America Bulletin, v. 102, p. 243-251, 6 figs., 1 table, February 1990.

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al outwash are present (Figs. 2a, 2b), but evidence of channelized subglacial drainage is rare, as are drumlins (Mooers, 1988). Therefore the meltwater that produced the outwash apron presumably came from surface melt, with little or no contribution from subglacial sources. The possibility that subglacial drainage features existed and are now buried is unlikely. Deposits of the St. Croix Phase upglacier from the moraines are relatively thin (2 to 5 m) and are composed largely of lodgment till with a veneer of glaciofluvial sediment in low areas (Goldstein, 1985, p. 47, 61; Mooers, 1988). Subglacial drainage features such as eskers and tunnel valleys should still have some surface expression, but none can be identified. Large-scale glacial thrust systems, which can be recognized by the presence of a high hill bounded on the up-glacier side by a deep lake basin of similar volume, are associated with all of the end moraines but are more numerous at recessional ice-margin positions (Fig. 2b). The thrust systems are composed of an interlayered sequence of displaced blocks of drift and bedrock separated by proglacial outwash, unlike the single thrusted masses of bedrock such as those in North Dakota (Moran and others, 1980). Be- cause of their complex nature, Mooers (1988), following the work of Boyer and Elliot (1982), referred to these features as "thrust systems." One such structure consists of a mass of Cre- taceous lignitic shale overlying glacial outwash (Mooers, 1988). The décollement between the shale and the underlying outwash is marked by a Figure 1. Location and general distribution of landforms in central Minnesota. shear zone ~1 m thick. The Cretaceous sediments, which compose only a part of the total thrust system, were apparently quarried from the of the St. Croix moraine and associated deposits hummocky stagnation topography, glacial thrust lake basin and then transported to their present under investigation here, however, is that south systems, or proglacial fans at the terminations of position by shearing in less-competent underly- of Leech Lake and north of the younger drift particular esker and tunnel-valley segments ing beds. cover, termed the "western sector" of the St. (Mooers, 1988, 1989a, 1989b). These retreatal In marked contrast to the northern part of the Croix moraine. This western sector can be di- features and the contrasts in landscape develop- St. Croix moraine, the southern part has smaller vided into two regions on the basis of sedimen- ment between areas traversed by the Rainy and amounts of frontal outwash, and the moraine is tology, morphology, and drainage relationships. Superior lobes are directly related to the differ- composed of a series of closely spaced transverse These two segments, and the features formed by ent glaciological regimes of the former ice ridges. Hummocky stagnation topography in- the recession of ice from them, are hereafter re- masses. dicative of a superglacial drift blanket is less ferred to as the "northern and southern regions Within the northern region, which is the area common there and is absent at recessional ice of the St. Croix-Phase deposits" (Fig. 1) glaciated by the Rainy lobe, the St. Croix mo- margins, which instead are marked by small (Mooers, 1988). raine is characterized by broad hummocky stag- outwash fans with eskers commonly ending at Previous investigations treated the recession nation topography with a maximum local relief fan apices. Throughout this region, there is of the Superior and Rainy lobes from their max- of 10 to 25 m (Fig. 2a). Recessional ice margins abundant evidence of a well-developed englacial imum at the St. Croix moraine as a single phase, are marked by bands of stagnation topography 2 and subglacial drainage system, for tunnel val- with successive formation of drumlins, tunnel to 3 km wide (Fig. 2b) that are similar in mor- leys and eskers are common, particularly at the valleys, and eskers during thinning of the ice phology to the terminal morainic zone. The recessional ice-margin positions (Fig. 3). The re- before later readvances (Wright, 1972). More deposits within these moraines are predomi- lationships among tunnel valleys, eskers, and detailed mapping in central Minnesota shows nantly composed of flow till, collapsed outwash, outwash fans at the recessional margins is well that the retreat from the St. Croix moraine was and lacustrine sediments, which have undergone illustrated in the vicinity of Ann Lake in central characterized rather by several distinct reces- slumping and redeposition during meltout of the Minnesota (Fig. 4). sional margins, which can be recognized by underlying stagnant ice. Large amounts of front- No glacial thrust systems are known to occur

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within the southern part of the St. Croix moraine or at the recessional ice-margin positions. Drumlins are particularly abundant and well formed (Fig. 3). The contrasts in landforms between the northern and southern regions are summarized in Table 1.

