BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 33, PP. 491-514 SEPTEMBER 1, 1922
STAGES OF THE ICE AGE 1
BY WARREN UPHAM
(Presented before the Society December 28, 1921)
CONTENTS Page Sequence of glacial and interglacial stages...... 491 Nebraskan' glaciation...... 493 The Aftonian interglacial stage...... 495 Kansan glaciation.... t...... 499 The Yarmouth interglacial stage...... 503 Illinoian glaciation...... 504 The Sangamon interglacial stage...... 505 Iowan glaciation and loess deposition...... 506 The Peorian interglacial stage...... 506 Wisconsin glaciation...... 507 The Champlain stage...... 510 Estimated time ratios...... 512
S e q u e n c e o p G l a c i a l a n d I nterglacial S t a g e s
Throughout the Glacial period of growth, culmination, and decline of the North American and European ice-sheets, the 'climate causing the snowfall and ice accumulation fluctuated to such an extent that the boundaries of the continental glaciation were alternately advanced and checked or drawn back. North America had five stages of widely extended icefields. First was the long time of Nebraskan glaciation, followed by a great recession of the ice-borders during an interglacial stage named Aftonian, from its stratified beds and fossils near Afton, in southwestern Iowa. The second and maximum extension of ice accumulation is named, in the Keewatin area of its outflow west of the Mississippi, the Kansan stage, which was broken by the Yarmouth interval of ice melting and retreat. With the Kansan glaciation, but in part spreading across the eastern edge of its
1 Manuscript received by the Secretary of the Society December 12, 1921. (491)
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drift sheet and melting later from the Mississippi, was the thirds or Illi- noian, icefield, belonging to the far extended Labradorian area of glacial outflow. After the Kansan and Illinoian stages, which appear to have been mainly contemporaneous, another retreat of the ice-sheet took place in the Sangamon interglacial stage. Both the Yarmouth and Sangamon areas of wavering glaciation, and the duration of their stages, were slight in comparison with the Aftonian stage, which was the most prolonged interruption of the Ice Age on this continent. Next were the Iowan and Wisconsin stages of glaciation, the fourth and fifth in the list of wide climatic fluctuations that successively amassed and melted back the North American icefields. The Iowan drift was followed by the chief deposit of the loess, a peculiar silt, which was weath ered and eroded in the vicinity of Peoria, in Illinois, previous to the re- advancing Wisconsin glaciation. Hence the name Peorian is applied to the interglacial time of recession of the ice-border between these stages of colder climate and ice-advance. Depression of the land during the Iowan glacial stage had given decreased slopes of the Missouri, Mississippi, and Ohio valleys, permitting them to receive the thick formation of the loess. The land subsidence appears to have remained, or even to have progressed for the northeastern part of our continent, during the Wisconsin stage, while the most prominent marginal moraines were amassed at times of pause or often some readvance that accompanied the general melting and final departure of the ice-sheet. After the ice had melted far back, the sea covered the depressed coast and reached inland along the Saint Lawrence, Ottawa, and Lake Cham plain valleys, forming fossiliferous marine beds, from which this latest and relatively short part of the Glacial period is named the Champlain stage. It is thus seen that the alternating Iowan, Peorian, Wisconsin, and Champlain stages, comprising the time of greatest subsidence of the continental glaciated area, were attended by the growth, fluctuations, and disappearance of the Iowan and Wisconsin icefields. These four late stages seem to have been somewhat closely continuous with the preceding Kansan and Illinoian maximum glaciation, which was widely separated from the Nebraskan stage by the Aftonian retreat of the ice boundaries. A similar sequence of glacial and interglacial stages has been traced in the British Isles, northern Europe, and the Alps, which indicates that the climatic conditions producing the Ice Age on the continental areas at opposite sides of the Atlantic were approximately contemporaneous, and that their fluctuations of glaciation were caused by like secular changes of climate.
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N e b r a s k a n G l a c i a t i o n
The earliest recognized stage of great extension of the ice-sheet upon Canada.and the United States has been named Albertan and Jerseyan, from its drift in Alberta and New Jersey. Following a suggestion of the late Prof. Samuel Calvin, the preferable name Nebraskan was proposed by Prof. B. Shimek2 for this stage. Thus all the series of North Amer ican ice advances and recessions, if the Champlain stage be excepted, are named from the glacial drift and alternating interglacial beds within the drainage basin of the Mississippi. Eroded river valleys have occasional exposures of Nebraskan drift, beneath the Kansan drift and formations of Aftonian age, in eastern Nebraska and in southern and eastern Iowa; but its most western and southern extension in that region was probably everywhere overspread and surpassed by the Kansan icefield. East of the Mississippi the early Patrician transportation of copper fragments and drift boulders from the region of Lake Superior to central and southern Ohio seems referable to the Nebraskan stage of glaciation. Belonging originally to Patrician drift, but later carried onward as a part of the Illinoian drift, Prof. G. F. Wright found boulders of red jasper conglomerate near Cincinnati, borne beyond the Ohio River into the north edge of Kentucky, where they were deposited at the extreme boundary of the ice-sheet after a southward glacial journey of about 500 miles from their parent ledges, east of Lake Superior and north of Lake Huron. In northeastern Pennsylvania and northern New Jersey the attenuated border of the drift, extending 10 to 25 miles south of the terminal mo raine, is referred by Chamberlin and Salisbury to the first great stage of our continental glaciation, named by them Jerseyan, probably correlative with the Nebraskan stage and the ancient Patrician drift. It is remark ably contrasted with the adjoining drift sheet of Wisconsin age, which includes the thick moraine and spreads thence far north upon New York, New England, and eastern Canada. The thin Jerseyan drift remains in patches, and indeed it may have been originally so deposited. Formed mainly by the local snowfall, the thin border of the early icefield probably had low surface slopes, without sufficient movement in many portions for drift erosion, transportation, and deposition. Reaching more than 1,200 miles from the Missouri River to the At lantic close south of New York City, the southern margin of the most ancient known stage of our continental glaciation fell not very far short of the later maximum icefield in Kansas and Missouri. It attained nearly
