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Retreat of the Laurentide ice sheet from 14,000 to 9000 years ago
QUATERNARY
RESEARCH
Retreat
1,
316-330
of
the
(1971)
Laurentide 9000
Ice Years
Sheet
From
14,000
to
Ago
H. E. WRIGHT,
JR.’
The intricate pattern of moraines of the Laurentide ice sheet in the Great Lakes region reflects the marked lobation of the ice margin in late Wisconsin time, and this in turn reflects the distribution of steam-cut lowlands etched in preglacial times in the weak-rock belts of gentle Paleozoic fold structures. It is difficult to trace and correlate moraines from lobe to lobe and to evaluate the magnitude of recession before readvance, but three breaks stand out in the sequence, with readvances at about 14,500, 13,000, and 11,500 years ago. The first, co’rresponding to the Cary advance of the Lake Michigan lobe, is represented to the west by distant advance of the Des Moines lobe in Iowa, and to the east by the overriding o,f lake beds by the Erie lobe. The 13,000-year advance is best represented by the Port Huron moraine of the Lake Michigan and Huron lobes, but by relatively little action to west and east. The ll,SOOyear advance is based on the Valders till of the Lake Michigan lobe, but presumed correlations to east and west prove to be generally older, and the question is raised that these and some other ice advances in the Great Lakes region may represent surges of the ice rather than regional climatic change. Surging may involve the buildup of subglacial meltwater, which can provide the basal sliding necessary for rapid forward movement. It would be most favored by the conditions in the western Lake Superior basin, where the Superior lobe had a suitable form and thermal regime, as estimated from geomorphic and paleoclimatic criteria. The Valders advance of the Lake Michigan and Green Bay lobes may also have resulted from a surge: the eastern part of the Lake Superior basin, whence the ice advanced, has a pattern of deep gorges that resemble subglacial tunnel valleys, which imply great quantities of subglacial water that may have produced glacial surges before the water became channeled.
INTRODUCTION
fectetl by irregularly distributed riiountain masses. and in many areas there the frontal moraines are poorly developed or at best are inconspicuous among the features of bedrock relief. But in the intervening area, between the Missouri River in South Dakota and the Ohio River in Ohio, which are both basically frontal streams, the ice margin was prominently lobate, with each lobe being localized by a previously existing lowland. These lowlands, now mostly occupied by the Great Lakes, were initially eroded by streams in relatively nonresistant sedimentary rocks, which in turn are distributed according to the gentle fold structures of the
The front of the Laurentide ice sheet during its Main Wisconsin maximum stretched from the Rocky Mountain Piedmont to the New England coast, with a deep penetration to the south in the Great Lakes region and the Mississippi Valley area-the major topographic low of the continent. In Montana and the western Dakotas the front was reasonably straight, for the northern Great Plains provided few lowlands to encourage irregular frontal advance. In New York and New England the front was severely af1 Limnological Research Center, Minnesota, Minneapolis, Minnesota
University 55455.
of
continental 316
interior
( Fig.
1 ‘\ . Lake
Ontario
RETREAT
OF
LAURENTIDE
ICE
SHEET
GEOLOGIC MAP OF THE
/l/\/i,.
‘1X/\/
\,
GREAT
FIG.
graphic
1. Geologic map of the Great Lakes region, showing the localization units in the Paleozoic bedrock. From Hough (1958).
and Lake Erie are localized respectively by Ordovician and Devonian shales at the north edge of the Allegheny structural basin, Lake Huron and Lake Michigan by Devonian shales on opposite sides of the Michigan structural basin, and Lake Superior by late-Precambrian red sandstone and shale in the center of the Superior syncline. The pattern continues westward with the Red River Valley and the James River valley on two different units of Cretaceous
LAKES
REGION
of the lake
basins
’ ’
by strati-
shale on the eastern side of the Great Plains syncline. During the time of maximum Wisconsin ice advance, the Great Lakes basins were completely inundated by ice, and the ice margin to the south was relatively regular as it reached its ultimate termination on the slightly differentiated low uplands that extend from centra1 Ohio to Illinois (Fig. 2). .\ctually the steady flow lines of thick ice across or along the Great Lakes basins at
318
RETREAT
OF
LAtiRENTIDE
this time may have resulted in appreciable deepening of these basins by glacial erosion, for the basin floors now extend below sea level, whereas in preglacial time they sloped to the sea. At any rate, when the ice sheet thinned and began its retreat, its frontal form became much more influenced by the major features of the subglacial topography (Fig. 2 ) Many distinct lobes developed, not only in the major Great Lakes basins but also in subsidiary weak-rock belts as well, such as the Green Hay lobe in eastern Wisconsin on Cambrian sandstones, and the Chippewa and other small lobes in different Precambrian rocks in northern Wisconsin. .\s the ice became still thinner, the lobation was locally controlled in part by ridges rep resenting the massive moraines of earlier phases of ice advance-moraines that were probably still cored by stagnant ice and thus even higher than they are today. A good example of this moraine control is in Minnesota, where one margin of the Grantsburg sublobe of the Des Moines lobe, for example, lapped onto the inner margin of the previously formed St. Croix Moraine of the Superior lobe along a distance of 70 miles. This extreme digitation of the margin of the ice sheet resulted in the formation of moraines, loop-like recording successive stages of ice retreat (Fig. 2). In cases the moraines are fairly uniform in size and spacing, and they appear to represent periodic still-stands of the ice, or even slight readvances. In the Lake Michigan lobe, for example, 30 mapped moraines cover a time range of 8000 years (Frye it ab., 1965) and in Ohio at least 12 moraines were formed in 1000 years (Goldthwait ct al., 1965). The implied periodicity is therefore a very few hundred years. Although the tills of successive moraines of the same ice lobe can rarely be distinguished lithologically as stratigraphic units (Willman and Frye, 1970)) because of steady ice flow from the same source area, the moraines themselves give
ICE
SEIEET
319
adequate indication of a fluctuating ice margin. Synchroneity of moraines from lobe to lobe is difficult to establish, however, especially when lobes are attenuated and the interlobate junction is topographically confused. In cases, time relations can be worked out from outwash deposits or proglacial lake features common to adjacent lobes. But generally dependence has been put on moraine trends, and the results are often unsatisfactory. Because of this, efforts h ave been made to identify major geomorphic or stratigraphic breaks in the sequence, so that the long time involved might be separated into stadial and interstadial units that may be of importance in continental or inercontinental climatic correlations. Many subdivisions have been made in the almost 100 years since Chamberlain first emphasized that the U’isconsin glaciation was not a simple event-subdivisions ranging from the Early-Middle-Late categorization of Leverett to the series of many named stadial and interstadial intervals of Leighton. One of the more recent classifications for the Lake Michigan lobe recognizes only two stadial subdivisions : the Valderan, at the end of the series, and the Woodfordian, which includes all the rest (Frye and \,Villman, 1960). The powerful tool of radiocarbon dating provides a continuing incentive to search for correlation and significant subdivisions, and newly refined techniques of pollen analysis make possible more accurate stratigraphic approaches to climatic reconstructions that may permit an evaluation of the climatic significance of ice-margin fluctuations. A break in the retreatal sequence might be considered of major climatic significance if the ice front readvanced a great distance and if the readvance can be shown to be synchronous for several ice lobes. Geologic evidence for distant retreat and readvance is generally difticult to find. Crosscutting mo-
320
II.
E.
WRIGHT,
JR.
recognized as a basis for subdividing the successionof moraines. The crosscutting implies a realignment of the Lake Michigan lobe before readvance. hut in this case the distance of retreat cannot be accurately estimated. The latest revision of the nomenclature in Illinois (Willman and Frye, 19701) recogntzes a stratigraphic distinction at a slightly younger position than that implied by the major topographic discontinuity. A radiocarbon date for a correlative moraine of the Frie lobe is 14.300 years ago (W198) , so the Cary ice advance can be estimated at about l-t,50 years ago. The magnitude of the 11,500-year advance is better documented in Minnesota. although the situation is complicated because two adjacent ice lobes did not advance a similar amount (Wright and others, 1971). The Superior lobe, advancing from the northeast. had produced the massive St. Croix moraine in southeastern Minnesota at an earlier time, and it retreated back into the Lake Superior basin at least 80 miles from the moraine, leaving abundant stagnant ice behind (Fig. 3). It reaclvanced a very short distance about 16,000 years ago, hut at this time the Des Moines Lobe advanced from the Red River Valley along the Minnesota River i’alley and into the Minneapolis lowland in the form of the Grantsburg strhlohe. occupying part of the area previously covered by the Superior lobe. THE 14.500-YEAR Continued expansion of the ice caused it to ADVANCE overflow a low divide on the Iowa The 14.500-year ice advance produced the -Minnesota border and spill down the Des Cary group of moraines in northern Illinois, Moines River valley to central Iowa as the which can be traced with only minor diffimain Des Moines lobe proper, reaching its culties to the Saginaw and Erie lobes to the terminus about 14.000 years ago (Ruhe. east (Zumberge, l%O; Wayne and Zum- 1969; Wright rt 171.. 1971). This great aclberge, 1965). The moraines sharply crosscut Vance of the Grantsburg sublobe after the several moraines of the previously formed distant retreat of the Superior lobe implies a Tazewell group (Fig. 2). Although the mo- major break in the climatic sequence, alraine sequencein Illinois reveals other cross- though the change in locus of major ice-lobe cutting relations (Frye and Willman, advance introduces suspicion that the re1960), the Tazewell/Cary topographic dis- gional glacial response to climatic change continuity is the sharpest and has long been may not be simple. raines of the same ice lobe imply a retreat, followed by a readvance with a somewhat different form, but the distance of retreat is usually difficult to estimate. Stratigraphic evidence for subaerial weathering or erosion or for widespread proglacial outwash between drifts of successive ice advances is significant. when found, but such evidence is rare in the Great Lakes region. The most valuable type of evidence involves proglacial lakes and their drainage relations. The most famous example of such a situation involves the Two Creeks/Valders sequence for the Lake Michigan lobe, described below. But it should be mentioned that topographic conditions are not favorable in all cases for the development of proglacial lakes or for the preservation of their features. Recent studies focus attention on three breaks possibly of major importance, i.e., three ice advances after significant retreat. Although it is the retreats (interstadials) that are usually emphasized, it is the advances that are potentially more significant climatically, in that they imply a reversal in the general ice retreat that had been going on since the maximum 18.000 years ago. These three possible regional readvances of the ice occurred about 14,500. 13.000, and 11.500 years ago. The evidence for the regional character of each of these can he evaluated.