GENESIS OF LANDFORMS

The formation of certain types of glacial landforms is often restricted to particular glaciological environments; therefore, individual landforms and suites of landforms can be used to reconstruct the prevailing glaciological conditions at the time of their formation (for example, Moran and others, 1980; Mickelson and others, 1983; Shreve, 1985a, 1985b). The contrasts in landform assemblages associated with the recessional ice margins can then be used to examine changes in the glaciological regime during ice retreat. For the purposes of this study, the landforms deemed most significant in glaciological reconstructions are hummocky stagnation moraine, glacial thrust systems, tunnel valleys and eskers, and drumlins.

Hummocky Stagnation Moraines and Glacial Thrust Systems

The hummocky stagnation topography in the study area consists largely of collapsed superglacial sediment; however, glacial thrust systems similar to those described by Moran (1971) and Bluemle and Clayton (1984) also contribute significantly to the morainic topography, particularly at the recessional ice margins within the northern region. The close association of these landforms suggests a genetic relationship. Mechanisms that result in the incorporation of large amounts of sediment and the subsequent transportation of the debris to the glacier surface near the margin of continental glaciers are relatively few. Without the contribution of sediment from nunataks or valley walls, the debris must come from the glacier bed. Weertman (1961) proposed that refreezing of basal meltwater in Figure 2. A. Portion of the St. Croix terminal moraine -40 km south of Leech Lake. the near-marginal zone could account for debris Ice-flow direction was from southeast to northwest (Oshawa and Backus, Minnesota, 7.5 bands seen in modern polar glaciers. Some dis- minute quadrangles, contour interval 10 ft). B. Segment of the Outing recessional moraine tance upglacier from the margin, where the ice located 55 km east-southeast of Leech Lake (Laura Lake, Minnesota, 7.5 minute quadrangle, was relatively thick, the combination of geo- contour interval 10 ft). Ice-flow direction from northeast to southwest. Much of the Outing thermal and frictional heat was greater than moraine consists of large hills located immediately adjacent to relatively deep lake basins of could be conducted down the temperature gra- similar size. Portions of these hills are composed of displaced blocks of Cretaceous shale. dient to the glacier surface. Meltwater produced by basal melting in this thawed-bed zone would be forced by the hydrostatic pressure gradient glacier sole, incorporating sediment into the objects on the bed, a process similar to that sug- toward the margin, until it encountered a region basal ice. The sediment would subsequently be gested by Lliboutry (1965, p. 689), is also a where the ice was thinner and the temperature transported to the glacier surface by marginal possible mechanism of debris entrainment. Boul- gradient allowed the heat to be conducted to the compressive flow. ton (1970, p. 221), however, pointed out that surface. The water would then refreeze to the Incorporation of debris by regelation around sediment entrained on the downglacier side of

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an object would be redeposited against the next irregularity, and there would be little or no cu- mulative addition of debris. Two other entrainment processes must be mentioned: first, the process of block incorporation (Moran, 1971; Moran and others, 1980), which is elaborated below; and second, the overriding of frontal aprons suggested by Hooke (1973) and modified by Shaw (1977). In cold climates, sediment, ice blocks, and seasonal snow drifts may accumulate at the ice margin. During glacial advance, much of this debris can be reincorporated into the ice and may contribute significantly to the debris load and therefore to the morainic topography. The processes of moraine development discussed above are common on polar and subpolar glaciers, where the marginal zone of the glacier is frozen to the bed. The surface of the ice may be subject to seasonal melting, but summer warmth cannot penetrate to the glacier sole. On temperate glaciers, where the ice is everywhere Figure 3. Southern region of the deposits of the St. Croix Phase, showing the location of at the pressure-melting point, processes that re- drumlins, tunnel valleys, and eskers (modified from Wright, 1972). sult in large accumulations of superglacial debris