2 Bulletin Geol. Soc. of America, vol. 20, December 24, 1909, p. 408 ; also in Science, January 14, 1910.
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the same lim it as the Illinoian and Ohio drift, which was early borne southward from the Patrician central area of outflow, but later from the more eastern Labradorian area. On the southwest edge of the very broad Quebec lobe of that eastern icefield, from the reentrant angle of the glacial boundary near the east end of Lake Erie to the coast of New Jersey, the earliest drift extends beyond the latest Wisconsin glaciation, which is bordered by a belt of prominent morainic ridges and hills. 'With so great southward extension of the ice-sheet in the- Nebraskan stage on the east half of our continent, it doubtless covered nearly all of Minnesota and the upper part of the Mississippi basin, excepting the large driftless area, mostly in Wisconsin, which was exempt from glacia tion through all the Ice Age. Across the Canadian provinces of Saskatchewan, Alberta, and British Columbia, and over the adjoining border of the United States, the Kee- watin and Cordilleran icefields probably reached nearly as far south in Nebraskan time as in any of the later and shorter stages of glaciation. For that western half of the continent, through the admirable studies of Gilbert and Russell on Lakes Bonneville and Lahontan in Utah and Nevada, and from the Quaternary history of Lake Mono, in California, it is ascertained that the somewhat moister climatic conditions producing the remarkable enlargement of the lakes in the Great Basin were attended with increased snowfall and glaciation upon the contiguous mountain ranges. The two stages of great rise of these lakes in the Pleistocene period, separated by the intervening stage of their desiccation, when they were nearly or wholly evaporated away, are very certainly correlative on more northern and eastern parts of this continent with two divisions of the Ice Age that were marked by abundant snowfall and ice accumula tion, while the interval of desiccation has its analogue eastward in an interglacial stage of fluctuating retreat and readvance of the ice bound ary. Gilbert wrote of Lake Bonneville as follows, noting the parallelism of its history with climatic changes that gave alternate stages of growth and decline of glaciers and icefields:3 “The Bonneville basin originated by distortion of the earth’s crust, and came intè existence long before the Bonneville epoch. Little is known of its earliest flimatic and physical conditions, but it was comparatively dry for a long period immediately preceding the formation of the great lake. During this period, alluvial cones were formed about the bases of all its greater mountain Panges, and the smaller ranges were wholly or partly buried by valley deposits. The valley deposits may have been entirely alluvial, but were probably also partly lacustral, the lakes being of small extent.
3TT. S. Geol. Survey, Monograph I, 1890. pp. 316-318.
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“There followed two epochs of high water, with an interval during wbiqh the basin was nearly or quite empty. The first of these epochs was at least five times as long as the second. The second scored its water mark 90.feet, higher than the first, and would have encroached still farther on the basin sides had it not been cheeked by outflow. During the epoch of outflow, the discharging current eroded the rim, and thus lowered the lake 375 feet; and after the outflow had ceased, the water fell by desiccation, with one notable interruption, to its present level in Great Salt Lake. The inter-Bonneville epoch of low water was of greater duration than the time that has elapsed since the final desiccation. “The history of the Bonneville oscillations is, moreover, closely paralleled by that of the Lahontan oscillations, and it is believed that they belong to a series of climatic changes affecting not only these two basins, but the adjacent subdivisions of the Great Basin. . . . “The moraines of three Pleistocene glaciers descend from the Wasatch Moun tains to the level of the Bonneville shoreline; the moraines of four glaciers descend from the Sierra Nevada to the level of the old shoreline of Mono Lake; and the relations of these moraines to the shores of the lakes and the associated deposits indicate that the maximum stag? of the lakes coincided closely with the epoch of maximum glaciation. “The phenomena sustain the theory that the Pleistocene lakes of the west ern United States were coincident with the Pleistocene glaciers of the same district, and were produced by the same climatic changes. It follows as a corollary that the glacial history of this region was bipartite, two maxima of glaciation being separated, not by a mere variation in intensity, but by a cessation of glaciation.” Comparing the fluctuations of the western lakes and glaciers with those of the North American ice-sheet, we may confidently refer the prolonged first high stage of the lakes to the Nebraskan stage of far extended ice accumulations; the time when the lakes disappeared under a drier clinjate is represented by the Aftonian stage of wide recession of the ice-sheet; and the second rise of the lakes occurred probably during the time of culmination and final disappearance of the icefields. This second great division of the Glacial period included the maximum Kansan and tlli- noian stages and the later Iowan and Wisconsin glaciation, being termi nated when subsidence of the formerly high ice-covered land caused the continental glacier to be melted away, with the formation of very remark able terminal and recessional moraines.
T h e A f t o n i a n intebglacial S t a g e
The present writer, in a paper read at the International Geological Congress in Toronto in 1913,4 referred the interval between the high
* Report of the Congress, published in 1914, pp. 455-466.
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stages of Lakes Bonneville and Lahontan to the time of the Sangamon recession of the iee-border. Chains of lakes in Martin County, Minne sota, mark courses of interglacial rivers that flowed southward into the basin of the East Pork of the Des Moines, which were likewise regarded in that paper as of Sangamon age, being thought to afford proof of a great recession of the ice-sheet between the Illinoian and Iowan stages of glaciation, so far, probably, as to uncover the southern half of Minnesota. More full consideration, however, of the relationship of the fluctuating western Quaternary lakes with the recognized diverse stages of growth and wane of the icefields, as here stated, has since led me to refer both the drying up of the western lakes and the erosion of interglacial river courses in southern Minnesota, with all the many occurrences of fossil- iferous interglacial formations showing a very great interruption of the Ice Age on this continent, to the Aftonian stage of glacial recession. In comparison with that time of far withdrawal of the ice boundary, its Sangamon oscillation was of brief duration and relatively narrow areas. Under this new review of the changeful and complex records of secular climatic conditions during the Ice Age, the Nebraskan and Aftonian stages appear very long; but the later stages, represented by drift sheets and interglacial beds that are far more extensively accessible to observa tion, forming the surface upon nearly all the glaciated area of this conti nent, are seen to have occupied less time than was before supposed. The most noteworthy revision of the sequence of changes in climate and oscil lations of the ice-margin, as shown by the drift and associated interglacial beds, consists in the reference of the four glacial stages after the Aftonian interval to a practically continuous prevalence of widespread snowfall, bringing the ice-sheet to its maximum area and thickness, from which it waveringly receded, but was at last, in a geologic sense, rapidly m eltei away. •Short distances and duration of ice recession and return, or of fluctua tion in adjacent or confluent icefields, one superseding or pushing back another, may sufficiently account for the observed Yarmouth, Sangamon, and Peorian land surfaces and fossiliferous beds that, were covered by ensuing glaciation. Even the moraine-forming Wisconsin stage had changes of glacial currents on some areafc during the final melting of the ice-sheet, which brought new outlines of its border and an overlapping of consecutive series of the marginal moraines. In Iowa the fauna of the Aftonian deposits included the extinct mam moth and mastodon, an extinct horse, and species of the deer and rabbit yet living in this region. The flora comprised the pine and other non if-