FIG.
3. Outline
of Wisconsin
RETREAT
OF
ST
CROIX
I.AUREN’I‘ILIE
ICE
ice lobes in Minnesota
III the eastern Great Lakes area, the 13.500-year ice advance was also preceded by a distant retreat. Here the evidence comes from relations north of Lake Erie. The Erie lobe had retreated far enough to allow the formation of an extensive glacial lake in which clay was deposited, and far enough to allow the lake to drain at a relatively low level to the east as well as to the west ( Dreimanis, 1969 ). The ice then readvanced almost 300 miles and built the Powell and younger moraines in northern Ohio out of the reworked lake beds. Farther east the 11,5OC-year advance cannot be traced so easily. -At least the magnitude of recession before this advance cannot be evaluated. Carbon dates indicate that in western New York the ice stood along the northern slope of the Appalachian Highlands south of Buffalo at about this time (Calkin, 1970). In New England it proha-
321
SHEE?
at times of their maximum
bly reached
the present
advances.
east coast near
Bos-
ton.
THE
13,000-YEAR ADVANCE
The 13.000-year advance of the Laurentide ice sheet left its most conspicuous record in the Michigan-Ontario area. The prominent Port Huron moraine of the Huron, Saginaw, and Lake Michigan lobes was formed at this time, as indicated by dates on the strandlines of its proglacial lakes. Recent studies in northern Michigan indicate that before the Port Huron advance the ice had receded enough to form a proglacial lake in which red clay was deposited, for drift dated there as about 13,000 years consists of red clayey till (Farrand et al., 1969). In the Erie basin. the ice had retreated to form Lake Arkona. which was temporarily drained eastward at a very low
322
H.
E.
WRIGHT,
level across the Niagara escarpment to the Lake Ontario basin (Dreimanis, 1969; Wall. 1968). The Erie lobe then readvanced as much as 100 miles. Realignment of the ice during readvance is suggested by the fact that the lithology and fabric of the Port Huron till differs from that of the older tills in the same area (Dreimanis, 1967 1. Farther east, the active ice front 13,000 years ago probably lay along the slopes rising southeast from the St. Lawrence valley to the mountains of Quebec and northern New England, although separate ice, largely stagnant, persisted in the mountains (McDonald, 1968). No distant recession preceded this stand. In the Minnesota area. evidence for a significant readvance of the ice 13,000 years ago is absent. The Des Moines lobe was in retreat from its 11,500-year maximum. Its recessional moraines are undistinguished and not crosscutting (Ruhe, 1969). The Superior lobe was confined to the Lake Superior basin. THE
11,500-YEAR ADVANCE
The Lake Michigan
Lobe
The third interruption in the general retreat of the Laurentide ice sheet is the Valders readvance, preceded by the Two Creeks interval. This famous succession is based on an exposure along the Lake Michigan coast of northeastern ‘\2’isconsin, where till is overlain by red lake sediments, at the top of which is a layer of organic detritus, including wood, dated here and at similar localities as about 11,550 years ago. The organic layer is capped by the Valders red clayey till. Apparently the Lake Michigan lobe retreated from its Port Huron position far enough to allow a proglacial lake to form and then drain eastward through the Straits of Mackinac. Readvance of the ice produced the Valders till, consisting of reworked lake beds. The distance of readvance
JR.
from the Straits of Mackinac to the southern terminus of Valders drift near Milwaukee is about 200 miles. The clarity of the stratigraphic relations and the magnitude of the shift in ice front has made the Two Creeks/Valders sequencejustifiably famous, and the type locality is now protected as a state park. The Two Creeks interstadial has been considered a key event in the history of Wisconsin glaciation, and in one popular classification the “Twocreekan” and “C’alderan” substagesare two of the three subdivisions recognized for the entire Main Wisconsin glaciation. Efforts have been made to trace this fluctuation east and west from the Lake Michigan lobe, and to find correlations throughout the continent and in fact the world. The regional and climatic significance of the Valders readvance has been recently challenged on a number of counts. ( 1 ) Radiocarbon dating in northeastern Wisconsin has shown that the Two Creeks interval ended about 11.850 years ago and thus is at least 850 years older than the famous Alleriid oscillation of northwestern Europe, with which it had generally been correlated (Broecker and Farrand, 1963j. (2) The distinctive red clayey till, originally assumed to be the hallmark of the I’alders advance in the western Great Lakes region, was also produced by earlier ice advances in northeastern Mimlesota (\Yright and Ruhe. 1965) , northern Wisconsin ( ljlack, 19691, and northern ;\Iichigan ( Farrand et al.. 1969). (3) Drainage of the Two Creeks lake through the Straits of Mackinac is incompatible with new carbon dates in the region, and an alternate route farther south through the Petoskey channel has been identified, thus requiring a less distant retreat before the Valders readvance (Farrant1 c,t ccl.,I %9 ) .
\Yest of the Lake Michigan and Green Bay lobes, red clayey till occurs around the
RETRLAT
OF
LAURENTIDE
fringes of the I,ake Superior basin, but it represents ice advances of the Superior lobe older than the \-alders phase of the Lake Michigan lobe, vi::., the Split Rock phase 16.000 years ago (Fig. 3) and the Nickerbon phase 12.000 years ago (Jl’right rt 01.. 1971). -4t the time of the \-alders phase the Superior lobe in Minnesota was confined to the Lake Superior basin, and thereafter various proglacial lakes formed in the basin. Meanwhile. a small bulge of the wasting Superior lobe expanded into Ontario just north of Minnesota, forming the Marks moraine, which. with the Dog Lake moraine of the adjacent Hudson Bay lobe, blocked the eastern outlet of Glacial Lake Agassiz. This outlet had temporarily lowered the lake level and produced the break between the sedintents of Agassiz I and Agassiz II. The stratigraphic connection between the two basins is made by a conspicuous red band in the :1gassiz IT clays, derived from outwash from the Ilarks moraine far to the east (Zoltai. 19h.i 1. The >farks moraine has been correlated with the 1-alders sequence (T
ICE
SHEET
32.3
much younger than \-alders, for wood from lx-e-Uuluth lake sediments is 10,220 years old ( M-359). The \Talders equivalent in the I,ake Superior basin may thus be unrecognizal)le, and there is some evidence that the ice front remained at or near the Nickerson moraine from 12.000 until perhaps 10.500 years ago ( U’asylikowa and Wright. 1970 j. Thus the secluence of moraines and other glacial features in Mimlesota and northwestern Ontario shows numerous ice-margin fluctuations, no one of which stands out as the obvious correlative of the Valders phase of the 1,nke Michigan lobe. Actually, the fact that one particular moraine or ice retreat can GLLIW the damming or drainage of :I progalcial lake is in large part the accident of unrelated topographical arrangements. Furthermore, the several movements of adjacent lobes in this region (Superior. Hudson IIav, and Patricia lobesj were not all synchronous (Zoltai, 196.5’) , implying a nonregional cause for ice-margin fluctuations (see below ) .
East of the Lake Michigan lobe the Valders drift border of 11,500 years ago is as difficult to trace as to the west. The red clayey till does not extend to the east, being diagnostic of drift originating ultimately in the bedrock of the Lake Superior basin. :1lso. the moraine forms are not strongly developed. Reliance must be placed largely on carbon dates. The low water level for Lake Michigan for Two Creeks time. necessar!: to explain the spread of the Two Creeks forest on the lake beds, requires drainage to the east, and thus similarly low levels for the eastern Great Lakes. Such low levels were postulatetl by Hough (19%). followetl by a rise again as ice advanced to close the eastern outlets in \‘alders time. But carbon dates now show that the ice did not readvance across the St. I,awrence \‘alley in Valders time, and that the lake levels never rose
324
H.
E.
WRIGHT,
again in the east (Hough, 1963 ; Dreimanis. 1964). The ice had stood at the Highland Front moraine on the south side of the St. Lawrence Valley 12,700 years ago, but marine waters invaded the vallev by 12,000 years ago hy rapid calving from the east, and the Champlain Sea had formed. Although the inception of the Champlain Sea coincided approximately in time with the Two Creeks interval of the Lake Michigan area (MCDonald, 196s ) , no glacial readvance equivalent to the ITalders can he recognized north of the valley.
Thus the Valders phase of the Lake Michigan lobe about 11,500 years ago involved an advance of almost 200 miles: yet in other lobes in the Great Lakes region a correlative ice advance is hardly recognizable. If the Valders phase reflected a regional climatic change, one would expect all lobes to have advanced more or less sychronously, and by comparable amounts. Of course the extent of marginal advance for a particular ice lohe and the timing of its maximum may be controlled in part by the distance from its margin to its accumulation area (and thus the lag in ice flow after increase in accumulation), but one would not expect such great discrepancies among adjacent ice lohes as are found in the Great Lakes region for the late Wisconsin (Wright, 1971). The lack of synchroneity prompts the suggestion that some of the ice-lobe activity may reflect local, nonclimatic factors. This leads to the hypothesis that some lobes may have been subject to surging, a mode of ilow described for dozens of mouritain glaciers and a few ice caps today (Ambrose, 1969) and postulated for Pleistocene ice sheets (IVeertman, 1966). The case for surging is strongest for the Superior lobe, because of the favorable morphology of the basin (Wright, 1969). The basin is deepest near the southwest
JR.
end, where a trough with 975 ft of water occurs close to the north shore near Silver Bay ( Fig. 4). A boring there shows ahnost 700 ft of sediment, and geoljhysical sounding indicates at least 300 ft more ( Farraintl. 1969). The Superior lobe, n-hen confined to the basin in the Split Rock phase, reached a lateral limit of at least 700 ft ahove the lake level near its terminus (Fig. 4 ) . The ice at this time over the deep trough was therefore at least 975 + 1000 + 700 ft thick (2675 ft ), and presumably thicker because of an asial gradient towards the terminus and a gradiellt
tcJ\Vard
the
lateid
margin
as
\vd.