Figure 4. Tunnel valley and eskers in the vicinity of Ann Lake, which is located along a prominent recessional margin of the Superior lobe. Large cone-shaped hills Ha and Hb are located at the heads of eskers Ea and Eb. Esker Eb terminates into a subaerial outwash fan (Fb), which has two apices. The trace of the tunnel valley is delineated by the swampy area. A portion of the tunnel valley, now occupied by Ann Lake, terminates at the apex of a large subaerial outwash fan Fc. Direction of flow on the fan surfaces is shown by arrows (redrawn from the Ann Lake and Ogilvie, Minnesota, 7.5 minute quadrangles; contour interval 20 ft with intermediate 10-ft contours shown by dotted lines).

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TABLE 1. SUMMARY OF THE CONTRASTS IN GLACIAL LANDFORMS BETWEEN THE NORTHERN AND SOUTHERN REGIONS OF THE DEPOSITS OF THE ST. CROIX PHASE terminate in subaerial outwash fans (Fig. 4). In addition, many tunnel valleys are associated

Northern region Southern region with only one specific recessional ice margin with no upglacier or downglacier connections to Hummocky stagnation topography at terminal Widespread hummocky stagnation topography and recessional moraines at maximum ice limit only other tunnel-valleys segments (Fig. 3). The

Large Irontal outwash aprons Small frontal outwash fans number of tunnel valleys is greatest about mid-