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erous trees, which were very abundant in northeastern Iowa, so that their wood is found in the sections of many wells, forming a broadly preserved forest bed. Leaves and wood of deciduous trees, as oak, elm, ash, walnut, and hickory, are also found, indicating a temperate climate similar to that of the present time. Near Toronto, about 750 miles east-northeast from Afton, extensive interglacial beds contain remains of numerous species of mammals and trees identical with those of the Aftonian stage in Iowa and Nebraska, so that the Toronto formation is referred by Prof. A. P. Coleman to the same time. He also well refers more northern Canadian forest beds and lignite to Aftonian time, of which he writes as follows: “Another widespread tract of interglacial deposits lies 350 or 400 miles north of Toronto, along river valleys of the James Bay region, with an extent from east to west of 100 miles and from north to south of 50. It includes 27 known outcrops of interglacial lignite or peat, scattered over an area of several thou sand square miles. Logs of wood 17 inches in diameter have been found in the beds.” 5
In twelve counties of southern Minnesota, wells in the glacial drifts encounter interglacial beds containing wood, layers of peat, and occasion ally shells of fresh-water and air-breathing mollusks, which layers are underlain and overlain by till, the direct deposit of an ice-sheet. These counties, described by the State geologist, the late Prof. N. H. Winchell, and by the present writer, in the first volume of the Final Report of the Geological and Natural History Survey of Minnesota, are, in their order from east to west, Fillmore, Mower, Freeborn, Steele, Faribault, Martin, Jackson, Cottonwood, Brown, Redwood, Lyon, and Rock counties. Professor Winchell recorded many such wells in Mower County and adjacent parts of Fillmore County that penetrate a bed of peat carrying pieces of wood thought to be pine and cedar. This interglacial peat bed, varying from about one foot to six or eight feet in thickness, lies at &0 to 25 feet beneath the surface of Le Roy and Grand Meadow townships and the city of Austin. In other parts of Mower County it occurs at depths of 35, 45, and 50 feet. It probably is continuous with the extensive inter glacial forest bed of Aftonian age in northeastern Iowa which was studied early and well reported by McGee. The other counties to the west have less abundant records of wells that pass through or into interglacial forest and peat beds, which would indicate that there they were largely removed by the erosion of the ensuing stages of glaciation. Evidence of a long interglacial stage, here regarded as the Aftonian,
5 Report of the International Geological Congress in Toronto, 1913, pp. 435-449.
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■ which interrupted the Ice Age in the southern half of Minnesota, is sup plied by the exceptional chains of lakes before noted in Martin County, oiie of the central counties in the southernmost tier of this State. These • chains of lakes are three in number and are named the East, Central, and West chains. South Creek receives the outflow from the East chain, which extends in a somewhat irregular northerly course for 12 miles from the Iowa line. This chain comprises eight lakes, varying from a half mile to two miles in length, with from a half to two-thirds as great widths. These lakes are bordered by rolling areas of till. Their shores ascend 30 to 40 feet, mostly by quite steep slopes. The spaces between the lakes are, in some cases, marshy and as wide as the narrower parts of the lakes, but other adjoining lakes are connected by contracted channels, such as might be cut by the outflowing stream. Thus the series does not occupy depres sions in any well marked and continuous valley. About twenty lakes are included in the Central chain, extending 22 miles in an almost perfectly straight north to south course, from Perch Lake, three miles south of Watonwan County, to Iowa Lake, which is crossed by the State line. This series of lakes lies three to six miles west of the East chain. Its outlets are South, Center, Elm, and Perch creeks, all flowing eastward. On both sides of the Central chain of lakes, as in the case of the East chain, the adjoining country consists of till, which rises to a moderately- undulating expanse, 30 to 40 or 50 feet above the lakes. Though form ing a very distinct, straight series, these lakes do not occupy a well de fined valley, for its width varies from one mile or more to less than an eighth of "a mile, and it is interrupted in three places by water-divides which are 10 to 13 feet above the adjoining lakes. Silver and Iowa,^£kes are the headwaters of South Creek, and have their outlet by a stream'that runs nearly along the State line to the south end of the East chain. The middle part of the Central chain, reaching 12 miles from Summit Lake to the Twin Lakes, is tributary to Center Creek, and its portion farther north, excepting Perch Lake, is within the belt drained by Elm Creek. The West chain of lakes is less distinctly connected than the East and Central chains, from which it also differs in having the longer axes of some of its lakes transverse to the course of the chain, and in having shorter series of lakes joined with it as branches. Tuttle’s Lake, at the south end of the chain, lies on the State line about four miles west of Iowa Lake, the south end of the Central chain. Thence the West chain reaches 20 miles northwesterly, then nine miles northerly, and then north
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west and west eight miles to Mountain Lake, in Cottonwood, County, its whole extent being 37 miles. Its successive portiops from ’south to north are; tributary to the East Pork of Des Moines River, to Center and Elm creeks, and to the South Fork of Watonwan River. This West chain has about 25 lakes, extending through a region of moderately undulating till, deposited by the ice-sheet, with no noteworthy areas nor unusually thick included layers of water-deposited gravel and sand, which, indeed, is true of all this county. An explanation of these series of lakes, which appears most probable, is that they mark avenues of drainage and occupy portions of valleys that were excavated in the till during the Aftonian interglacial stage, after, ice had long covered this region and had deposited the greater part of the drift, but before other glacial stages again enveloped this area beneath the ice-sheet, partially refilling these valleys and leaving along their courses the present chains of lakes. Detailed description has therefore seemed needful, that we may compare the great amount of early glacial erosion and drift transportation and deposition, which preceded the Aftonian stage, with the relatively small amount that can be attributed bore to later glaciation. Why a widespread and long amelioration of the cold climate melted back the ice-sheet in Aftonian time so far from the former Nebraskan border, with which the later Kansan and Illinoian border nearly coin cided, it is hardly possible to conjecture, unless there was a subsidence of the continent during the later part of the Nebraskan stage, followed after the Aftonian stage by a second high continental uplift.
