Introduction

The Quaternary Period of the Cenozoic Era includes the time from about 2 Ma to the present. The interval of time from about 2 Ma to 10,000 years b.p. is called the Pleistocene Epoch, and includes the history of the great continental ice sheet in North America. Nearly all the landscape of Minnesota was developed during this time, either by glacial erosion and deposition, or by stream erosion and deposition. That landscape development continues through the Holocene Epoch to the present (10,000 years b.p. to now).

Glaciation

• Formation
  • permanent snowfields build up as the result of climatic cooling in latitudes/altitudes of sufficient precipitation
  • snowflakes at the bottom of a snowfield are converted to balls of ice called firn
  • more pressure causes firn to be recrystallized into a large-scale puzzle-like mosaic to form glacial ice, anlogous to metamorphism of sediment brought about by increased pressure
  • when glacial ice builds up to sufficient thickness, it will deform under its own weight and will flow.
  • body of ice that shows evidence of present or former movement is then called a glacier
• Glacial Budget
  • snowline=elevation at which permanent snowfields exist
  • area above snowline=zone of accumulation
  • area below snowline=zone of melting or wastage
  • if area of accumulation for a glacier is greater than zone of melting, the glacier's front will advance
  • if area of accumulation is less than zone of melting, the glacier's front will retreat
  • no matter whether a glacier's front is advancing or retreating, the actual flow of ice within the glacier will always be downslope, or outward from the center of thickness for an ice sheet - the ice will never flow uphill
• Zones of different behavior of glacial ice
  • brittle or rigid zone includes the upper 50' or so of the glacier, where the ice behaves as a brittle solid and will fracture to form crevasses
  • at depth, there is enough pressure for ice to deform under its own weight and behave like a plastic solid. Ice in this plastic zone will flow at rates of feet to tens of feet per year
  • flow of glacial ice will be downslope or outward from the center of maximum thickness of the ice sheet
  • flow of ice will transport sediment through the glacier, like a giant conveyor belt, and carry the sediment to the surface and margins of the ice sheet