Little channelized subglacial drainage Abundant evidence of large volumes of way along the axis of the Pierz drumlin field, subglacial meltwater (tunnel valleys and and none of the valleys can be unequivocally eskers) traced to the St. Croix moraine. The eskers that -ew drumlins Large drumlin fields frequently lie within the valleys are also com- jlacial thrust systems abundant at terminal and No documented glacial thrust systems recessional ice margins posed of short segments, some of which begin abruptly at large cone-shaped hills composed of glaciofluvial sediment, and end a short distance are limited. On Malaspina Glacier, Alaska, Gus- Sugden (1977) suggested that the frozen mar- away in subaerial outwash fans (Fig. 4). These tavson and Boothroyd (1987, p. 197) suggested ginal zone along parts of the southern margin of outwash fans, along with ice-collapse slopes and that most of the superglacial sediment near the the Laurentide Ice Sheet may have been as prominent gaps in the drumlin field, mark the margin is from the meltout of lateral and medial much as several tens of kilometers wide. Numer- recessional ice-margin positions of the Superior moraines, with relatively little material derived ical modeling by Mooers (1988), however, using lobe (Figs. 3 and 4). Mooers (1989b) therefore from the glacier bed. Such morainic features, a modified version of the computer program proposed that the tunnel valley and esker system however, were extremely rare or absent on con- described by Hooke (1977), indicates that a fro- formed relatively near the ice margin during re- tinental ice sheets of the midcontinent region of zen marginal zone wider than 1 to 2 km would treat of the Superior lobe. The major source of North America. Another mechanism that results require specification of unreasonable climatic water was melting at the glacier surface that in the accumulation of superglacial debris on parameters or virtual stagnation of the ice. reached the bed through moulins and crevasses. Malaspina Glacier is the discharge of sediment Moran and others (1980) presented abundant The cone-shaped hills located at the heads of from subglacial and englacial channels onto field evidence indicating that glacial thrust sys- some esker segments are interpreted as sediment stagnant ice near the margin (Gustavson and tems rarely occur more than 3 km from the deposited at the base of moulins (Mooers, Boothroyd, 1987). This process is likely to have former ice margins. A few such features have 1989b). As the Superior lobe retreated, the en- operated on the Laurentide Ice Sheet wherever been reported as much as 5 km from the margin, glacial and subglacial drainage system became large subglacial streams discharged. but these are generally streamlined, indicating better developed, accounting for the larger number of tunnel valleys at the first few reces- The process of block incorporation of drift they were quarried in the frozen marginal zone sional ice margins. Further retreat, however, re- and bedrock is also favored in a near-marginal and later overridden by the thawed-bed region. sulted in a narrowing of the lobe and an position where the toe of the glacier is frozen to Within the present study area, there are no associated reduction in the number of channels. the substrate (Moran, 1971; Clayton and Moran, topographic features that could have produced The apparent continuity of the tunnel valleys 1974; Moran and others, 1980). Mathews and lateral or medial moraines, and there is little was explained by headward development of the Mackay (1960) attributed the failure of sedi- or no evidence of channelized subglacial drain- englacial drainage system during retreat (Moo- ment beneath ice to the role of high fluid pres- age associated with the stagnation topography. ers, 1989b). Nye (1953) has shown that tunnels sures, as suggested in the process of thrust Therefore, it is likely that most of the de- in the ice will remain open during the winter faulting (Hubbert and Rubey, 1959). Permafrost bris composing the stagnation topography was even though greatly reduced in size. After con- was considered necessary for thrust-system for- incorporated by the freeze-on and thrusting duits were established, they were reoccupied mation, as it forms a large brittle sheet. The processes and was associated with a frozen during subsequent ablation seasons, and newly stress imposed by the overriding ice then re- glacier toe. formed englacial drainage conduits preferen- moves parts of the frozen material, which are tially connected with pre-existing channels subsequently transported to the glacier margin. Tunnel Valleys and Eskers (Mooers, 1988, 1989b). Moran (1971) and Clayton and Moran (1974) also discussed the role of fluid pressure in thrust- Wright (1973) postulated that the tunnel val- ing and incorporation of large blocks of the gla- ley and esker system formed while the ice was Timing of Drumlin Formation cier bed. They considered a situation where standing at its maximum limit at the St. Croix permafrost is present beyond the ice margin. moraine. At the head of the tunnel-valley sys- The Pierz drumlin field, originally interpreted Water produced by basal melting in the thawed- tem, 200 km from the moraine, the ice was pre- as a single integrated feature (Wright, 1956, bed zone is forced toward the margin by the sumed to have been nearly 2,000 m thick. The 1973), has prominent breaks associated with the hydrostatic gradient of the ice-surface slope, ice at this elevation would have been too cold to recessional ice margins (Mooers, 1989a). With such as suggested by Weertman (1961). If the allow surface melt to penetrate to the glacier each successive recessional position, drumlins escape of the water is inhibited either by pinch- bed. Wright (1973) therefore proposed that the are more abundant and more regular in outline. ing out of more permeable units or by a frozen valleys were excavated by a catastrophic out- Drumlin axes behind the recessional margins are toe at the margin, a zone of excess fluid pressure burst of subglacially stored water. This interpre- perpendicular to these ice limits but oblique to will exist, reducing the effective normal stress. If tation, however, is inconsistent with field the St. Croix moraine. Mooers (1988, 1989a) the shear stress applied by the ice exceeds the evidence presented by Mooers (1988, 1989b). therefore proposed that drumlins were formed strength of the sediment, failure occurs at the Detailed examination of the tunnel valley and within ~20 to 30 km of the ice margin as the base of the permafrost layer or impermeable esker system reveals that the valleys are discon- Superior lobe retreated. strata. tinuous, having prominent breaks that often To test this model of time-transgressive drum-