K ansan Glaciation
It is reliably proven that’the Ice Age was much diversified and even very complex. Yet I believe it more reasonable to ascribe all our North American drift formations to one prolonged and continuous Glacial period, with great fluctuations of the ice-border, especially in the interior of the continent, than to regard our Ice Age as twofold or threefold, in the sense of having had its vast ice-sheet wholly melted away, or even nearly so, with ensuing renewal of snow and icefields. Geologically very rare, an ice age would scarcely be duplicated with so nearly the same limits of ice extension upon half of our continent. The same general conclusion is also, as I think, applicable to the European glaciation. Almost inconceivable geologic duration divided the Permian and Pleistocene ice ages. In this most recent ice age, which continued to the threshold of the historic period, I can not think that the stupen-
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dons climatic changes implied in the glaciation could permit complete' repetition of the continental ice-sheets in America and Europe, and ex tend them in each case to so nearly the same maximum limits in the earlier and later parts of the Glacial period. It is better, until proofs are obtained more fully in the central regions of the drift areas on each con tinent, to regard their time of glaciation as one and continuous, with much areal oscillation, such as is proverbial of weather, during both the general stages of growth and departure. ’ All the ground that was relinquished by the ice-sheet during its Afto- nian recession, which doubtless reached north to the southern part of the Red River .valley, nearly to the head of the Mississippi, and to the vicinity of James Bay, became again deeply enveloped by the snowfall and ice fields of the Kansan and Illinoian stages, spreading in the basin of the- Missouri and Mississippi rivers somewhat farther than the previous Ne braskan limit. The Keewatin glacial outflow southward extended at the maximum of the Kansan stage to the lower part of the Kansas Riv.er, to' the Missouri below Kansas City, and to the Mississippi on the northeast boundary of Missouri. In the States of Illinois, Indiana, and Ohio the Labradorian glaciation later attained its maximum of the Illinoian stage,: depositing the outermost drift over the northern watershed of the Ohio River, and in the vicinity of Cincinnati it extended beyond this river into the edge of Kentucky. Along the upper course of the Missouri the border of the Keewatin ice-' field at its Kansan maximum limit crossed that river to the drift bound ary in Nebraska, South and North Dakota, and eastern Montana. Far ther west the Cordilleran outflow of both the Nebraskan and Kansan stages seems generally to have been surpassed by the Wisconsin stage, which is thought also to have produced the greatest extension of local glaciers upon the more southern Cordilleran ranges, to Colorado, and . California. ... In the greater part of New York and New England, where interglacial formations are very scantily developed or absent, the icefield of Nebraskan time may have persisted through the Aftonian interval and forward dur ing the Kansan and later glacial stages to the Champlain stage, in which the Glacial period ended. The maximum thickness and area of the ice- sheet east of the Hudson River was reached by the late Wisconsin mo raine-forming stage. Much drift that had been eroded and borne far from its sources during the very long Nebraskan glaciation, having afterward in Aftonian time rested from its travel, was again in part gathered up into the Kansan,
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Illinoian, and later icefields, to be carried forward until deposited by the final melting and retreat of the ice-border. Only where rock formations of restricted area have peculiar characters by which their boulders can be identified with certainty as distinct from all similar boulders supplied by- other formations and districts is it possible to affirm positively the dis tance of their transportation. Big Stone and Otter Tail counties, of western Minnesota, have rare granite boulders with included fragments of hornblende schist,6 like much of the granite forming the Giants’ or Mesabi range, which was probably their source. These boulders appear to have been carried about 200 miles southwest and west-southwest. Their glacial journey ran athwart the southeastward courses of the predominant Keewatin ice currents and out flow during the Kansan and later glaciation in that district, so that we must ascribe their transportation to the earlier Nebraskan stage. It was probably contemporaneous with the Patrician ice currents which spread from north of Lake Superior and carried their drift nearly or quite to th? Ohio River, as before noted. In Lucas County, of southern Iowa, a mass of drift copper weighing more than 30 pounds7 undoubtedly was borne by the currents of the ice- sheet about 600 miles, from the present copper mining region south of Lake Superior or from Isle Roy ale, first south westward and later south ward through eastern and southern Minnesota, passing west of the Wis consin driftless area. Its journey probably was accomplished mostly dur ing Nebraskan time. The farthest recognized origin of rock fragments in the drift of our continental ice-sheet, recorded in part by Dr. Robert Bell, of the Cana dian Geological Survey, whose observations are supplemented by the present writer, is from James Bay southwest to North Dakota and Minne sota. The rock is “dark grey, granular, siliceous felsite or greywacke, . . . characterized by round spots, from the size of a pea to that of a cricket ball or larger, of a lighter color than the rest of the rock, which weather out into pits of the same form.” It occurs as the bedrock, ac cording to Dr. Bell, on Long Island, off Cape Jones, on the east coast of Hudson Bay, where it is narrowed to form James Bay, having there a southwestward strike and probably continuing under the sea for some distance in that direction. He noted that the abundance of pebbles and boulders of this rock is the most remarkable feature of the drift on the
8 Geological Survey of Minnesota, Final Report, vol. i, 1884, p. 626; vol. ii, 1888, p. 551. 7 C. A. W hite: Geology of Iowa, 1870, vol. i, p. 96.
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west coast of James Bay and along the Attawapishkat, Albany, and Kenogami rivers, and that its fragments were found by him as far west as^to Lonely Lake and south to Lake Superior.8 Farther to the southwest and south I have observed fragments of it, usually measuring only a few inches, but in some instances a foot or more in diameter, occurring very rarely in the drift in the northeastern part of North Dakota, where the largest piece ever found by me was about 30 miles south of the interna tional boundary and 50 miles west of the Red River, and at numerous localities in Minnesota, where it extends at least as far south as Steele County, 75 miles south of Saint Paul and 1,000 miles southwest of its outcrop north of James Bay. The drift travel so known, like the instances before cited, is referable mainly to the Nebraskan glaciation and to a great Labradorian ice cur rent, for the later Keewatin currents, during the Kansan and ensuing glacial stages, could not permit drift derived from Hudson and James bays to be borne so far to the west. Moreover, the far southward trans portation into southeastern Minnesota covered too great a distance to be due to the Kansan and later glaciation, although it may partly be so explained. After Nebraskan travel to northwestern Minnesota, the Steele County boulder may have been carried southeast during Kansan time, taking then a course nearly at right angles with its earlier journey. From a revie'w of these notes of boulders carried far by glacial currents, it appears that during an early part of Nebraskan time the Patrician radial outflow reached west across northern Minnesota as well as south across Michigan and Ohio. Afterward, probably during the greater part of that very long stage, its directions of glacial flow respectively upon the Keewatin and Labradorian areas in, these States were nearly like the currents of the later Kansan and Illinoian icefields. Chains of lakes in southern Minnesota, described in preceding pages, show that the drift there was mostly deposited before a great recession of the ice-sheet which is referred to the Aftonian stage. Three localities farther north in this State supply definite measures of the’moderate thick ness of the drift referable in these places to the Kansan and later stages, agreeing with the evidence of these lakes. At New Ulm, about 30 miles distant northward, the later drift sheet, overlying a sand and gravel deposit that is probably Aftonian, has a thick ness of 16 to 18 feet. Rushseba township, in Chisago County, about 50 miles north of Saint Paul and Minneapolis, has a surface sheet of till 10
8 Geol. and Nat. Hist. Survey of Canada, Annual Report, new series, vol. ii, for 1886, p. 36G ; compare Report of Progress for 1878-79, pp. 22, 23C.