Glacial history in North America

• History of the advance and retreat of the Laurentide Ice Sheet, and more isolated mountain ice caps and valley glaciers that existed in the mountains of the western third of the continent.
  • The Laurentide Ice Sheet was part of a larger ice cap that included the Scandinavian Ice Sheet
  • Evidence of several major advances and retreats, and many many minor advances and retreats, is present in Canada and the U.S.
  • The most recent major advance reached its maximum about 18,000 years ago. Rapid retreat followed.
  • The Laurentide Ice Sheet has almost disappeared. The Greenland Ice Sheet survives today but is in active retreat.
  • Greenland 1
  • Greenland 2
  • Vatnajokull in Iceland is another good example of a continental ice sheet
• Glacial Erosion
  • Landscapes in northeastern Minnesota were shaped largely by glacial erosion. In this area, which was closer to the source of the ice, glaciers were scouring the bedrock, creating basins in more easily erodible material, and leaving behind knobs or ridges in more resistant material. Many of the basins have been occupied by lakes since the retreat of the ice.
  • plucking
    • frost wedging in fractured bedrock beneath the glacier will loosen blocks or fragments of rocks
    • loosened rocks will be frozen into the base of the glacier and can then be transported by flow of the ice to the glacier's margins
  • abrasion
    • rocks frozen into the glacier act as sandpaper or a file to scrape the bedrock beneath
    • abrasion produces glacial polish and striations in underlying bedrock
    • striations are long scratches in the bedrock, which can be used to indicate the sense of ice movement
  • landforms produced by erosion
    • bedrock of different resistance will be eroded at different rates by the ice. Easily eroded rocks will form low places, while less easily eroded rocks will form high places
    • knob and basin topography will result. This is the sort of topography that dominates the arrowhead region of MN, for example, in the Boundary Waters Canoe Area Wilderness
    • basins formed by glacial erosion are often occupied by lakes
• Glacial Deposition
  • Most of the landscape of Minnesota has been shaped by glacial deposition.
    • Where glaciers deposited sediment beneath the ice, gently rolling topography is developed. Lakes may be present in broad depressions.
    • Where glaciers piled up sediment around their edges, irregular hummocky topography is developed on broad ridges called moraines. The low places (kettles) are often occupied by lakes
    • Where water draining the ice produced broad floodplains, relatively flat topography with sometimes visible dry channels and terraces is present.
    • Where lakes filled by meltwater covered large areas in front of the glaciers, flat lake plains are developed in clay deposits.
    • Where the wind carried sand and silt beyond the edges of the ice, sand dunes were formed near to the source, and a covering of fine silt mantled the surface far from the source.
  • sediment carried by the ice will be deposited upon retreat of the glacier to form glacial till
  • Till is material deposited directly by the ice
    • because is is very viscous (not very fluid), and because sediment cannot settle through the ice,
      • till is either not layered or stratified, or is very weakly layered or stratified
      • till does not have gravel-sized particiles in contact with one another throughout the deposit
      • till is not well sorted, but rather has a wide range of particle sizes
    • examples of till from Rochester and the Boundary Waters Canoe Area show several of these characteristics. Till from the North Shore of Lake Superior shows the typical stony character of glacial deposits of the Superior Lobe.
    • erratics are boulders that are out of place, having been carried great distances from their original sources to be deposited on bedrock of a different type, such as this Sioux Quartzite erratic sitting atop an area of Cretaceous sediments in South Dakota
    • the composition of till depends on the rocks that serve as its source. Therefore, one can use till composition to tell from where a glacier came as it advanced over a particular area
    • Landforms developed on till
      • ridges formed by deposition of till at the end or sides of a glacier are called end or lateral moraines. Here is a photo of the margin of a glacier in Iceland, showing the active development of an end moraine
        • These moraines may be broad and very irregular in topography, having numerous hills and depressions present, such as this moraine in Glacial Lakes State Park in western Minnesota.
        • Lakes often occupy the depressions.
        • These lakes in depressions within and between end moraines may be quite large, such as Lake Millelacs in north central Minnesota.