:!t the southwest end of the basin the l)etlrock rises sharply and forms 3 divide IKtweet1 this basin and the llinneapolis basin to the southwest ; the elevation of the divide, which is near the town of Sandstone, is about 500 it above the lake level. \Z%ere the ice terminated at the divide, it was presumably only a few hundred feet thick. The ice thickness thus decreased abruptly from almost 300 ft to a thin edge over a distance of ahout 100 miles. Pollen and plant-macrofossil studies in northeastern Minnesota indicate that the vegetation of the region was tundra until at least 11.50 years ago (\Vright and Watts, 1969). Ijecause tundra today in broad areas of northern Canada is roughly coincident with continuous permafrost and with a mean annual atmospheric temperature of -S”C or below (Rowe, 1959; I
RETREAT
OF
I.AURENTIIlE
ICE
325
SITEEl
ssw
NNE
Lake Superior
basin
-
Q@ &ii Minneapolis 2000 -z E
ul
-_---
1000
dold
c90 I ,Cold
,$
I
a,
---_-_
-_---__
--__--_
lowland ice, frozen
ice
400
Miles FIG. 4. Longitudinal section of Superior lobe at time of Split Rock phase, as borings and postulated thermal relations. Field control for the surface slope for glaciation can be inferred from the slope of the Highland lateral moraine along the Superior. But for the Split Rock phase no such control is available, and the surface shown as much gentler, on the assumption that the ice tongue became distended the gentle surface slope, the great depth of the basin makes the thickness profile some more steeply sloping modern glaciers.
rcconstructetl from an earlier phase of north shore of Lake slope is therefore by surging. Despite similar to that of
rock is insufficiently permeable. Ice flow 1966), the search for a mechanism might continues from the source area to the mar- well focus on the thermal relations at the gin. facilitated by the water film that per- baseof the ice. mits lmal sliding. The thin toe of the ice, The last two recognizable advances of the however. being frozen to its bed, moves Superior lobe in Minnesota barely renchecl much more slowly, and it resists the ice the rock divide at the end of the basin pressure behind it like a dam. Eventually (Wright and Watts. 1969). They are both the pressure exceeds the strength of the represented 1)~red clayey till derived largely dam, and the front then advances rapidly in from overridden lake beds. The first of the a surge-a sequence of events similar to two, the Split Rock phase, left a thin and that postulated for the recent surge of part discontinuous sheet of till at elevations of an ice cap in Spitzbergen (Schytt, 1969). below about 1300 ft. The second, the NickAlthough the conditions or mechanisms for erson phase, produced a sharp and rugged surging are hardly esplained in a fully satis- moraine. These two advances may represent factory way (Ambrose, 1969) the appar- surges, with the Kickerson moraine perhaps ently common occurrence of surging in cer- being the product of multiple surges. The tain glaciers today implies some type of timing of the advances therefore may not be periodically unstable flow conditions. Inas- related to any particular climatic event. much as flow in temperate glaciers is con- I3oth of them predated the Valtlers advance trolled largely hy basal sliding (~Veertman. of the Lake 1’lichigan lohe.
326
1%.
E.
WRIGHT,
Surging in tllr Lake Mic-lriyun-G~rrn KU>! Lobr Other basins in the Great Lakes region do not have a nlorphometry so favorable for surging as the Lake Superior basin, but the posibilities must he considered, especially in cases where the advance of adjacent ice lobes are thought to be nonsynchronous. The uniquely great advance of the Lake Michigan and Green Bay lobes in the \‘alers phase may be such a case, ad certain special features of the basin morphometry may in fact support this hypothesis. The ice for the Lake Michigan and Green Bny lolm came out of the eastern portion of
JR.
the I .ake Superior lmsin. This portion of the lake has a general depth of only a few hundred feet, hut the bottom is marked 1)~ about n dozen subpxxllel north-south chnnnels that branch and rejoin ( Fig. 5 ). The channels are as much as I SO miles long, 2 miles wide, and 1000 it deep and are cut into bedrock. The cover of glacial sediments in this part of the basin is generally less than 100 it thick (Farrand, 1969). The pattern has every resemblance to gorges eroded hy high-velocity streams. The channels are tentatively attributed to preglacial stream cutting by Farrand (1969), hut this explanatim fails to nccowt for the nhupt tenni-
FIG. 5. Bottom topography of eastern 1,ake Superior hasin, showing pattern of submerged gorges, here interpreted as tunnel valleys cut into bedrock. Note the abrupt termination of the pattern against the south shore, which is underlain by bedrock at shallow depth. Distance across the lake from north to south through Michipicoten Island is about 80 miles. From Farrand (1969).
RETREAT
OF
LAURENTIDE
nation of all of them against a bedrock shelf at the south shore of the lake. An alternate explanation relates them to tunnel valleys produced by subglacial streams. Tunnel valleys are believed to form during the earlv stages of wastage of ice lobes under thermal conditions that provide abundant water by basal melting or (if the glacier is temperate up to the surface) by the downward penetration of surficial meltwater (Wright, 1971). If the water is sufficient in volume, it could be concentrated into streams beneath the ice rather than simply move as a basal film. The hydrostatic pressure of the still-active ice lobe provides the subglacial streams with the velocity necessary (1) to transport all the detritus supplied by the ice, (2 j to erode its bed, thereby cutting the tunnel valley deep into bedrock, and (‘3’) to flow uphill if the morphometry of the subglacial basin demands it. Excellent examples of tunnel valleys can be found in eastern Minnesota, where they were formed beneath the Superior lobe in the St. Croix phase (Wright, 1971). Here there are about a dozen subparallel channels, as much as 100 miles long and 200 ft deep. Most of them contain eskers, formed during later stages of wastage when the ice was thin and stagnant. Tunnel valleys indicate abundant subglacial water, the same condition favorable for rapid basal sliding of the ice, i.e., surging. In the case of the Superior lobe in its St. Croix phase, surging may not have been possible because the frozen ice toe formed too thick an ice dam. But when the ice withdrew into the deep Lake Superior basin, the water could be trapped and could build up its basal film until rapid basal gliding was possible, Some water must have escaped. becausehuge eskers and fans formed locally at the ice edge. In the eastern Lake Superior basin, the buildup of basal water may have been enough not only to cut tunnel valleys but, at a different time, to permit surging of the
ICE
327
SHEET
Lake Michigan-Green Bay lobe. The relative age of the two events cannot be determined. Perhaps the postulated 200-mile Valders surge of the ice came first after a long accumulation of basal water. The expanded lobe then wasted rapidly, especially with active calving into proglacial lakes. Basal meltwater could build up again in the eastern Lake Superior basin, but to greater amounts because the steadily warming lateWisconsin climate permitted penetration of surficial meltwater to the base. The basal water could then be channeled to produce the tunnel valleys, and it could escapeat the front along the south shore, building the great sandplainsnorth of Lake Michigan. This hypothetical sequenceof events is of course highly speculative, but it has enough evidence to undermine the assumption that the major ice advance of one lobe is accompanied by a major advance of adjacent lobes. The case is strongest for the Superior and Lake Michigan-Green Bay lobes in late Wisconsin time. Whether earlier advances of these or other ice lobes in the Great Lakes region might reflect similar nonclimatic events cannot be determined at present. Evidence for general 14.500- and 13,000-year advances across at least part of the Great Lakes region implies some regional controls, but the correlation and dating are not sufficiently refined to be certain. The best confirmatory evidence might come from independent paleoclimatic studies, such as pollen analysis. So far one climatic reversal has been suggested for the pollen sequence in Minnesota and Wisconsin, but it is not reflected in all diagrams, it is not we11 dated, and it is not uniformly interpreted (Cushing. 1967 ; Wright, 1968). Post-17alders
Glacial
Events
The glacial history of the Laurentide ice sheet did not end with the Valders advance of the Lake Michigan-Green Bay lobe. But the subsequent events are not well recorded or well studied north of the Great Lakes. A
328
H.
E.
WRIGHT,
sequence of moraines in northwestern Ontario has been mapped by Zoltai (1965), bringing the record there up to about 9000 years ago, and relating it to the history of Lake Agassiz and its eastern outlets to Lake Superior. But eastward from here the terrain is marked by no distinctive moraines. Once the height of land was crossed by the retreating ice front, proglacial Lake Barlow-Ojibway covered the terrain with clay, and the principal distinctive feature is the Cochrane till, which is composed of lake clays deformed by readvancing ice. This feature is dated as about 8000 years ago, and it has long been used to indicate a significant break in the climatic sequence. In fact, one classification (Frye et nl., 1968) places the end of the Valderan substage and thus of the Wisconsinan Glaciation at the Cochrane moraine. Hughes (1965)) however, considers the Cochrane readvance to be of minor and local importance, and Prest (1970) attributes it to a surge of ice from the west side of Hudson Bay, where the sea had invaded through the Hudson Straits and provided the excess water. In any case, it is clear from the pollen sequence that the vegetation and thus the climate in southern Canada after 10,1>00 years ago was much like that of today (Terasmae, 1960) and the 3000 years of ice wastage that followed, until Hudson Bay was cleared, was merely a result of the long time required to melt the huge ice mass, even under warm climatic conditions. CONCLUSIONS Review of the chronology of late-wisconsin moraines and other glacial features at the margin of the Laurentide ice sheet indicates that the ice front experienced many fluctuations. The deep lobations in the Great Lakes region provide the opportunity to examine the synchroneity of fluctuations from lobe to lobe. Ice-margin fluctuations are generally assumed to represent climatic changes, espe-
JR.