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to 20 feet deep lying similarly on interglacial beds which include a thin layer of peat with decaying fragments of wood next beneath the till. More distant northwestward, at Barnesville, in Clay County, on the southern part of the area of the Glacial Lake Agassiz, a well penetrated through 12 feet of till into quicksand “containing several branches and trunks of trees, thought to be tamarack, up to eight inches in diameter, which lie across the well, and which, together with the inflow of water, prevented further digging.” This well is in the till area of the village of Barnesville and about 80 feet below the highest beach of Lake Agassiz, which passes from south to north about three miles east of this village. In 1888, at the time of my survey of that region, I considered the. occurrence of this interglacial bed within the area of the glacial lake as good evidence that the ice-sheet had been melted back at least far enough to give outflow into Hudson Bay from the Eed River valley, thus draining off the forerunner of Lake Agassiz. Subsequent studies, however, lead me to hold now the different view that probably an inter glacial lake here may have cut its southern outlet, at the site of Brown’s Valley, to a lower level than the well at Barnesville; or that the attitude of the land was unlike what it is now, having then such an ascent from south to north that the Barnesville locality during the interglacial time was above the general surface of the region at Brown’s Valley, into which the River Warren, outflowing from Lake Agassiz, cut its deep, continuous valley. So I now think that we have in the section of this well only evidence of an ice-retreat (that is, a departure of the outer part of the ice-sheet) northward a little more than halfway between the south and. north boundaries of Minnesota. On the extensive area of the three chains of lakes and at the more north ern places where the thickness of drift above the old interglacial land surface is known, the Kansan, Iowan, and Wisconsin glaciation, to the end of the Ice Age, effected little or no erosion and made only scanty deposits of till beneath the ice-sheet. Each of the three recorded sec tions, in New Ulm, Rushseba, and Barnesville, appears to have received its upper till, lying on the interglacial beds, from the drift gathered up into the ice-sheet, borne along in its basal portion, and laid down when the ice in which it had been held was melted away. This englacial drift appears, therefore, to have been equivalent generally to a deposit 10 to 20 feet thick.
T h e Y a rm o u th interglacial Stage
During its maximum Kansan extension the east margin of the Kee- watin icefield probably coincided nearly with the present course of the
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Mississippi from the Wisconsin driftless area south to northeastern Mis souri. An ensuing return of a more temperate climate melted the Keewatin ice far back previous to the culmination of the Illinoian glacial stage, when an eastern icefield advanced into southeastern Iowa, cross ing the present valley of the Mississippi along a distance of over 100 miles and covering a tract in Iowa about 10 to 20 miles wide. Rem nants of a black soil, peat, branches of trees, and bones of the rabbit and skunk, found there on the Kansan drift, overlain by the western border of the Illinoian drift, show that for a considerable time this tract had a land surface before it was again covered by ice. This time, of glacial .recession from the farthest eastern limit of the Kansan glaciation, fol lowed by the most western advance of the Labradorian icefield in its Illi noian stage, is named by Leverett the Yarmouth interglacial stage, from Yarmouth, Iowa, about 20 miles northwest of Burlington. At that vil lage interglacial beds 43 feet in thickness were penetrated by a well, with 30 feet of overlying Illinoian till and 33 feet of underlying Ktosan till.
I l l i n o i a n G l a c i a t i o n
Although the glacial drift has a width of about 300 miles between the driftless area and the boundary of glaciation in Missouri and south ern Illinois, the Keewatin and Labradorian icefields were not confluent there. An ice-dammed lake would thus have been held on the large unglaciated area of southwestern Wisconsin and its smaller adjacent parts in Minnesota, Iowa, and Illinois, whether such confluence occurred during the Nebraskan stage or in Kansan and Illinoian time. The absence of lacustrine beaches and deltas testifies that the Mississippi flowed through the driftless area during all the Ice Age without being obstructed to form a lake. By the Illinoian ice-advance into Iowa when it attained its greatest westward extent, the Mississippi was displaced and caused to flow tempo rarily outside the ice-border, crossing the lower courses of the Maquoketa, Wapsipinicon, Cedar, Iowa, and Skunk rivers. Along this part of the boundary of Illinoian glaciation its drift is amassed in a wide but low marginal ridge, 40 to 60 feet above the adjoining surface. Stream erosion on the Kansan drift sheet has much exceeded its amount on the Illinoian drift, indicating, as estimated by Leverett, a greater interval between their times of deposition than from the latter to the present time. The difference in contour, however, may be partly due to an originally patchy condition and inequalities of the Kansan deposition for the outer region of attenuation of its icefield, in contrast
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with the more uniform sheet of drift laid down later from a thicker Illinoian ice-border, which could form a low terminal moraine. The Keewatin and Labradorian icefields are here regarded as very cer tainly attaining respectively their greatest southward and southwestward areas in one general stage of prevalent snowfall and ice accumulation, called west of the Mississippi the Kansan stage, and from this river east ward the Illinoian stage. After the melting of each of these icefields from large southern tracts of their maximum extension, the subsequent more northern drift formations of the mainly receding continental glacier, which yet at many times halted or readvanced, with much complexity of its outlines, drift sheets, and marginal moraines, have been referred to Iowan and Wisconsin stages of glaciation.- To the last belongs far the greater part of the area of the ice-sheet, including nearly all its surface deposits in Canada; but underlying the Wisconsin drift are seen on many tracts deposits referable to the preceding glacial and interglacial stages. Wherever the northern and central icefields may have remained un melted from thé earliest Nebraskan stage onward to the close of the Glacial period in the* Champlain stage, only a single' sheét of drift re sulted, such being probably true for much of the interior of Canada and for parts of New York and New England.'