        • Moraines on the Couteau de Missouri that extends from SW Minnesota to the northwest through the Dakotas are often well developed.
      • sheets of till deposited at the base of a glacier form ground moraine
        • Ground moraine is a gently undulating or rolling to hummocky surface covering a large area.
        • Glacial boulders carried from far off sources are often present on the gently rolling surface.
        • Ground moraine can have kettles with lakes present, where blocks of ice frozen into the till eventually melt to form depressions.
        • This is especially true when the ice becomes stagnant and no longer flows, because the ice sheet has become too thin.
        • Here is a good example of ground moraine with kettles in Saskatchewan
      • in areas of ground moraine, glaciers may streamline and shape and sculpt the till and create elongate hills called drumlins. The orientation of drumlins can be used to determine the direction of ice movement. The steep side of the drumlin faces in the direction from which the ice came.
        • drumlins in Wisconsin
        • drumlin in Alberta
        • aerial view of drumlin field
  • Outwash or glacio-fluvial sediment
    • deposited by meltwater in braided rivers draining the ice sheet such as this example from Iceland. (Braided refers to the intertwining or splitting and rejoining of many channels in the river, separated by sand and gravel bars.)
    • outwash is layered or stratified because it is laid down in a fluid medium. The deposits are often cross bedded.
    • outwash is well sorted because it is washed by the flow of the water
    • outwash is composted of sand and gravel, and the particles are all in contact with one another, with no mud in between them, unlike till
    • Landforms developed on outwash
      • Outwash spreads out beyond the till in the form of a broad outwash plain, created by the merging of the floodplains of many braided streams draining the ice. Long after the glacier melts, these plains are criss-crossed by shallow abandoned channels and may also be pock-marked by depressions or kettles formed by ice blocks which were rafted into the outwash, then melted away
      • outwash may exist in more narrow valleys which cut through the sheets of till, in which case the resulting landform is referred to as a valley train. The Mississippi Valley in the Winona area contains glacial outwash in the form of a valley train.
      • Outwash plains and valley trains generally have different terrace levels developed along their margins, reflecting formation of floodplains and downcutting of the rivers into those floodplains as the volume of meltwater changes through time. Terraces along the Mississippi River between Winona and Lake City are especially well-developed, and many have gravel pits that mine the outwash deposits.
        • Terrace near Weaver, MN
        • Terrace south of Kellogg, MN
        • Terrace surface south of Kellogg, MN
        • Two terrace levels at Kellogg, MN
        • Terrace at Wabasha, MN
        • Gravel pit in upper terrace at Wabasha, MN
        • Two terrace levels just north of Lake City, MN
      • Eskers are elongate s-shaped or sinuous ridges in areas of ground moraine, formed by deposition of glaciofluvial material in tunnels in the ice. When the glacier melts, the deposits that fill the tunnels are let down onto the surface as ridges.
      • Kames are cone-shaped hills in areas of ground moraine, formed when depressions in the glacier are filled with glacio-fluvial sediment (outwash). When the ice melts, the sediment filling these depressions is let down onto the surface as a pile of sediment forming the kame.
  • Wind-blown silt (loess)
    • The surface of glacial deposits such as outwash, or till recently exposed by retreating ice, is subject to considerable wind erosion, because vegetation is not present to stabilize the material. Sand is reworked into sand dunes near to the source of the sediment. Silt, on the other hand, is blown for great distances by the prevailing wind, and is deposited as a blanket over widespread areas.
    • This blanket of loess may reach 10's of feet thickness in Minnesota, and generally produces a subdued effect in the topography, much like new-fallen snow will smooth out an irregular surface. In southeastern Minnesota, the loess cover reaches as much as 10 feet in thickness, and helps to produce a relatively rich soil.
    • Where thick loess deposits are exposed along major rivers, such as the Missouri River in the vicinity of Omaha, Nebraska and Council Bluffs, Iowa, steep bluffs are formed. The fine silt is very cohesive, helping the particles to hold together to create such steep slopes.
• Maps of glacial deposits and landforms reveal the former extent of ice sheets.