cially when evidence exists for distant retreat of the ice front before readvance. The case for regional climatic control is strengthened when moraines can be traced from lobe to lobe across a broad area. In the late-Wisconsin history of the Great Lakes region, the geomorphic and stratigraphic relations, along with radiocarbon dating, point to cases of both synchroneity and nonsynchroneity. The ice advance at about 14,500 years ago is identifiable from the Des Moines lobe of Minnesota in the west (and probably the James lobe still farther west) to the Erie-Ontario lobe in the east, as an event that followed ice retreat of almost 100 miles, although in some areas, such as eastern Minnesota, all ice lobes did not advance an equal amount. The 13,0OU-year advance is manifested especially around the Lake Michigan and Huron basins as the Port Huron moraine; it also was preceded by a distant ice retreat, but its representation east and west from these basins is weak. The 11.500-year Valders advance of the Lake Michigan-Green Bay lobe occurred after very distant retreat, but the advance is confined to this double lobe, which emerged from the eastern part of the Lake Superior basin. The relations may be explained by postulating a rapid surge of the ice after long accumulation of water at the base of the ice. The case for surging is even stronger for the Superior lobe for earlier advances, because of the especially favorable morphometry of the basin. The key to some ice-lobe advances may therefore be in the morphometric. thermal, and hydrologic relations as much as in the climatic. In deep basins the ice-lobe morphometry can sometimes be estimated from the elevations of lateral moraines, along with borings from the center; hydrologic relations can be inferred from the presence of subglacial tunnel valleys and proglacial outwash bodies; and at least some of the thermal properties of the ice can be postulated from knowledge of
RETREAT OF LAURENTIDE the contemporaneous periglacial vegetation through pollen analysis. l+‘hat is required for further development of a surge hypothesis is field evidence from drift morphology and structure, along with a comparison with data from glaciers that are known to have surged in recent decades. REFERENCES J. W. (Ed.) (1969). Seminar on the causes and mechanics of glacier surges, and symposium on surging glaciers. Canadian Jourxal of Earth Scicr~ccs 6, 807-1018. BLACK, R. F. (1969). Valderan glaciation in western upper Michigan. Proceedings of the 12th AMBROSE,
Conferemc of the Illtematioxal Great Lakes Resrurch, Am
Association r2rbor 1969,
for
pp.
116-123. BROECKER, W. S., and FARRAND, W. R. (1963). Radiocarbon age of the Two Creeks forest bed, Wisconsin. Geological Society of .4ruerica Btdletilt
74, 795-802.
BROWN, R. J. E. (1967). Permafrost in Canada. Geological Survey of Canada Map 1246A. CALKIN, P. E. (1970). Strand lines and chronology of the glacial Great Lakes in northwestern New York. Ohio Jowxal of Science 70, 78-96. CASHING, E. J. (1967). Lake-Wisconsin pollen stratigraphy and the glacial sequence in Minnesota. In “Quaternary Paleoecology” (E. J. Cushing and H. E. Wright, Jr., eds.), pp. 59-88. Yale University Press, New Haven. DREIMANIS, ALEKSIS (1964). Lake Warren and the Two Creeks interval. Joumnl of Geology 72, 247-250. DREIMANIS, ALEKSIS (1967). Cary-Port Huron interstade in eastern North America and its correlatives. Geological Society of America, Northwestern Section, Second Annual Meetings (Boston, 1967) I Abstract. DREIMANIS, ALEICSIS (1969). Late-Pleistocene lakes in the Ontario and Erie basins. Proceediugs of the 12th Confrrolce of the Iutemational Association for Great Lakes Rcscarch, Amt A1-bor 1969 pp. 170-180.
ELSON, J. A. (1967). Geology of glacial Lake Agassiz. Ilt “Life, Land and Water,” Conference on Environmental Studies of the Glacial Lake Agassiz Region (W. J. Mayer-Oakes, ed.), Proceedings, pp. 37-96. University of Manitoba Press, Winnipeg. FARRAND, W. R. (1969). The Quaternary history of Lake Superior. Proceedings of the 12th Con-
ICE SHEET
329
ference of the Illterlcatiorwl Lakes Research, .4nn Arbor
Association for Great 1969, pp. 181-197.
FAKRAND, W. R., ZAHNER, R., and BENNINGHOFF, W. S. (1969). Cary-Port Huron interstade : Evidence from a buried bryophyte bed, Cheboygan County, Michigan. Geological Society of -4werica
Special
Proper
123,
249-258.
FLINT, K. F., COLTON, R. B., GOLDTHWAIT. R. P., and WILLMAN, H. B. (1959). Glacial map of the United States east of the Rocky Mountains. Geological Society of Plmerica, Boulder, Colorado. FRYE, J. C., and WILLMAN, H. B., (1960). Classification of the Wisconsinan Stage in the Lake Michigan glacial lobe. Illimis State Geological Survey
Circular
285.
FRYE, J. C., WILL~IAN, H. B., and BLACK, R. F. (1965). Outline of glacial geology of Illinois and Wisconsin. I+t “The Quaternary of the United States” (H. E. Wright and D. G. Frey, eds.), pp. 43-61. Princeton University Press, Princeton, New Jersey. FRYE, J. C., WILLMAN, H. B., RUBIN, M., and BLACK, R. F. (1968). Definition of the Wisconsinan Stage. U.S. Geological Suwey Bulletin 1274-E, El-E22. GOLDTHWAIT, R. P., DREIMANIS, A., FORSYTH, J. L., KARNOW, P. F., and WNITE, G. F. (1965). Pleistocene deposits of the Erie lobe. 112 “The Quaternary of the United States” (H. E. Wright, Jr. and D. G. Frey, eds.), pp. 85-97. Princeton University Press, Princeton, New Jersey. Hou(;H, J. L. (1958). “Geology of the Great Lakes.” University of Illinois Press, Urbana, Illinois. HOEGFI, J. L. (1963). The prehistoric Great Lakes of North America. .4mevican Scientist 51, W-109. HUGHES, 0. L. (1965). Surficial geology of part of the Cochrane District, Ontario, Canada. IN International Studies on the Quaternary” (H. E. Wright, Jr. and D. G. Frey, eds.), pp. 535-565. Gcologicrrl Society of American Special Paper 84. M’CDONALI), B. C. (1968). Deglaciation and differential postglacial rebound in the Appalachian region of southeastern Quebec. Jowml of Geology 76, 664-677. PREST, V. K. (1969). Retreat of Wisconsin and recent ice in North America. Geoloqical Survey of Canada Map 125’?A, scale 1 :5,000,000. PREST, V. K. (1970). Quaternary geology of Canada. 112 “Geology and Economic Minerals of Canada,” pp. 676764. Canada Department of Energy, Mines & Resources, Ottawa.
H. E. WRIGHT,
330 ROWE,
J. C. (1959).
Canada Department Resources, Forestry
Forest regions Northern
Affairs
of Canada. Natural
Brawct, Bulletin 123. R. V. (1969). “Quaternary Landscapes in Iowa.” Iowa State University Press, Ames, Iowa. SCHYTP, VALTER (1969). Some comments on glacier surges in eastern Svalbard. In “Seminar on the Causes and Mechanics of Glacier Surges, and Symposium on Surging Glaciers” (J. W. Ambrose, ed.), pp. 867-874. Capkadian Journal of Earth Sciences 5, 807-1018. TERASMAE, JAAN (1960). A palynological study of post-glacial deposits in the St. Lawrence lowlands, Geological Survey of Canada Bulletin 56, RUHE,
l-22.
R. E. (1968). A sub-bottom reflection survey in the central basin of Lake Erie. Geological Society of America Bdletin 79, 91-106. WASYLIKOWA, KRYSTYNA, and WRIGHT, H. E., JR. (1969). Late-glacial plant succession on an abandoned drainageway, northeastern Minnesota. Krakow, Acta Palaeobotanica 11, 23-43. WAYNE, W. J., and ZUMBERGE, J. H. (1965). Pleistocene geology of Indiana and Michigan. In “The Quaternary of the United States” (H. E. Wright, Jr. and D. G. Frey, eds.), pp. 63-82. Princeton University Press, Princeton, New Jersey. WEERTMAN, J. (1966). Effect of a basal water layer on the dimensions of ice sheets. Journal of Glaciology 6, 191-208. WILLMAN, H. B., and FRYE, J. C. (1970). Pleistocene stratigraphy of Illinois. Illinois State Geological Survey Bdletin 64, 204 pp. WALL,
JR.
H. E., JR. (1968). The roles of pine spruce in the forest history of Minnesota and adjacent areas. Ecology 49, 937-955. WRIGHT, H. E., JR. (1969). Glacial fluctuations and the forest succession in the Lake Superior region. Proceedings of flu 12th Coltference of the Internatiorcal dssociation for Great Lakes Research, Am Arbor 1969, pp. 397-405. WRIGHT, H. E., JR. (1971). Tunnel valleys, glacial surges, and the subglacial hydrology of the Superior lobe, Minnesota. Geological Society of America Special Paper, in press. WRIGHT, H. E., JR., MATSCH, C. L., and GUSHING, E. J. (1971). The Superior and Des Moines lobes. Geological Society of ,4wterica Special Paper, in press. WRIGHT, H. E., JR., and RUHE, R. V. (1965). Glaciation of Minnesota and Iowa. In “The Quaternary of the United States” (H. E. Wright, Jr. and D. G. Frey, eds.), pp. 29-41. Princeton University Press, Princeton, New Jersey. WRIGHT, H. E., JR., and WATTS, W. A. (with contributions by Saskia Jelgersman, Jean C. B. Waddington, T. C. Winter, and Junko Ogawa) (1969). Glacial and vegetational history of northeastern Minnesota. Minnesota Geological Survey Special Publication 11, 59 pp. ZOLTAI, S. C. (1965). Glacial features of the Quetico-Nipigon area. Calbadian Journal of Earth Sciences 2, 247-269. ZUMBERGE, J. H. (l%O) . Correlation of Wisconsin drifts in Illinois, Indiana, Michigan, and Ohio. Geological Sociefy of A4werica Balletin 71, 11771188. WRIGHT,
RESEARCH
Retreat
1,
316-330
of
the
(1971)
Laurentide 9000
Ice Years
Sheet
From
14,000
to
Ago
H. E. WRIGHT,
JR.’