: T h e Sangamon" interglacial Stage
An interval separating the times of formation of the Illinoian and Iowan drift, the latter being closely coincident with the chief loess deposi tion, is shown by an old soil and leached subsoil of the original Illinoian surface, and occasionally by scanty beds of peat, overlain by the loess. ,It was-named by Leverett in 1897 the Sangamon interglacial stage, from the county and river of this name in Illinois where such evidence of it was first observed. In a paper before cited, written in 1913, I referred the arid climate by which Lakes Bonneville and Lahontan were dried up, between their times of high water, to this Sangamon stage of interruption in the con tinental glaciation ; but such correlation in the records of these lakes and the Glacial period seems instead to belong surely with the very widely extended Aftonian recession of the ice-sheet. My erroneous study em bodied in that paper served as a stepping-stone to reach this conclusion, from which the general time relationships in the series of secular climatic changes during the Ice Age are, as I believe, rightly ascertained.
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I owan Glacia tion and L oess D epo sitio n
During the closing part- of Illinoian time and continuing into the Iowan stage, a gradual subsidence of the northern ice-laden half of this continent gave low gradients to the Mississippi and its tributaries. A temperate climate then returned upon the previously high and cold region adjoining the ice-border. Bains washed into the valleys much silt from erosion of the drift sheets and from the formerly englacial drift that became exposed on the surface of the melting and receding Iowan ice fields. Deep and wide floodplains of this alluvial silt, similar to the loess of the Rhine in Germany, whence it is so named, were spread along the Missouri, Mississippi, and Ohio valleys. Its time of most abundant deposition is marked by its northern boundary in northeastern Iowa, found by McGee to coincide with the margin of the icu-sheet at a stage of glaciation which has been named the Iowan stage. Within the area that was then ice-covered are many boulders, remaining from the en glacial and at last superglacial drift, its finer part having been carried away by the streams from the melting ice to be deposited as loess. Winds sweeping across the valleys bore far over the uplands a large portion of this fine silt from its floodpkins when the rivers were reduced in size, as they doubtless flowed in restricted channels for the greater part of each year. Thus the loess was spread as a general mantle, usually a few feet thick, upon nearly all the extensive area occupied by the Kansan and Illinoian drift.
T h e P eoeian interglacial S tage
After the principal deposition of loess and preceding the advance of a lobe of the Wisconsin icefields near Peoria, in Illinois, an interglacial surface of the loess there was exposed to weathering, which has caused the name Peorian to be given to that interval of some glacial recession and readvance. It was apparently of less duration or areal extent than the Yarmouth and Sangamon stages, though the lobation of the morainic border of the later drift in the Mississippi basin presents a very remark able change from the more regular previous boundaries of the ice-sheet. Loess continued to be plentifully deposited in northwestern Iowa until the time of formation of the outermost Altamont moraine, of Wisconsin age, which in 1880 I observed to be bordered on its west side along a distance of 75 miles, from Guthrie County northwestward to Storm Lake, by an expanse of loess as high as the crests of the morainic hills. The sheet of Wisconsin till east of the moraine is 50 to 75 feet lower than
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the loess, showing that the icefield there was a barrier preventing the loess from being spread over the lower area. Immediately after southern Illinois and Indiana were bared by the recession of the ice in late Illinoian and Sangamon time, the early loess there was being laid down, being contemporaneous in part with Illinoian till. It was more abundantly supplied during the Iowan glacial stage, and its later deposition was of the same age as the Altamont moraine.
W i s c o n s i n G l a c i a t i o n
The very diversified drift sheet of the latest grand division of the Ice Age was named in 1894 by Chamberlin for the State of Wisconsin, where in 1877 he mapped and described its interlobate Kettle moraine. In 1879 the present writer traced its terminal moraines on Long Island, Martha’s Vineyard, Nantucket, and Cape Cod. During the next fifteen years, in, the service of the Minnesota and United States Geological Surveys, I mapped the courses of twelve marginal moraines formed by the Wisconsin icefields at their maximum extension and at times of pause or readvance interrupting their general retreat in Minnesota, Iowa, and South and North Dakota. The scanty eroding action of the glacial currents while depositing the relatively smooth expanses of the outer and earlier drift, during each of the preceding stages, was strongly contrasted with the vigor of erosion displayed by the planed and striated bedrocks of many districts inclosed by the later moraines. When the ice-sheet heaped these morainic ridges and hills, it had worn into its adjacent bed of rock or of the old drift deposits, accomplishing much erosion on extensive tracts covered by the later and uneven Wisconsin drift, which has plentiful lakes and lakelets, reaching northward from the outermost large and continuous moraine and comprising the far greater part of our drift-bearing area. Drainage from the Wisconsin ice-border in its lobate course across the basin of the Mississippi was also more vigorous than when the loess was deposited. The land depression of that time had been succeeded by a moderate reelevation, giving to this interior region of the continent nearly its present height. Wide and deep channels in the loess flood plains were soon eroded by the rivers, which brought down and deposited much sand and gravel from the Wisconsin drift, their finer silt being then mostly borne far southward to the lower Mississippi or even to its delta. But the more temperate climate which had been restored, probably by a great subsidence of the continent after the Kansan and Illinoian glaciation, bringing at first much ice-melting and loess deposition, con-
XXXIV—B o l l . Geo l. S oc. A m ., V ol. 33, 1921