The Glacial Theory

In 1837, Louis Agassiz proposed that the exotic boulders strewn across Europe, together with striated bedrock, were the result of glaciers which have since melted. Agassiz based his idea on observations of the effects of modern glaciers on the landscape of the Swiss Alps.

In 1872, Winchell, the first head of the Minnesota Geological Survey, began a program of mapping glacial deposits in Minnesota, together with Warren Upham, a New England glacial geologist. By 1883, they outlined the extent of the major moraines and lobes of ice that advanced into Minnesota from the Laurentide Ice Sheet which was centered over Hudson Bay in Canada. They were also able to place Minnesota's glacial deposits into a framework of glacial history for the mid-continent of North America.

Vertical sequences of glacial strata in North America indicate four major advances and retreats of the Laurentide Ice Sheet. These advances and retreats are given names in the calendar of the Pleistocene Epoch

• modern interglaciation
• Wisconsinan glaciation
  • Sangamon interglaciation
• Illinoian glaciation
  • Yarmouth interglaciation
• Kansan glaciation
  • Aftonian interglaciation
• Nebraskan glaciation

Modern studies of glacial strata suggest that even this calendar must be revised, as more complicated vertical sequences of interbedded glacial and non-glacial deposits are being discovered and integrated into the framework of glacial history and climate change.

Minnesota's Glacial History

Minnesota's landscape and the landscape of adjacent Wisconsin owe their origin to the brush strokes that glaciation put on the canvas of the bedrock. Even where glacial ice was not primarily responsible for development of the landscape, such as in southeastern Minnesota, climate change and effects of the nearby ice, such as meltwater from the ice sheet, profoundly affected the topography.

The Laurentide Ice Sheet, centered over Hudson Bay in Canada, grew in size and shrank with alternating cooling and warming of the climate during the Pleistocene. Cooling a warming cycles appear to have been caused by cyclic changes in the ellipticity of the earth's orbit about the sun, together with changes in the tilt of the earth's rotational axis and the degree of wobble of the rotational axis. The ice sheet formed in a climatic belt where not only was cold weather the rule, but also where sufficient precipitation was present to form the glaciers which coalesced to form the ice sheet. The ice advanced outward from the maximum center of thickness of the ice sheet, and in the US, that direction was generally from north to south. Lobes or tongues of ice advance outward at the edge of the ice sheet and moved through lowlands which channeled the flow.