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lin formation behind a retreating ice margin, a modified version of the numerical technique originally described by Hooke and others (1976) and Hooke and Mooers (1986) was used to reconstruct the surface topography of the Superior lobe. A complete description of the modified Figure 5. Computer- technique and the results are given by Mooers generated reconstructions (1989a). In brief, input to the model consisted of of the Rainy and Superior X, Y, and Z (elevation) coordinates of former lobes, (A) at the maxi- ice-margin positions and elevations of the glacier mum limit at the St. Croix bed. Basal shear stress (r) was then specified to moraine, and at two re- prepare a contour map of the ice surface, using cessional positions (B) the relationship r = pgh dh/ds, where p is the and (C). The trend of density of ice, g the acceleration of gravity, and drumlin axes is nearly dh/ds is the glacier surface slope. A uniform perpendicular to ice-sur- Minneapolis Minneapolis and St. Paul basal shear stress was used in several preliminary face contours near the and St. Paul runs; in succeeding simulations, the basal shear glacier margin for each stress was allowed to vary with bed position as position, but farther back, described by Mooers (1989a). Assuming that ice the orientations deviate flow was parallel to drumlin axes and perpen- quite strongly, suggesting dicular to the ice-surface contours and moraines, that they were adjusted to the trend of drumlin axes was best explained by successive recessional po- Ice surface time-transgressive drumlin formation behind a sitions (modified from contours retreating ice margin (Fig. 5). The alternative, Mooers, 1989a). that drumiins were formed while the ice stood at /, Drumiins the St. Croix moraine, requires large variations V in the basal shear-stress distribution over small distances, implying large spatial variations of ice N 1 100 km velocity within the Superior lobe. No field evi- Minneapolis and St. Paul dence could be found to support differential flow within the lobe. The broad arcuate trend of the former ice margins exhibits no small-scale lobation that would typically be expected with The large coalescing apron of outwash fans sures. The incorporated sediment was subse- large variations in ice velocity or discharge along the margin of the St. Croix moraine and quently transported to the glacier surface by (Holdsworth, 1973; Mickelson and others, the absence of evidence for eskers and tunnel marginal compressive flow and accumulated 1983, p. 13). valleys indicate that seasonal meltwater was over stagnating ice. Seasonal melt draining from draining from the ice surface. The ice was ap- the ice surface reworked much of that material parently too cold to allow surface melt to reach into a series of large coalescing outwash fans GLACIAL-PROCESS MODEL the glacier bed (Mooers, 1988). In this case, the beyond the glacier margin. sediment in the fans would have been derived The situation described above persisted The contrasts in landforms and landform as- from the abundant superglacial material that throughout the St. Croix Phase, at the maximum semblages between the recessional feature of the now composes the moraine. ice limit and during recessional events. Rainy and Superior lobes, along with informa- Subglacial meltwater drained through the sed- tion on landform genesis, can now be used to iments, and thrusting occurred where pore-fluid Southern Region develop a conceptual model of changes in pressures became elevated (Mooers, 1988). glacial-processes as functions of space and time. Drumlin formation, however, was apparently The terminal moraine in the southern region not favored, as these features are extremely rare. is predominantly a feature of the Superior lobe Northern Region The glacial system can be represented as in and therefore reflects the glaciological condi- Figure 6A. In the near marginal zone, the ice tions that existed within this portion of the ice The hummocky stagnation moraine and was frozen to the substrate, forming a frozen toe sheet. Although the St. Croix moraine in this large-scale glacial thrust systems in the St. Croix up to about 2 km wide (Fig. 6A, zone 1). region is wider than the section of the moraine moraine and the recessional moraines indicate Farther upglacier, a thawed-bed region existed, associated with the Rainy lobe, the hummocky that climatic conditions were cold enough to where water produced by basal melting was stagnation topography is restricted to a narrow allow a frozen toe to exist at each successive evacuated from the glacier sole through the sub- band (0.25 to 0.5 km) along the moraine's outer retreatal position. The minimum width of the glacial aquifer system (Fig. 6A, zone 3). The margin. Much of the morainic topography, frozen toe, as indicated by the distance of trans- transition from a thawed bed to the frozen mar- however, was reshaped by glacial meltwater port of thrusted blocks of sediment, is similar at gin (Fig. 6A, zone 2) was an area where sedi- draining through large channels within the mo- all recessional ice margins, ~ 1 to 2 km (Mooers, ment was incorporated by freezing on and by raine parallel to the ice front. This reshaping and 1988). thrusting that was facilitated by high fluid pres- the absence of glacial thrust systems makes it