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tinned and was the principal cause of the lobation of the Wisconsin ice fields, their changing, stronger, and faster currents and their frequent accumulation of marginal moraines. The'edge of the ice was then melted more rapidly, on the average, than in any previous part of the Glacial period, so that it acquired generally a steeper gradient of the frontal slope, due to the predominantly marginal melting and to much snowfall upon the border for a width of many miles. Whenever the increased marginal currents carried the ice forward to its edge at a rate equalling or sometimes exceeding that of its melting there, the mainly receding boundary was held stationary or was even pushed forward during several or many years, permitting the englacial and superglacial drift to be amassed in conspicuous terminal and interlobate series of ridges, knolls, and hills. During this moraine-forming stage the southern portion of the ice- sheet from Lake Erie to North Dakota consisted of vast lobes, one of which reached from central and western Minnesota south to central Iowa. This lobe in its early maximum extent ended near Des Moines, and its margin was marked by the Altaniont moraine, the first and outer most in the series of twelve successive marginal moraines of this time which are found in Minnesota. When the second, or Gary, moraine was formed, it terminated on the south at Mineral. Ridge, in Boone County, Iowa. At the time of the third, or Antelope, moraine it had farther retreated to Forest City and Pilot Mound. The fourth, or Kiester, moraine was formed when the southern extremity of the ice-lobe had retreated across the south line of Minnesota and halted a few miles from it, in Freeborn and Fairbault counties of this State. The fifth, or Elysian, moraine, crossing southern Le Sueur County, marks the next halting place of the ice. At the time of forma tion of this moraine the south end of the lobe had been melted back 180 miles from its farthest extent, and its southwest side, which first restecf on the Coteau des Prairies, had retired 30 to 50 miles to the east side of Big Stone Lake and the east part of Yellow Medicine County. In its next recession this ice-lobe was melted away from the whole of Le Sueur County, and its southeast extremity was withdrawn to Waconia, in Carver County, where it again halted, forming its sixth, or Waconia, moraine. The seventh, or Dovre, moraine marks a pause in the retreat when the southeast end of the lobe lay on Kandiyohi County. Probably nearly all of the south half of Minnesota was at that time divested of. its ice mantle, but nearly all of the north half was yet ice-covered. The glacial boundary across the State had an approximately east to west
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course not far from Little Falls, where primitive men made quartz implements before the upper Mississippi was wholly uncovered from glaciation. By its next recessions the ice-border was withdrawn to the eighth, or Fergus Falls, moraine and the ninth, or Leaf Hills, moraine. These are merged together in the prominent accumulations of the Leaf Hills, which reach in a semicircle from Fergus Falls to the southeast, east, and. northeast, a distance of 50 miles, marking the southern limits of this ice-lobe when it terminated nearly due west of Little Falls and halfway between the south and north borders of Minnesota. Conspic uous moranic hills a few miles east of Little Falls and others in the north part of Morrison County and along its west side seem to be corre lated with the Fergus Falls moraine. Much of the modified drift of the Mississippi Valley at Little Falls was deposited when the ice-sheet terminated at these hills, 5 to 15 miles distant to the east, north, and west. When the tenth, or Itasca, moraine and the eleventh, or Mesabi, moraine were amassed, crossing the lake region at the head of the Mississippi, probably the gravel and sand of the modified drift were entirely deposited north of Little Falls. The twelfth, or Vermilion, moraine, the most northern mapped in Minnesota, skirting the south side of Net, Pelican, and Vermilion lakes, consists of relatively small drift hills and ridges, with very abundant boulders. Thence to the central gathering grounds of the ice-sheet, west, south, and east of Hud son Bay, doubtless many other moraines were formed by its wavering but mainly receding borders, which await discovery when the glacial drift of those wooded and very scantily inhabited regions shall be fully explored. Contemporaneously with the accumulation of the Dovre and later moraines, the Glacial Lake Agassiz was formed by the retreating Kee- watin ice-barrier in the basin of the Bed River and Lake Winnipeg, with outlet across the continental watershed and along the Minnesota and Mississippi rivers. The. earliest and highest Herman beach, much up lifted above its original level, has now, on the west side of this very large glacial lake, a northward ascent of about 35 feet in the first 75 miles north from its mouth, at Lake Traverse, about 60 feet in the second 75 miles, and about 80 feet in the next 74 miles to the inter national boundary. Farther to the north, in Manitoba, the rate of ascent of the early beaches is increased to two feet or more per mile, whereas the latest and lowest beaches are nearly horizontal. The uplift is found
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to have advanced like a wave from Minnesota and North Dakota north ward through Manitoba and northeast to Hudson Bay, permanently elevating the country as now, mostly about 500 feet above the height which it held when first uncovered by the late Wisconsin glacial melting. Soon after the ice-front receded, and while it yet continued to be the northern barrier of Lake Agassiz, the uplift along the whole extent of this ancient lake, more than 600 miles from south to north, was prac tically completed. The southern half of the lake area was first raised nearly to its present height; later its northern half was elevated, while the southern part received only slight increase of height; and last the. basin of Hudson Bay, near the center of the glaciated area of North America, has been raised from its Champlain marine submergence of 300 to 500 feet. A part of this elevation on the shores of Hudson Bay was shown by Dr. Robert Bell to have been very recent, and it is even probably still in progress. Nearly the same amount and a similar gradual advance of this Wis consin uplift, with continuation through Champlain time, are attested likewise by the northward ascent of the beaches of a very complex series of glacial lakes held in the basins of the present Great Lakes tributary to the Saint Lawrence by the barrier of the receding Labradorian icefield.