The intricate pattern of moraines of the Laurentide ice sheet in the Great Lakes region reflects the marked lobation of the ice margin in late Wisconsin time, and this in turn reflects the distribution of steam-cut lowlands etched in preglacial times in the weak-rock belts of gentle Paleozoic fold structures. It is difficult to trace and correlate moraines from lobe to lobe and to evaluate the magnitude of recession before readvance, but three breaks stand out in the sequence, with readvances at about 14,500, 13,000, and 11,500 years ago. The first, co’rresponding to the Cary advance of the Lake Michigan lobe, is represented to the west by distant advance of the Des Moines lobe in Iowa, and to the east by the overriding o,f lake beds by the Erie lobe. The 13,000-year advance is best represented by the Port Huron moraine of the Lake Michigan and Huron lobes, but by relatively little action to west and east. The ll,SOOyear advance is based on the Valders till of the Lake Michigan lobe, but presumed correlations to east and west prove to be generally older, and the question is raised that these and some other ice advances in the Great Lakes region may represent surges of the ice rather than regional climatic change. Surging may involve the buildup of subglacial meltwater, which can provide the basal sliding necessary for rapid forward movement. It would be most favored by the conditions in the western Lake Superior basin, where the Superior lobe had a suitable form and thermal regime, as estimated from geomorphic and paleoclimatic criteria. The Valders advance of the Lake Michigan and Green Bay lobes may also have resulted from a surge: the eastern part of the Lake Superior basin, whence the ice advanced, has a pattern of deep gorges that resemble subglacial tunnel valleys, which imply great quantities of subglacial water that may have produced glacial surges before the water became channeled.
INTRODUCTION
fectetl by irregularly distributed riiountain masses. and in many areas there the frontal moraines are poorly developed or at best are inconspicuous among the features of bedrock relief. But in the intervening area, between the Missouri River in South Dakota and the Ohio River in Ohio, which are both basically frontal streams, the ice margin was prominently lobate, with each lobe being localized by a previously existing lowland. These lowlands, now mostly occupied by the Great Lakes, were initially eroded by streams in relatively nonresistant sedimentary rocks, which in turn are distributed according to the gentle fold structures of the
The front of the Laurentide ice sheet during its Main Wisconsin maximum stretched from the Rocky Mountain Piedmont to the New England coast, with a deep penetration to the south in the Great Lakes region and the Mississippi Valley area-the major topographic low of the continent. In Montana and the western Dakotas the front was reasonably straight, for the northern Great Plains provided few lowlands to encourage irregular frontal advance. In New York and New England the front was severely af1 Limnological Research Center, Minnesota, Minneapolis, Minnesota
University 55455.
of
continental 316
interior
( Fig.
1 ‘\ . Lake
Ontario
RETREAT
OF
LAURENTIDE
ICE
SHEET
GEOLOGIC MAP OF THE
/l/\/i,.
‘1X/\/
\,
GREAT
FIG.
graphic
1. Geologic map of the Great Lakes region, showing the localization units in the Paleozoic bedrock. From Hough (1958).
and Lake Erie are localized respectively by Ordovician and Devonian shales at the north edge of the Allegheny structural basin, Lake Huron and Lake Michigan by Devonian shales on opposite sides of the Michigan structural basin, and Lake Superior by late-Precambrian red sandstone and shale in the center of the Superior syncline. The pattern continues westward with the Red River Valley and the James River valley on two different units of Cretaceous
LAKES
REGION
of the lake
basins
’ ’
by strati-
shale on the eastern side of the Great Plains syncline. During the time of maximum Wisconsin ice advance, the Great Lakes basins were completely inundated by ice, and the ice margin to the south was relatively regular as it reached its ultimate termination on the slightly differentiated low uplands that extend from centra1 Ohio to Illinois (Fig. 2). .\ctually the steady flow lines of thick ice across or along the Great Lakes basins at
318
RETREAT
OF
LAtiRENTIDE
this time may have resulted in appreciable deepening of these basins by glacial erosion, for the basin floors now extend below sea level, whereas in preglacial time they sloped to the sea. At any rate, when the ice sheet thinned and began its retreat, its frontal form became much more influenced by the major features of the subglacial topography (Fig. 2 ) Many distinct lobes developed, not only in the major Great Lakes basins but also in subsidiary weak-rock belts as well, such as the Green Hay lobe in eastern Wisconsin on Cambrian sandstones, and the Chippewa and other small lobes in different Precambrian rocks in northern Wisconsin. .\s the ice became still thinner, the lobation was locally controlled in part by ridges rep resenting the massive moraines of earlier phases of ice advance-moraines that were probably still cored by stagnant ice and thus even higher than they are today. A good example of this moraine control is in Minnesota, where one margin of the Grantsburg sublobe of the Des Moines lobe, for example, lapped onto the inner margin of the previously formed St. Croix Moraine of the Superior lobe along a distance of 70 miles. This extreme digitation of the margin of the ice sheet resulted in the formation of moraines, loop-like recording successive stages of ice retreat (Fig. 2). In cases the moraines are fairly uniform in size and spacing, and they appear to represent periodic still-stands of the ice, or even slight readvances. In the Lake Michigan lobe, for example, 30 mapped moraines cover a time range of 8000 years (Frye it ab., 1965) and in Ohio at least 12 moraines were formed in 1000 years (Goldthwait ct al., 1965). The implied periodicity is therefore a very few hundred years. Although the tills of successive moraines of the same ice lobe can rarely be distinguished lithologically as stratigraphic units (Willman and Frye, 1970)) because of steady ice flow from the same source area, the moraines themselves give
ICE
SEIEET
319
adequate indication of a fluctuating ice margin. Synchroneity of moraines from lobe to lobe is difficult to establish, however, especially when lobes are attenuated and the interlobate junction is topographically confused. In cases, time relations can be worked out from outwash deposits or proglacial lake features common to adjacent lobes. But generally dependence has been put on moraine trends, and the results are often unsatisfactory. Because of this, efforts h ave been made to identify major geomorphic or stratigraphic breaks in the sequence, so that the long time involved might be separated into stadial and interstadial units that may be of importance in continental or inercontinental climatic correlations. Many subdivisions have been made in the almost 100 years since Chamberlain first emphasized that the U’isconsin glaciation was not a simple event-subdivisions ranging from the Early-Middle-Late categorization of Leverett to the series of many named stadial and interstadial intervals of Leighton. One of the more recent classifications for the Lake Michigan lobe recognizes only two stadial subdivisions : the Valderan, at the end of the series, and the Woodfordian, which includes all the rest (Frye and \,Villman, 1960). The powerful tool of radiocarbon dating provides a continuing incentive to search for correlation and significant subdivisions, and newly refined techniques of pollen analysis make possible more accurate stratigraphic approaches to climatic reconstructions that may permit an evaluation of the climatic significance of ice-margin fluctuations. A break in the retreatal sequence might be considered of major climatic significance if the ice front readvanced a great distance and if the readvance can be shown to be synchronous for several ice lobes. Geologic evidence for distant retreat and readvance is generally difticult to find. Crosscutting mo-
320
II.
E.
WRIGHT,
JR.
recognized as a basis for subdividing the successionof moraines. The crosscutting implies a realignment of the Lake Michigan lobe before readvance. hut in this case the distance of retreat cannot be accurately estimated. The latest revision of the nomenclature in Illinois (Willman and Frye, 19701) recogntzes a stratigraphic distinction at a slightly younger position than that implied by the major topographic discontinuity. A radiocarbon date for a correlative moraine of the Frie lobe is 14.300 years ago (W198) , so the Cary ice advance can be estimated at about l-t,50 years ago. The magnitude of the 11,500-year advance is better documented in Minnesota. although the situation is complicated because two adjacent ice lobes did not advance a similar amount (Wright and others, 1971). The Superior lobe, advancing from the northeast. had produced the massive St. Croix moraine in southeastern Minnesota at an earlier time, and it retreated back into the Lake Superior basin at least 80 miles from the moraine, leaving abundant stagnant ice behind (Fig. 3). It reaclvanced a very short distance about 16,000 years ago, hut at this time the Des Moines Lobe advanced from the Red River Valley along the Minnesota River i’alley and into the Minneapolis lowland in the form of the Grantsburg strhlohe. occupying part of the area previously covered by the Superior lobe. THE 14.500-YEAR Continued expansion of the ice caused it to ADVANCE overflow a low divide on the Iowa The 14.500-year ice advance produced the -Minnesota border and spill down the Des Cary group of moraines in northern Illinois, Moines River valley to central Iowa as the which can be traced with only minor diffimain Des Moines lobe proper, reaching its culties to the Saginaw and Erie lobes to the terminus about 14.000 years ago (Ruhe. east (Zumberge, l%O; Wayne and Zum- 1969; Wright rt 171.. 1971). This great aclberge, 1965). The moraines sharply crosscut Vance of the Grantsburg sublobe after the several moraines of the previously formed distant retreat of the Superior lobe implies a Tazewell group (Fig. 2). Although the mo- major break in the climatic sequence, alraine sequencein Illinois reveals other cross- though the change in locus of major ice-lobe cutting relations (Frye and Willman, advance introduces suspicion that the re1960), the Tazewell/Cary topographic dis- gional glacial response to climatic change continuity is the sharpest and has long been may not be simple. raines of the same ice lobe imply a retreat, followed by a readvance with a somewhat different form, but the distance of retreat is usually difficult to estimate. Stratigraphic evidence for subaerial weathering or erosion or for widespread proglacial outwash between drifts of successive ice advances is significant. when found, but such evidence is rare in the Great Lakes region. The most valuable type of evidence involves proglacial lakes and their drainage relations. The most famous example of such a situation involves the Two Creeks/Valders sequence for the Lake Michigan lobe, described below. But it should be mentioned that topographic conditions are not favorable in all cases for the development of proglacial lakes or for the preservation of their features. Recent studies focus attention on three breaks possibly of major importance, i.e., three ice advances after significant retreat. Although it is the retreats (interstadials) that are usually emphasized, it is the advances that are potentially more significant climatically, in that they imply a reversal in the general ice retreat that had been going on since the maximum 18.000 years ago. These three possible regional readvances of the ice occurred about 14,500. 13.000, and 11.500 years ago. The evidence for the regional character of each of these can he evaluated.