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tunnel-valley segments also appears to be great- A. est midway through the drumlin field. Phases a-c of Figure 6B show the postulated Maximum limit and « 1000 spatial distribution of glacial processes for the recessional phases meters maximum and for two recessional positions of the Superior lobe. While the ice was standing at its maximum at the St. Croix moraine, a frozen toe may have existed, the width of which is uncertain (Fig. 6B, phase a, zone 4). As stated above, however, it is unlikely that it could have B. exceeded 2 km, and it was probably narrower T than the frozen toe of the Rainy lobe. Evidence indicates that a poorly developed subglacial and (phase a) = 900 englacial drainage system existed close to the meters Maximum limit glacier margin, and the source of the water was probably surface melt originating in the area immediately upglacier from the moraine (Fig. 6B, phase a, zone 5) (Mooers, 1989b). The presence of collapsed eskers and discontinuous frontal outwash aprons not associated with 5 n -—— tunnel valleys suggests that meltwater was also (phase b) »800 draining directly from the ice surface without A First recessional meters access to the englacial channel system (Mooers, position 1988). At this stage, a few drumlins were being formed (Fig. 6B, phase a, zone 6), but conditions were not as favorable for drumlin formation as they were later during continued retreat. Zones 5 and 6, as well as the region upglacier from zone 6, were presumably areas of net basal (phase c) melting. • 700 Second recessional meters As the ice thinned and the margin retreated, position the width of the frozen toe became narrower. Minor hummocky stagnation topography is present at the first recessional ice margin, but T- —r~ -r~ -1 15 30 45 60 75 only locally present at later recessional positions. Distance (km) Even when present, the deposits are very thin, rarely exceeding about 2 m. The absence of hummocky moraine reflects the changing ther- Figure 6. Conceptual models of the spatial and temporal variations in glacial landform mal character of the Superior lobe, as conditions development. A. The northern region, where conditions varied little during ice recession. Zone did not favor the development of the frozen toe. 1 was an area of debris-covered ice where thrusting of subglacial sediments was occurring. Little debris was incorporated by freezing on, Zone 2 was an area of freezing-on of sediment at the glacier bed, and zone 3 was the and there is no evidence of thrusting. Without thawed-bed region. B. The southern region, where suites of landforms suggest pronounced contributions of sediments by the aforemention- variations in landform development during ice recession. Zone 4 was the ice marginal area with ed processes, the superglacial debris blanket was a sediment cover of variable thickness. A frozen toe was present at the maximum limit but relatively thin (Fig. 6B, phases b and c, zone 4). absent at recessional positions. Zone 5 was the area of tunnel-valley and esker formation, while During continued retreat, the englacial and drumlin formation was occurring in zone 6. subglacial drainage system became better developed, and the surface meltwater was able to difficult to estimate the width of the frozen toe The tunnel valley and esker system also penetrate to the bed farther upglacier from the that may have been associated with the Superior changes in character with distance from the St. margin (Fig. 6B, phases b and c, zone 5). The lobe. Croix moraine. Near the outer limit, there are length of tunnel-valley segments increased, and Near the St. Croix moraine, the Pierz drum- few tunnel valleys. Where present, they are typi- the larger, more prominent channels indicate lins are widely spaced and have diffuse poorly cally narrow and relatively short. Few eskers are that discharge was greater. The water produced defined outlines. The density of drumlins asso- present, and generally they are not associated by melting at the ice surface was virtually sedi- ciated with each successive recessional ice mar- with tunnel valleys. To the east, the number and ment free, and upon reaching the bed, it had gin increases; their shapes become more elon- size of tunnel valleys and eskers increases to a great erosive potential. Apparently at this stage, gated, and the outline of the drumlins is easily maximum about midway along the length of the little meltwater was draining directly off the ice distinguished. Pierz drumlin field (Fig. 3). The length of surface. Instead, most of the water must have