T h e C h a m p l a i n S t a g e
Subsidence of the heavily ice-laden northern half of our continent from its great preglacial elevation brought during the Sangamon inter glacial stage, and more remarkably at the ensuing Iowan stage of glacia tion, low slopes and very gentle currents of the Mississippi and its tribu taries, with restoration of a temperate climate at the ice-margin, permit ting much loess to be deposited along these valleys. Warm summers, favorable to the accumulation of marginal moraines, continued during the Wisconsin glaciation, although evidence of freer drainage from the ice-front, with deposition of much water-borne sand and gravel, indicates that in the Mississippi basin the maximum subsidence had been followed by a slight reelevation. Toward the close of the Ice Age and coincident with the existence of Lake Agassiz and the Laurentian glacial lakes, the moderate uplift of the glaciated region from its Sangamon and Iowan depression extended far ther north, advancing in general as fast as the ice-borders receded, but apparently it is not yet entirely completed. Indeed, considerable parts of the drift-bearing areas of North America and Scandinavia are still
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undergoing small and slow oscillatory movements, not having fully reached isostatic repose. That the land northward from Boston was lower than now while the ice-sheet was being melted away is proved by the occurrence of fossil mollusks of far northern range, including Yoldia (Leda) arctica Gray, which is now found living only in the Arctic seas, preferring localities that receive muddy streams from glaciers and from the Greenland ice- sheet. This species is plentiful in the stratified clays resting on the till in the Saint Lawrence Valley and in New Brunswick and Maine, extend ing southward to Portsmouth, New Hampshire. But it is known that the land was elevated from this depression to about its present height before the sea here became warm and before the southern mollusks, which exist as colonies in the Gulf of Saint Lawrence, migrated thither, for these southern species are not included in the extensive lists of the fossil fauna found in the b'eds overlying the till. In the Saint Lawrence Valley the marine deposits reach to the south ern end of Lake Champlain, whence the beds and this stage ending the Ice Age are named, to Ogdensburg and Brockville, and at least to Pem broke and Allumette Island, in the Ottawa River, about 75 miles above the citj of Ottawa. The isthmus of Chiegnecto, connecting Nova Scotia with New Brunswick, was submerged, and the sea extended 50 to 100 miles up the valleys of the chief rivers of Maine and New Brunswick. The uplift from the Champlain sealevel was 10 to 25 feet in the vicinity of Boston and northeastward to Cape Ann; about 150 feet near Ports mouth, New Hampshire; from 150 to about 300 feet along the coast of Maine and southern New Brunswick; about 40 feet on the northwestern shore of Nova Scotia, thence increasing westward to 200 feet in the Bay of Chaleurs, 375 feet in the Saint Lawrence Valley opposite the Saguenay, and about 560 feet at Montreal; 150 to 400 or 500 feet, increasing from south to north, along the basin of Lake Champlain; about 275 feet at Ogdensburg, and 450 feet near Ottawa. The differential elevation was practically completed, as we have seen from the boreal character of the Champlain marine molluscan fauna, shortly after the departure of the ice-sheet. With the areas of the glacial lakes in the interior of the. conti nent, this coastal region gives testimony of a wavelike elevation of the formerly ice-burdened portion of the earth’s crust, proportionate with the glacial melting and closely following the retreat of the ice from its bound aries of greatest extent inward to the areas on which its waning remnants lingered the latest. From the Champlain submergence' our Atlantic coast was raised some
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what higher than now, and its latest movement from New Jersey to south ern Greenland has been a moderate depression. The vertical amount of this Postglacial elevation above the present height, and of the recent subsidence, on the coast of New Jersey, New England, and the eastern provinces of Canada, is known to have ranged from 10 feet to a maximum of at least 80 feet at the head of the Bay of Fundy, attested in many places by stumps of forests, rooted where they grew, and by peat beds now submerged by the sea. As in Scandinavia, the restoration of isostatic equilibrium is attended by minor oscillations, the conditions requisite for repose having been overpassed by the early reelevation of outer portions of each of these great glaciated areas. The end of the Ice Age was not long ago, geologically speaking, for equilibrium of the disturbed areas has not yet been completely attained.
E s t i m a t e d T i m e R a t i o s
In correlating our continental glaciation with the two stages of high water of Lake Bonneville, as noted by Gilbert, Nebraskan time appears to have far exceeded the combined Kansan, Illiuoian, Iowan, and Wis consin glacial stages, in a ratio of not less than five to one.. The Afto- nian stage, when the border of the ice-sheet melted over wide areas, but afterward advanced even beyond its previous extent, probably coincided with the long time when Lake Bonneville was reduced by evaporation to an area similar to the Great Salt Lake, or may indeed have been wholly dried up, that time of desiccation having been longer than the Postglacial period. The latest Wisconsin stage, with the formation of its many recessional moraines, saw the ice-sheet gradually melted away. Since the retreat of its border from central Minnesota, the Mississippi River has eroded its gorge below the Falls of Saint Anthony, which Prof. N. II. Winchell, by comparing its recent rate of erosion with the length of the gorge, esti mated to have required about eight thousand years. Similarly the Niagara Falls and gorge supply evidence, according to G. Lv. Gilbert, Prof. G. F. Wright, and the present writer, that the northward recession of the ice-border uncovered the region of Lakes Erie and Ontario about seven thousand years ago. These measures of late Glacial and Post glacial time in America accord well with recent surveys by Baron De Geer in Sweden, where the last twelve thousand years have witnessed the melt ing of the latest part of the European ice-sheet. During the early part of the time represented by erosion of the Saint Anthony gorge, the later seven marginal moraines in the series of twelve
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mapped in Minnesota were accumulated, marking pauses or slight re advances in the wavering general recession of the glacial boundary. The seventh to the twelfth of these moraines were formed contemporaneously with the Glacial Lake Agassiz, besides probably numerous others farther north, beyond the twelfth, or Vermilion, moraine. But the entire dura tion of Lake Agassiz, estimated from the ratio between the erosion and beach deposits of its shores and those of the Great Lakes tributary to the Saint Lawrence, can not comprise more than two thousand years, the ratio being certainly no greater than as one to four or five. Therefore it may be inferred that the Wisconsin stage of glaciation, from its earliest Altamont moraine to the end of the existence of Lake Agassiz, did not exceed four or five thousand years, having begun about ten thousand years ago. The principal uplift of the continent from its subsidence beneath the weight of the ice-sheet took place while Lake Agassiz yet existed, the Champlain submergence being thus mainly terminated at the same time with the melting of the latest remnants of the vast continental icefields. Recurring to Gilbert’s estimate of the ratio for the high stages of Lake Bonneville, we may refer the Kansan and later stages of the ice-sheet, with the Yarmouth, Sangamon, and Peorian recessions and readvances of the glacial boundary, to a relatively late part of the Ice Age, during the second high rise of that lake, measured probably by about 40,000 years and ending nearly 5,000 years ago. Under this view the maximum area of the Kansan icefield was perhaps attained, as thought by Leverett, twice as long ago as the farthest advance of the Ulinoian icefield, which over lapped the edge of the Kansan drift. The successive farthest limits of the ice-border in these stages, noted approximately, may have been about 40,000, 20,000, 12,000, and 10,000 years ago, respectively, for the Kansan, lllinoian, Iowan, and Wisconsin glaciation. The long Aftonian interval, bringing far retreat of the outer part of the ice-sheet, was probably about 50,000 years ago. Earlier the very pro longed Nebraskan glaciation, correlated with the first great rise of Lake Bonneville, appears to have extended through fully 150,000 years. Strict acceptance, however, of the ratio from the lacustrine record, as in the proportion of five to one, would give even 200,000 years, on this pro visional time scale, for the formation of the Nebraskan drift sheet, ac counting very amply for its observed thickness beneath the Aftonian beds and the later glacial drift in Iowa and southern Minnesota. Within the first half or third of the Nebraskan stage very thick ice accumulation southwest of Hudson and James bays, upon the present area of Patricia, a district of Canada, caused currents of glaciation tu
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reach thence southward, over the basins of Lakes Huron and Michigan, nearly or quite to the Ohio River and westward across Minnesota. Through all the later and longer part of the Ice Age the Labradorian and Keewatin glacial currents passed transversely across those early courses of dispersion of Patrician drift.
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