FIG.
3. Outline
of Wisconsin
RETREAT
OF
ST
CROIX
I.AUREN’I‘ILIE
ICE
ice lobes in Minnesota
III the eastern Great Lakes area, the 13.500-year ice advance was also preceded by a distant retreat. Here the evidence comes from relations north of Lake Erie. The Erie lobe had retreated far enough to allow the formation of an extensive glacial lake in which clay was deposited, and far enough to allow the lake to drain at a relatively low level to the east as well as to the west ( Dreimanis, 1969 ). The ice then readvanced almost 300 miles and built the Powell and younger moraines in northern Ohio out of the reworked lake beds. Farther east the 11,5OC-year advance cannot be traced so easily. -At least the magnitude of recession before this advance cannot be evaluated. Carbon dates indicate that in western New York the ice stood along the northern slope of the Appalachian Highlands south of Buffalo at about this time (Calkin, 1970). In New England it proha-
321
SHEE?
at times of their maximum
bly reached
the present
advances.
east coast near
Bos-
ton.
THE
13,000-YEAR ADVANCE
The 13.000-year advance of the Laurentide ice sheet left its most conspicuous record in the Michigan-Ontario area. The prominent Port Huron moraine of the Huron, Saginaw, and Lake Michigan lobes was formed at this time, as indicated by dates on the strandlines of its proglacial lakes. Recent studies in northern Michigan indicate that before the Port Huron advance the ice had receded enough to form a proglacial lake in which red clay was deposited, for drift dated there as about 13,000 years consists of red clayey till (Farrand et al., 1969). In the Erie basin. the ice had retreated to form Lake Arkona. which was temporarily drained eastward at a very low
322
H.
E.
WRIGHT,
level across the Niagara escarpment to the Lake Ontario basin (Dreimanis, 1969; Wall. 1968). The Erie lobe then readvanced as much as 100 miles. Realignment of the ice during readvance is suggested by the fact that the lithology and fabric of the Port Huron till differs from that of the older tills in the same area (Dreimanis, 1967 1. Farther east, the active ice front 13,000 years ago probably lay along the slopes rising southeast from the St. Lawrence valley to the mountains of Quebec and northern New England, although separate ice, largely stagnant, persisted in the mountains (McDonald, 1968). No distant recession preceded this stand. In the Minnesota area. evidence for a significant readvance of the ice 13,000 years ago is absent. The Des Moines lobe was in retreat from its 11,500-year maximum. Its recessional moraines are undistinguished and not crosscutting (Ruhe, 1969). The Superior lobe was confined to the Lake Superior basin. THE
11,500-YEAR ADVANCE
The Lake Michigan
Lobe
The third interruption in the general retreat of the Laurentide ice sheet is the Valders readvance, preceded by the Two Creeks interval. This famous succession is based on an exposure along the Lake Michigan coast of northeastern ‘\2’isconsin, where till is overlain by red lake sediments, at the top of which is a layer of organic detritus, including wood, dated here and at similar localities as about 11,550 years ago. The organic layer is capped by the Valders red clayey till. Apparently the Lake Michigan lobe retreated from its Port Huron position far enough to allow a proglacial lake to form and then drain eastward through the Straits of Mackinac. Readvance of the ice produced the Valders till, consisting of reworked lake beds. The distance of readvance
JR.
from the Straits of Mackinac to the southern terminus of Valders drift near Milwaukee is about 200 miles. The clarity of the stratigraphic relations and the magnitude of the shift in ice front has made the Two Creeks/Valders sequencejustifiably famous, and the type locality is now protected as a state park. The Two Creeks interstadial has been considered a key event in the history of Wisconsin glaciation, and in one popular classification the “Twocreekan” and “C’alderan” substagesare two of the three subdivisions recognized for the entire Main Wisconsin glaciation. Efforts have been made to trace this fluctuation east and west from the Lake Michigan lobe, and to find correlations throughout the continent and in fact the world. The regional and climatic significance of the Valders readvance has been recently challenged on a number of counts. ( 1 ) Radiocarbon dating in northeastern Wisconsin has shown that the Two Creeks interval ended about 11.850 years ago and thus is at least 850 years older than the famous Alleriid oscillation of northwestern Europe, with which it had generally been correlated (Broecker and Farrand, 1963j. (2) The distinctive red clayey till, originally assumed to be the hallmark of the I’alders advance in the western Great Lakes region, was also produced by earlier ice advances in northeastern Mimlesota (\Yright and Ruhe. 1965) , northern Wisconsin ( ljlack, 19691, and northern ;\Iichigan ( Farrand et al.. 1969). (3) Drainage of the Two Creeks lake through the Straits of Mackinac is incompatible with new carbon dates in the region, and an alternate route farther south through the Petoskey channel has been identified, thus requiring a less distant retreat before the Valders readvance (Farrant1 c,t ccl.,I %9 ) .
\Yest of the Lake Michigan and Green Bay lobes, red clayey till occurs around the
RETRLAT
OF
LAURENTIDE
fringes of the I,ake Superior basin, but it represents ice advances of the Superior lobe older than the \-alders phase of the Lake Michigan lobe, vi::., the Split Rock phase 16.000 years ago (Fig. 3) and the Nickerbon phase 12.000 years ago (Jl’right rt 01.. 1971). -4t the time of the \-alders phase the Superior lobe in Minnesota was confined to the Lake Superior basin, and thereafter various proglacial lakes formed in the basin. Meanwhile. a small bulge of the wasting Superior lobe expanded into Ontario just north of Minnesota, forming the Marks moraine, which. with the Dog Lake moraine of the adjacent Hudson Bay lobe, blocked the eastern outlet of Glacial Lake Agassiz. This outlet had temporarily lowered the lake level and produced the break between the sedintents of Agassiz I and Agassiz II. The stratigraphic connection between the two basins is made by a conspicuous red band in the :1gassiz IT clays, derived from outwash from the Ilarks moraine far to the east (Zoltai. 19h.i 1. The >farks moraine has been correlated with the 1-alders sequence (T
ICE
SHEET
32.3
much younger than \-alders, for wood from lx-e-Uuluth lake sediments is 10,220 years old ( M-359). The \Talders equivalent in the I,ake Superior basin may thus be unrecognizal)le, and there is some evidence that the ice front remained at or near the Nickerson moraine from 12.000 until perhaps 10.500 years ago ( U’asylikowa and Wright. 1970 j. Thus the secluence of moraines and other glacial features in Mimlesota and northwestern Ontario shows numerous ice-margin fluctuations, no one of which stands out as the obvious correlative of the Valders phase of the 1,nke Michigan lobe. Actually, the fact that one particular moraine or ice retreat can GLLIW the damming or drainage of :I progalcial lake is in large part the accident of unrelated topographical arrangements. Furthermore, the several movements of adjacent lobes in this region (Superior. Hudson IIav, and Patricia lobesj were not all synchronous (Zoltai, 196.5’) , implying a nonregional cause for ice-margin fluctuations (see below ) .
East of the Lake Michigan lobe the Valders drift border of 11,500 years ago is as difficult to trace as to the west. The red clayey till does not extend to the east, being diagnostic of drift originating ultimately in the bedrock of the Lake Superior basin. :1lso. the moraine forms are not strongly developed. Reliance must be placed largely on carbon dates. The low water level for Lake Michigan for Two Creeks time. necessar!: to explain the spread of the Two Creeks forest on the lake beds, requires drainage to the east, and thus similarly low levels for the eastern Great Lakes. Such low levels were postulatetl by Hough (19%). followetl by a rise again as ice advanced to close the eastern outlets in \‘alders time. But carbon dates now show that the ice did not readvance across the St. I,awrence \‘alley in Valders time, and that the lake levels never rose
324
H.
E.
WRIGHT,
again in the east (Hough, 1963 ; Dreimanis. 1964). The ice had stood at the Highland Front moraine on the south side of the St. Lawrence Valley 12,700 years ago, but marine waters invaded the vallev by 12,000 years ago hy rapid calving from the east, and the Champlain Sea had formed. Although the inception of the Champlain Sea coincided approximately in time with the Two Creeks interval of the Lake Michigan area (MCDonald, 196s ) , no glacial readvance equivalent to the ITalders can he recognized north of the valley.
Thus the Valders phase of the Lake Michigan lobe about 11,500 years ago involved an advance of almost 200 miles: yet in other lobes in the Great Lakes region a correlative ice advance is hardly recognizable. If the Valders phase reflected a regional climatic change, one would expect all lobes to have advanced more or less sychronously, and by comparable amounts. Of course the extent of marginal advance for a particular ice lohe and the timing of its maximum may be controlled in part by the distance from its margin to its accumulation area (and thus the lag in ice flow after increase in accumulation), but one would not expect such great discrepancies among adjacent ice lohes as are found in the Great Lakes region for the late Wisconsin (Wright, 1971). The lack of synchroneity prompts the suggestion that some of the ice-lobe activity may reflect local, nonclimatic factors. This leads to the hypothesis that some lobes may have been subject to surging, a mode of ilow described for dozens of mouritain glaciers and a few ice caps today (Ambrose, 1969) and postulated for Pleistocene ice sheets (IVeertman, 1966). The case for surging is strongest for the Superior lobe, because of the favorable morphology of the basin (Wright, 1969). The basin is deepest near the southwest
JR.
end, where a trough with 975 ft of water occurs close to the north shore near Silver Bay ( Fig. 4). A boring there shows ahnost 700 ft of sediment, and geoljhysical sounding indicates at least 300 ft more ( Farraintl. 1969). The Superior lobe, n-hen confined to the basin in the Split Rock phase, reached a lateral limit of at least 700 ft ahove the lake level near its terminus (Fig. 4 ) . The ice at this time over the deep trough was therefore at least 975 + 1000 + 700 ft thick (2675 ft ), and presumably thicker because of an asial gradient towards the terminus and a gradiellt
tcJ\Vard
the
lateid
margin
as
\vd.