• Boundaries of ice advance in the mid-continent
  • Nebraskan and Kansan glaciations were primarily western glaciations, advancing southward into Missouri, Nebraska and northeast Kansas
  • Illinoian glaciation reached as far south as the southern tip of Illinois
  • Wisconsin glaciation was the least extensive advance, but because it is the youngest, it is at the surface throughout most of the midcontinent and the northeast
  • Minnesota was affected by all four glacial advances, but in southeastern Minnesota, the Wisconsinan ice never covered the area. Instead, it was diverted by an upland to the north in Wisconsin, and because the ice sheet wasn't thick enough to advance up over the upland, it flowed around it, leaving southeastern Minnesota and western Wisconsin as an island in the ice. Because older glacial sediments have been removed from this area, and because no young glacial deposits were ever present, this area is referred to as the "driftless area". Winona is located within the driftless area. Note that the topography of the driftless area is very different from that of adjacent areas covered by Wisconsinan glacial deposits. All one has to do is to drive west from Winona to Rochester to see how the deeply eroded landscape of the driftless area gives way to the flatter till plains surrounding it.
  • Older deposits are less extensive, having been been removed by erosion, or covered by younger deposits.
• Effects of Minnesota's bedrock on pattern of glaciation
  • previous stream erosion topography will determine the direction of ice advance at the margins of the ice sheet
  • till deposits will be thicker along these valley systems. In fact, you can use the thickness pattern of glacial deposits to map the pattern of preglacial river valleys.
  • kind of bedrock being eroded by the glacier will determine the composition of the till - different rock types in the till can then be traced back to the source from which the glacier advanced
  • Two major topographic lows or troughs channeled the pattern of ice advance in Minnesota
    • Superior-Minneapolis Lowland
      • channeled a lobe of ice from the Lake Superior region south and west across Minnesota
      • this lowland underlain by non-resistant rocks of deposited in the mid-continent rift
    • Red River - Minnesota River Lowland
      • channeled a lobe of ice largely from the north in the area of Winnepeg, Manitoba
      • underlain mainly by easily eroded Cretaceous and Paleozoic sediments and weathered Precambrian rocks with plateaus of more resistant rocks on its flanks
    • Lobes of ice at the margins of the ice sheet developed in these lowlands
      • The lobes can be distinguished from one another on the basis of the composition of their tills and the landforms developed on the deposits such as drumlins, which indicate the direction of advance of the ice from its source area
      • Superior and Rainy Lobes advanced from northeastern Minnesota in the area of the Superior Basin and produced deposits of reddish-brown to dark brown or black bouldery, coarse-grained till rich in fragments of igneous and metamorphic rocks and iron formations
      • Wadena and Des Moines Lobes advanced from northwestern Minnesota to the south and produced deposits of tan to yellow-brown till dominated by Paleozoic and Cretaceous sedimentary rock fragments
      • Careful mapping of tills and their rock fragments and landforms enables geologists to reconstruct the pattern and history of glacial advance and retreat across Minnesota
• Nebraskan, Kansan and Illinoian Glaciations
  • Details sketchy because of burial beneath youngest Wisconsinan till, or because of removal by erosion
  • erratics from Minnesota have been found as far south as Topeka, Kansas, so Minnesota was glaciated during this time
  • ice thickness over Minnesota probably reached more than 3,000' !
  • southeastern and southwestern Minnesota lack glacial lakes beyond the edges of the Wisconsinan moraines. These are areas of older glacial deposits but the landscape does not bear the imprint of glaciation.
    • streams have become well-established, draining the land
    • topography consists of hills and intervening stream valleys, indicative of stream erosion of the older tills which must have covered this area and which are still present in scattered localities
    • valleys have bedrock outcrops along their sides
    • thin blanket of loess covers the bedrock and the thin scattered tills with scattered erratics. Loess is wind-blown silt, derived from outwash plains during more arid interglacial episodes and blown over the landscape in great dust storms.
    • Dates of organic material in the loess suggests conditions like this prevailed from 29,000-14,500 years b.p.
  • tills of older glaciations primarily derived from sedimentary rock sources feeding glaciers which advanced through the Red River-Minesota River Lowland
  • dates from organic remains in these tills are older than 40,000 years and volcanic ash in equivalent tills from areas west and south of Minnesota indicate ages of 1.2 million years with still older till beneath. Tills usually assigned to Nebraskan and younger glaciations are all above ash layers yielding dates from 600,000-700,000 years. Considerable study is going on to develop revised calendars of glacial history in the mid-continent and the final chapter in the story may never be fully written
  • vertical sections of interlayered glacial deposits along the Minnesota River Valley near Redwood Falls show a complex glacial history and attest to the ancient character of the older tills
    • two glacial tills separated by an interglacial deposit of sandy outwash lie below a 40,000 year old bog deposit
    • one of the tills was deeply weathered by chemical processes during interglacial time to form a thick ancient soil where even the most resistant rock fragments have turned to clay.
  • old tills raise many questions: How long does it take to form an ice sheet? How much time is encompassed by the advance and retreat of an ice sheet? How long does an interglacial period last? How long does it take for an ancient soil to form? How much till was once present and how much has been removed by erosion during interglacial time? This is an exciting area of continuing study, all facilitated by careful mapping of glacial deposits and their landforms.
• Wisconsinan Glaciation
  • 75,000 yrs.b.p. climate cooled and Laurentide Ice Sheet advanced
  • from 75,000 to 12,000 yrs.b.p., ice sheet fluctuated in position as the result of smaller-scale warming-cooling cycles, and Wisconsinan ice advanced and retreated many times over Minnesota.
  • Wisconsinan advances and retreats of lobes of ice from the advancing and retreating edge of the Laurentide ice sheet resulted in the glacial sediment and glacial landforms present today at the surface across the state of Minnesota