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entered the englacial and subglacial drainage SUMMARY AND CONCLUSIONS thrusting) periodically broke through, cata- network and discharged at the margin through strophically cutting tunnel channels. the subglacial channels. As illustrated in Figure Glacial landform assemblages have been used In the present study area, these same relation- 4, outwash fans occur at the terminations of to examine changes in the glaciological regime ships exist on a gross scale. In the southern re- tunnel-valley and esker segments. The eskers, of two adjacent ice lobes of the Laurentide Ice gion, a large drumlin field and a tunnel-valley which formed after the tunnel valleys, record a Sheet during ice recession. At their maximum system exist behind the St. Croix moraine. The change from an erosional to a depositional extent, the Rainy and Superior lobes had a fro- evidence presented above and by Mooers (1988, mode, the reason for which is not clear. Thin- zen toe ~1 to 2 km wide, and they are best 1989a, 1989b), however, indicates that the tun- ning of the ice (Wright, 1972), reduction of characterized as subpolar glaciers. Englacial and nel valleys and drumlins developed during ice water input to individual conduits by redistribu- subglacial drainage was poorly developed, and recession, relatively close to the glacier margin tion of surface runoff during ice wastage, or in- ice-marginal thrusting of large blocks of drift where no frozen toe was present. In the northern creased sediment load supplied by the melting of and bedrock was common. Within the northern region, however, a frozen margin existed during debris-laden basal ice (R. LeB. Hooke, 1988, region glaciated by the Rainy lobe, apparently the recessional events, and ice-marginal thrust- personal commun.) seem to be the best possibili- little change occurred in the glacial thermal re- ing was common. The thrusting was presumably ties (Mooers, 1989b), however. gime with time. Ice retreat occurred as a series of facilitated by elevated pore-water pressures in Drumlin formation was favored within 20 to recessions followed by stillstands, and the glacio- the substrate. Few streamlined features are pres- 30 km of the margin (Fig. 6B, phases b and c, logical characteristics that existed at the maxi- ent, however, indicating that elevated fluid pres- zone 6), and conditions became more favorable mum limit persisted throughout ice recession. sure alone may not be a criterion for drumlin at successive recessional positions. Few features The climatic conditions, and therefore the glaci- formation. seen in the field can be used to determine the ological regime, may have changed relatively lit- The results of this study illustrate that distinct degree of overlap of the zones of tunnel-valley tle in this region. The local climate was likely zones, dominated by particular glaciological formation and drumlinization. In some in- dominated by the effects of the large ice mass processes (Fig. 6), existed within the near- stances, the tunnel valleys appear to crosscut (Wright, 1973, p. 259; Kutzbach, 1987). To the marginal (outer 100 km) areas of the Laurentide adjacent drumlins, but these relationships are south, however, glacial processes and subse- Ice Sheet. The spatial relationship of the zones hard to discern because drumlins are generally quent landform development in the area gla- and the dominant processes operating within parallel to channels, and distinguishing a poorly ciated by the Superior lobe varied with time. them were largely functions of the thermal re- formed drumlin from a well-formed, partially Tunnel-valley, esker, and drumlin formation gime of the ice, which was controlled by local eroded feature is difficult. It is possible, however, were favored during retreat, when the ice is pos- and regional climate. The glacial processes var- that the zones of tunnel-valley and drumlin for- tulated to have been largely temperate (Mooers, ied in intensity and dominance as the thermal mation overlapped considerably. 