:!t the southwest end of the basin the l)etlrock rises sharply and forms 3 divide IKtweet1 this basin and the llinneapolis basin to the southwest ; the elevation of the divide, which is near the town of Sandstone, is about 500 it above the lake level. \Z%ere the ice terminated at the divide, it was presumably only a few hundred feet thick. The ice thickness thus decreased abruptly from almost 300 ft to a thin edge over a distance of ahout 100 miles. Pollen and plant-macrofossil studies in northeastern Minnesota indicate that the vegetation of the region was tundra until at least 11.50 years ago (\Vright and Watts, 1969). Ijecause tundra today in broad areas of northern Canada is roughly coincident with continuous permafrost and with a mean annual atmospheric temperature of -S”C or below (Rowe, 1959; I
RETREAT
OF
I.AURENTIIlE
ICE
325
SITEEl
ssw
NNE
Lake Superior
basin
-
Q@ &ii Minneapolis 2000 -z E
ul
-_---
1000
dold
c90 I ,Cold
,$
I
a,
---_-_
-_---__
--__--_
lowland ice, frozen
ice
400
Miles FIG. 4. Longitudinal section of Superior lobe at time of Split Rock phase, as borings and postulated thermal relations. Field control for the surface slope for glaciation can be inferred from the slope of the Highland lateral moraine along the Superior. But for the Split Rock phase no such control is available, and the surface shown as much gentler, on the assumption that the ice tongue became distended the gentle surface slope, the great depth of the basin makes the thickness profile some more steeply sloping modern glaciers.
rcconstructetl from an earlier phase of north shore of Lake slope is therefore by surging. Despite similar to that of
rock is insufficiently permeable. Ice flow 1966), the search for a mechanism might continues from the source area to the mar- well focus on the thermal relations at the gin. facilitated by the water film that per- baseof the ice. mits lmal sliding. The thin toe of the ice, The last two recognizable advances of the however. being frozen to its bed, moves Superior lobe in Minnesota barely renchecl much more slowly, and it resists the ice the rock divide at the end of the basin pressure behind it like a dam. Eventually (Wright and Watts. 1969). They are both the pressure exceeds the strength of the represented 1)~red clayey till derived largely dam, and the front then advances rapidly in from overridden lake beds. The first of the a surge-a sequence of events similar to two, the Split Rock phase, left a thin and that postulated for the recent surge of part discontinuous sheet of till at elevations of an ice cap in Spitzbergen (Schytt, 1969). below about 1300 ft. The second, the NickAlthough the conditions or mechanisms for erson phase, produced a sharp and rugged surging are hardly esplained in a fully satis- moraine. These two advances may represent factory way (Ambrose, 1969) the appar- surges, with the Kickerson moraine perhaps ently common occurrence of surging in cer- being the product of multiple surges. The tain glaciers today implies some type of timing of the advances therefore may not be periodically unstable flow conditions. Inas- related to any particular climatic event. much as flow in temperate glaciers is con- I3oth of them predated the Valtlers advance trolled largely hy basal sliding (~Veertman. of the Lake 1’lichigan lohe.
326
1%.
E.
WRIGHT,
Surging in tllr Lake Mic-lriyun-G~rrn KU>! Lobr Other basins in the Great Lakes region do not have a nlorphometry so favorable for surging as the Lake Superior basin, but the posibilities must he considered, especially in cases where the advance of adjacent ice lobes are thought to be nonsynchronous. The uniquely great advance of the Lake Michigan and Green Bay lobes in the \‘alers phase may be such a case, ad certain special features of the basin morphometry may in fact support this hypothesis. The ice for the Lake Michigan and Green Bny lolm came out of the eastern portion of
JR.
the I .ake Superior lmsin. This portion of the lake has a general depth of only a few hundred feet, hut the bottom is marked 1)~ about n dozen subpxxllel north-south chnnnels that branch and rejoin ( Fig. 5 ). The channels are as much as I SO miles long, 2 miles wide, and 1000 it deep and are cut into bedrock. The cover of glacial sediments in this part of the basin is generally less than 100 it thick (Farrand, 1969). The pattern has every resemblance to gorges eroded hy high-velocity streams. The channels are tentatively attributed to preglacial stream cutting by Farrand (1969), hut this explanatim fails to nccowt for the nhupt tenni-
FIG. 5. Bottom topography of eastern 1,ake Superior hasin, showing pattern of submerged gorges, here interpreted as tunnel valleys cut into bedrock. Note the abrupt termination of the pattern against the south shore, which is underlain by bedrock at shallow depth. Distance across the lake from north to south through Michipicoten Island is about 80 miles. From Farrand (1969).
RETREAT
OF
LAURENTIDE
nation of all of them against a bedrock shelf at the south shore of the lake. An alternate explanation relates them to tunnel valleys produced by subglacial streams. Tunnel valleys are believed to form during the earlv stages of wastage of ice lobes under thermal conditions that provide abundant water by basal melting or (if the glacier is temperate up to the surface) by the downward penetration of surficial meltwater (Wright, 1971). If the water is sufficient in volume, it could be concentrated into streams beneath the ice rather than simply move as a basal film. The hydrostatic pressure of the still-active ice lobe provides the subglacial streams with the velocity necessary (1) to transport all the detritus supplied by the ice, (2 j to erode its bed, thereby cutting the tunnel valley deep into bedrock, and (‘3’) to flow uphill if the morphometry of the subglacial basin demands it. Excellent examples of tunnel valleys can be found in eastern Minnesota, where they were formed beneath the Superior lobe in the St. Croix phase (Wright, 1971). Here there are about a dozen subparallel channels, as much as 100 miles long and 200 ft deep. Most of them contain eskers, formed during later stages of wastage when the ice was thin and stagnant. Tunnel valleys indicate abundant subglacial water, the same condition favorable for rapid basal sliding of the ice, i.e., surging. In the case of the Superior lobe in its St. Croix phase, surging may not have been possible because the frozen ice toe formed too thick an ice dam. But when the ice withdrew into the deep Lake Superior basin, the water could be trapped and could build up its basal film until rapid basal gliding was possible, Some water must have escaped. becausehuge eskers and fans formed locally at the ice edge. In the eastern Lake Superior basin, the buildup of basal water may have been enough not only to cut tunnel valleys but, at a different time, to permit surging of the
ICE
327
SHEET
Lake Michigan-Green Bay lobe. The relative age of the two events cannot be determined. Perhaps the postulated 200-mile Valders surge of the ice came first after a long accumulation of basal water. The expanded lobe then wasted rapidly, especially with active calving into proglacial lakes. Basal meltwater could build up again in the eastern Lake Superior basin, but to greater amounts because the steadily warming lateWisconsin climate permitted penetration of surficial meltwater to the base. The basal water could then be channeled to produce the tunnel valleys, and it could escapeat the front along the south shore, building the great sandplainsnorth of Lake Michigan. This hypothetical sequenceof events is of course highly speculative, but it has enough evidence to undermine the assumption that the major ice advance of one lobe is accompanied by a major advance of adjacent lobes. The case is strongest for the Superior and Lake Michigan-Green Bay lobes in late Wisconsin time. Whether earlier advances of these or other ice lobes in the Great Lakes region might reflect similar nonclimatic events cannot be determined at present. Evidence for general 14.500- and 13,000-year advances across at least part of the Great Lakes region implies some regional controls, but the correlation and dating are not sufficiently refined to be certain. The best confirmatory evidence might come from independent paleoclimatic studies, such as pollen analysis. So far one climatic reversal has been suggested for the pollen sequence in Minnesota and Wisconsin, but it is not reflected in all diagrams, it is not we11 dated, and it is not uniformly interpreted (Cushing. 1967 ; Wright, 1968). Post-17alders
Glacial
Events
The glacial history of the Laurentide ice sheet did not end with the Valders advance of the Lake Michigan-Green Bay lobe. But the subsequent events are not well recorded or well studied north of the Great Lakes. A
328
H.
E.
WRIGHT,
sequence of moraines in northwestern Ontario has been mapped by Zoltai (1965), bringing the record there up to about 9000 years ago, and relating it to the history of Lake Agassiz and its eastern outlets to Lake Superior. But eastward from here the terrain is marked by no distinctive moraines. Once the height of land was crossed by the retreating ice front, proglacial Lake Barlow-Ojibway covered the terrain with clay, and the principal distinctive feature is the Cochrane till, which is composed of lake clays deformed by readvancing ice. This feature is dated as about 8000 years ago, and it has long been used to indicate a significant break in the climatic sequence. In fact, one classification (Frye et nl., 1968) places the end of the Valderan substage and thus of the Wisconsinan Glaciation at the Cochrane moraine. Hughes (1965)) however, considers the Cochrane readvance to be of minor and local importance, and Prest (1970) attributes it to a surge of ice from the west side of Hudson Bay, where the sea had invaded through the Hudson Straits and provided the excess water. In any case, it is clear from the pollen sequence that the vegetation and thus the climate in southern Canada after 10,1>00 years ago was much like that of today (Terasmae, 1960) and the 3000 years of ice wastage that followed, until Hudson Bay was cleared, was merely a result of the long time required to melt the huge ice mass, even under warm climatic conditions. CONCLUSIONS Review of the chronology of late-wisconsin moraines and other glacial features at the margin of the Laurentide ice sheet indicates that the ice front experienced many fluctuations. The deep lobations in the Great Lakes region provide the opportunity to examine the synchroneity of fluctuations from lobe to lobe. Ice-margin fluctuations are generally assumed to represent climatic changes, espe-
JR.
cially when evidence exists for distant retreat of the ice front before readvance. The case for regional climatic control is strengthened when moraines can be traced from lobe to lobe across a broad area. In the late-Wisconsin history of the Great Lakes region, the geomorphic and stratigraphic relations, along with radiocarbon dating, point to cases of both synchroneity and nonsynchroneity. The ice advance at about 14,500 years ago is identifiable from the Des Moines lobe of Minnesota in the west (and probably the James lobe still farther west) to the Erie-Ontario lobe in the east, as an event that followed ice retreat of almost 100 miles, although in some areas, such as eastern Minnesota, all ice lobes did not advance an equal amount. The 13,0OU-year advance is manifested especially around the Lake Michigan and Huron basins as the Port Huron moraine; it also was preceded by a distant ice retreat, but its representation east and west from these basins is weak. The 11.500-year Valders advance of the Lake Michigan-Green Bay lobe occurred after very distant retreat, but the advance is confined to this double lobe, which emerged from the eastern part of the Lake Superior basin. The relations may be explained by postulating a rapid surge of the ice after long accumulation of water at the base of the ice. The case for surging is even stronger for the Superior lobe for earlier advances, because of the especially favorable morphometry of the basin. The key to some ice-lobe advances may therefore be in the morphometric. thermal, and hydrologic relations as much as in the climatic. In deep basins the ice-lobe morphometry can sometimes be estimated from the elevations of lateral moraines, along with borings from the center; hydrologic relations can be inferred from the presence of subglacial tunnel valleys and proglacial outwash bodies; and at least some of the thermal properties of the ice can be postulated from knowledge of
RETREAT OF LAURENTIDE the contemporaneous periglacial vegetation through pollen analysis. l+‘hat is required for further development of a surge hypothesis is field evidence from drift morphology and structure, along with a comparison with data from glaciers that are known to have surged in recent decades. REFERENCES J. W. (Ed.) (1969). Seminar on the causes and mechanics of glacier surges, and symposium on surging glaciers. Canadian Jourxal of Earth Scicr~ccs 6, 807-1018. BLACK, R. F. (1969). Valderan glaciation in western upper Michigan. Proceedings of the 12th AMBROSE,
Conferemc of the Illtematioxal Great Lakes Resrurch, Am
Association r2rbor 1969,
for
pp.
116-123. BROECKER, W. S., and FARRAND, W. R. (1963). Radiocarbon age of the Two Creeks forest bed, Wisconsin. Geological Society of .4ruerica Btdletilt
74, 795-802.
BROWN, R. J. E. (1967). Permafrost in Canada. Geological Survey of Canada Map 1246A. CALKIN, P. E. (1970). Strand lines and chronology of the glacial Great Lakes in northwestern New York. Ohio Jowxal of Science 70, 78-96. CASHING, E. J. (1967). Lake-Wisconsin pollen stratigraphy and the glacial sequence in Minnesota. In “Quaternary Paleoecology” (E. J. Cushing and H. E. Wright, Jr., eds.), pp. 59-88. Yale University Press, New Haven. DREIMANIS, ALEKSIS (1964). Lake Warren and the Two Creeks interval. Joumnl of Geology 72, 247-250. DREIMANIS, ALEKSIS (1967). Cary-Port Huron interstade in eastern North America and its correlatives. Geological Society of America, Northwestern Section, Second Annual Meetings (Boston, 1967) I Abstract. DREIMANIS, ALEICSIS (1969). Late-Pleistocene lakes in the Ontario and Erie basins. Proceediugs of the 12th Confrrolce of the Iutemational Association for Great Lakes Rcscarch, Amt A1-bor 1969 pp. 170-180.
ELSON, J. A. (1967). Geology of glacial Lake Agassiz. Ilt “Life, Land and Water,” Conference on Environmental Studies of the Glacial Lake Agassiz Region (W. J. Mayer-Oakes, ed.), Proceedings, pp. 37-96. University of Manitoba Press, Winnipeg. FARRAND, W. R. (1969). The Quaternary history of Lake Superior. Proceedings of the 12th Con-
ICE SHEET
329
ference of the Illterlcatiorwl Lakes Research, .4nn Arbor
Association for Great 1969, pp. 181-197.
FAKRAND, W. R., ZAHNER, R., and BENNINGHOFF, W. S. (1969). Cary-Port Huron interstade : Evidence from a buried bryophyte bed, Cheboygan County, Michigan. Geological Society of -4werica
Special
Proper
123,
249-258.
FLINT, K. F., COLTON, R. B., GOLDTHWAIT. R. P., and WILLMAN, H. B. (1959). Glacial map of the United States east of the Rocky Mountains. Geological Society of Plmerica, Boulder, Colorado. FRYE, J. C., and WILLMAN, H. B., (1960). Classification of the Wisconsinan Stage in the Lake Michigan glacial lobe. Illimis State Geological Survey
Circular
285.
FRYE, J. C., WILL~IAN, H. B., and BLACK, R. F. (1965). Outline of glacial geology of Illinois and Wisconsin. I+t “The Quaternary of the United States” (H. E. Wright and D. G. Frey, eds.), pp. 43-61. Princeton University Press, Princeton, New Jersey. FRYE, J. C., WILLMAN, H. B., RUBIN, M., and BLACK, R. F. (1968). Definition of the Wisconsinan Stage. U.S. Geological Suwey Bulletin 1274-E, El-E22. GOLDTHWAIT, R. P., DREIMANIS, A., FORSYTH, J. L., KARNOW, P. F., and WNITE, G. F. (1965). Pleistocene deposits of the Erie lobe. 112 “The Quaternary of the United States” (H. E. Wright, Jr. and D. G. Frey, eds.), pp. 85-97. Princeton University Press, Princeton, New Jersey. Hou(;H, J. L. (1958). “Geology of the Great Lakes.” University of Illinois Press, Urbana, Illinois. HOEGFI, J. L. (1963). The prehistoric Great Lakes of North America. .4mevican Scientist 51, W-109. HUGHES, 0. L. (1965). Surficial geology of part of the Cochrane District, Ontario, Canada. IN International Studies on the Quaternary” (H. E. Wright, Jr. and D. G. Frey, eds.), pp. 535-565. Gcologicrrl Society of American Special Paper 84. M’CDONALI), B. C. (1968). Deglaciation and differential postglacial rebound in the Appalachian region of southeastern Quebec. Jowml of Geology 76, 664-677. PREST, V. K. (1969). Retreat of Wisconsin and recent ice in North America. Geoloqical Survey of Canada Map 125’?A, scale 1 :5,000,000. PREST, V. K. (1970). Quaternary geology of Canada. 112 “Geology and Economic Minerals of Canada,” pp. 676764. Canada Department of Energy, Mines & Resources, Ottawa.
H. E. WRIGHT,
330 ROWE,
J. C. (1959).
Canada Department Resources, Forestry
Forest regions Northern
Affairs
of Canada. Natural
Brawct, Bulletin 123. R. V. (1969). “Quaternary Landscapes in Iowa.” Iowa State University Press, Ames, Iowa. SCHYTP, VALTER (1969). Some comments on glacier surges in eastern Svalbard. In “Seminar on the Causes and Mechanics of Glacier Surges, and Symposium on Surging Glaciers” (J. W. Ambrose, ed.), pp. 867-874. Capkadian Journal of Earth Sciences 5, 807-1018. TERASMAE, JAAN (1960). A palynological study of post-glacial deposits in the St. Lawrence lowlands, Geological Survey of Canada Bulletin 56, RUHE,
l-22.
R. E. (1968). A sub-bottom reflection survey in the central basin of Lake Erie. Geological Society of America Bdletin 79, 91-106. WASYLIKOWA, KRYSTYNA, and WRIGHT, H. E., JR. (1969). Late-glacial plant succession on an abandoned drainageway, northeastern Minnesota. Krakow, Acta Palaeobotanica 11, 23-43. WAYNE, W. J., and ZUMBERGE, J. H. (1965). Pleistocene geology of Indiana and Michigan. In “The Quaternary of the United States” (H. E. Wright, Jr. and D. G. Frey, eds.), pp. 63-82. Princeton University Press, Princeton, New Jersey. WEERTMAN, J. (1966). Effect of a basal water layer on the dimensions of ice sheets. Journal of Glaciology 6, 191-208. WILLMAN, H. B., and FRYE, J. C. (1970). Pleistocene stratigraphy of Illinois. Illinois State Geological Survey Bdletin 64, 204 pp. WALL,
JR.
H. E., JR. (1968). The roles of pine spruce in the forest history of Minnesota and adjacent areas. Ecology 49, 937-955. WRIGHT, H. E., JR. (1969). Glacial fluctuations and the forest succession in the Lake Superior region. Proceedings of flu 12th Coltference of the Internatiorcal dssociation for Great Lakes Research, Am Arbor 1969, pp. 397-405. WRIGHT, H. E., JR. (1971). Tunnel valleys, glacial surges, and the subglacial hydrology of the Superior lobe, Minnesota. Geological Society of America Special Paper, in press. WRIGHT, H. E., JR., MATSCH, C. L., and GUSHING, E. J. (1971). The Superior and Des Moines lobes. Geological Society of ,4wterica Special Paper, in press. WRIGHT, H. E., JR., and RUHE, R. V. (1965). Glaciation of Minnesota and Iowa. In “The Quaternary of the United States” (H. E. Wright, Jr. and D. G. Frey, eds.), pp. 29-41. Princeton University Press, Princeton, New Jersey. WRIGHT, H. E., JR., and WATTS, W. A. (with contributions by Saskia Jelgersman, Jean C. B. Waddington, T. C. Winter, and Junko Ogawa) (1969). Glacial and vegetational history of northeastern Minnesota. Minnesota Geological Survey Special Publication 11, 59 pp. ZOLTAI, S. C. (1965). Glacial features of the Quetico-Nipigon area. Calbadian Journal of Earth Sciences 2, 247-269. ZUMBERGE, J. H. (l%O) . Correlation of Wisconsin drifts in Illinois, Indiana, Michigan, and Ohio. Geological Sociefy of A4werica Balletin 71, 11771188. WRIGHT,