1989b). regime changed during retreat. The examples Figure 6B illustrates the postulated spatial dis- Other studies in the midcontinent region have described above demonstrate the importance of tribution in glacial processes during the early discussed spatial relationships of glacial land- time-dependent changes in glacial regime in the stages of glacial recession. As ice retreat con- forms and associated glacial processes. Most over-all process of landform development. tinued, the Superior lobe became narrower. notable are the studies of Wright (1973), Clay- Many previous studies involving the spatial dis- Headward development of the englacial drain- ton and Moran (1974), Whittecar and Mickel- tribution of glacial landforms should be re- age system resulted in the merging of the subgla- son (1977), Nelson and Mickelson (1977), evaluated with time-dependent considerations in cial drainage conduits, and the tunnel valleys Moran and others (1980), Mickelson and others mind. became fewer (Fig. 3) (Mooers, 1988, 1989b). (1983,1986), Bluemle and Clayton (1984), and The size of individual valleys also decreased, Clayton and others (1985). Throughout these ACKNOWLEDGMENTS probably in response to a decrease in the ice- studies, certain consistent relationships have surface area contributing meltwater. The lobe been noted. Nelson and Mickelson (1977, This work was supported in part by grants retreated to the north, however, where closer p. 51), Bluemle and Clayton (1984, p. 280, from the Graduate School, Academic Comput- position to other ice lobes may have resulted in 296), and Mickelson and others (1983, 1986) ing Services and Systems, and the Department cooler conditions and therefore less surface discussed the association of drumlins and glacial of Geology and Geophysics of the University of melting. thrust systems. They proposed that thrusting oc- Minnesota; the Geological Society of America; The eskers are considered to have formed curred near the margin, whereas streamlining of and the Chevron Corporation. Thanks are ex- near the margin, possibly under stagnant ice subglacial material occurred in the thawed-bed tended to H. E. Wright, Jr. and R. LeB. Hooke (Mooers, 1988, 1989b). The active margin may zone farther upglacier, possibly also under con- for their contributions to this study. I would also have been several kilometers away. The pres- ditions of elevated pore-water pressure. In Wis- like to thank Lee Clayton, E. H. Muller, D. M. ence of the cone-shaped hills at the head of some consin, Mickelson and others (1983, p. 26-27) Mickelson, and J. C. Boothroyd for their critical eskers indicate that a sediment source was avail- also reported tunnel channels (tunnel valleys) reviews of this manuscript. able near the ice surface. A superglacial sedi- associated with apparent thrust systems and REFERENCES CITED ment veneer would be expected fairly near the drumlins. The tunnel channels are found in the near-marginal zone, and it has been proposed Andrews, J. T., 1987, The Late Wisconsin glaciation and déglaciation of the margin and possibly farther back if the ice sur- Laurentide Ice Sheet, in Ruddiman, W. F., and Wright, H. 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M., 1977, Landform distribution and genesis REVISED MANUSCRIPT RECEIVED JULY 6,1989 ternary Association, Program and Abstracts of the 9th biennial meeting, in the Langlade and Green Bay glacial lobes, north-central Wisconsin: MANUSCRIPT ACCEPTED JULY 18,1989 University of Illinois, Champaign, p. 34- 36. Wisconsin Academy of Science, Arts, and Letters, v. 65, p. 41-57. PUBLICATION NO. 1118 OF DEPARTMENT OF GEOLOGY AND GEOPHYSICS, AND Hooke, R. LeB,, Matsch, C. L., and Gasser, M., 1976, Implications of computer Nye, J. F., 1953, The flow law of ice from measurements in glacier tunnels, CONTRIBUTION NO. 365 OF THE LIMNOI.OGICAL RESEARCH CENTER, UNIVERSITY reconstructions of the Pleistocene DesMoines lobe, Minnesota and laboratory experiments and the Jungfraufirn borehole experiment: OF MINNESOTA

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