  • Vertical sections of glacial and interglacial sediments of Wisconsinan age attest to these fluctuations (Hawk Creek in Renville County is a good example). Intervals between glacial advances are represented by ancient soils (paleosols) or lacustrine sediments or concentrations of stony material at the top of a till sheet (stone lines).
    • four tills appear at many localities throughout Minnesota
    • note fluctuation in lobes and source areas that contributed sediment to the record.
    • Oldest till deposited by the Wadena lobe moving southward from the Winnipeg area through the Red River Valley, and lies beneath sediments dated at 40,000 years (see advance 1, accompanying figure)
    • Second oldest till deposited by Superior lobe moving west and soutwest from the Superior Lowland (see advance 2, accompanying figure).
      • older than 40,000 years
      • igneous and metamorphic rock fragments
      • landforms including moraines largely obliterated by later ice advances
      • only surface exposures are in Dakota County south of the Twin Cities beyond the Late Wisconsinan moraines
    • Third oldest till of the Wadena Lobe made of sandy material with limestone rock fragments, derived from sedimentary rocks in the Winnipeg area (see advance 3, accompanying figure). No Cretaceous shale indicates no ice from the west. This till lies above lacustrine sediments in the Hawk Creek section.
      • buried everywhere by younger till, but exposed in open pits in the Iron Range, in quarries and river valleys in the Twin Cities area, and along the Minnesota River Valley. Also found in the eastern Dakotas.
      • C14 dates indicate ages greater than 34,000 years and perhaps as old as 72,000
      • Slight retreat and readvance formed major landform of the Alexandria moraine (see advance 4, accompanying figure). This moraine was overridden by later advances, but owes its great height to this Wadena readvance. Behind the moraine lies a major drumlin field showing radiating flow of the ice to the south and sto the outhwest and to the west. When flow stopped and the ice stagnated, the sediment-covered ice margin collapsed, forming a broad and irregular and hummocky moraine with many lakes in the depressions between the hummocks. Note that this readvance is not represented by a till in the Hawk Creek section.
    • A later advance of the Superior-Rainy Lobe is also not recorded in the Hawk Creek section. This advance marks a shift in the centers of accumulation of snow and ice in the Laurentide Ice Sheet, so that now once again, ice advanced from northeastern Minnesota through the Superior Lowland (see advance 5, accompanying figure).
      • Began about 30,000 years ago to about 15,000 years ago.
      • Produced many of the surface landforms of central and northeastern Minnesota
      • Ice advanced, stopped, then deteriorated and stagnated and disappeared
      • Lobe joined with the thiner Wadena Lobe to the west
      • maximum advance marked by the St. Croix moraine, together with the Itasca moraine of the Wadena Lobe. The St. Croix moraine is a ribbon of a lake-dotted hummocky ridge about 250 miles long. Drumlin fields are present behind the moraine especially in the Brainerd area.
      • During stagnation of the ice, a network of tunnels developed at the base of the ice and eskers were eventually formed by the sediment deposited in the valleys carved at the base of the tunnels
      • Retreat of the Superior Lobe created a series of nested moraines east from the St. Croix moraine back to the east into the Superior Basin
    • About the time the Superior-Rainy Lobe was deteriorating, Ice in the Winnipeg area was advancing down the Red River Lowland and the Minnesota River Valley. This lobe, called the Des Moines Lobe, reached central Iowa by 14,000 years b.p. Offshoots or sublobes of the Des Moines lobe advanced into the area left bare by the wasting of the Superior Lobe (see advance 6, accompanying figure).
      • Grantsburg sublobe moved northeast into the Twin Cities Basin, breaking through part of the St. Croix moraine, and blocking south-flowing drainage to form Glacial Lake Grantsburg.
      • 12,000 years b.p., St. Louis sublobe advanced eastward into Superior Basin
      • disintegration of Des Moines Lobe left a distinctive fine-grained till dominated by Paleozoic limestone and Cretaceous Shale fragments. This till is at the surface of most of western and southern Minnesota and forms the parent material for the rich soils in these areas.
      • Two important moraines were formed by the Des Moines Lobe - the Bemis moraine marks the maximum advance, and the Altamont moraine marks a recessional standstill.
      • Ice melted back into the Red River Lowand by 12,000 years b.p. and no more ice was to cover Minnesota
• Glacial Retreat at end of Wisconsinan
  • 13,000 years b.p. marks beginning of climatic warming
  • sea level began to rise, all told, a total of about 400 feet!
  • meltwater flooded the mid-continent and an intricate drainage system of braided outwash streams flowed across Minnesota
  • glacial retreat produced a series of nested recessional moraines
  • ice stagnation produced slow melting of the so-called "dead ice" with large amounts of sediment covering its surface. When this sediment was finally "let down" onto the surface of the ground, it produced very irregular topography called "kame and kettle topography" with the kettles filled by numerous lakes
  • glacial retreat was rapid - terminus in central Iowa 14,000 years ago, and ice disappeared up the Red River Valley into Canada by 12,000 years ago. And by 10,000 years ago, ice was also gone from the Superior Basin. By 8,000 years ago, ice bergs were calving into Hudson Bay
• Glacial Lakes
  • Patterns of meltwater flow were influenced by morainal or drift-covered bedrock topography
  • during glacial retreat, front of retreating glacier acts as dam, ponding water between the moraine and the retreating ice front to form glacial lakes
  • water was ponded behind the Bemis moraine and the southwestern side of the Des Moines Lobe as it retreated and diminished in size
  • lakes formed in the Minnesota River Valley lowland as the Des Moines retreated to the north. These lakes were called Lake Minnesota
  • when Des Moines Lobe melted north across a drainage divide in the topography near Browns Valley in western Minnesota, water began to pond between the divide and the ice front to the north, forming the beginnings of Glacial Lake Agassiz
    • lake began to grow in size around 12,000 years b.p.
    • evidence in form of ancient shorelines and muddy laminated lake sediments is present over an area of 320,000 square kilometers
    • many different lake levels are recorded in sandy beach deposits which form ridges in the otherwise flat muddy lake sediments. These levels indicate that the size of the lake fluctuated. The surface of the lake at any one time did not cover more than 128,000 square kilometers, and was about 400 feet deep at its maximum.
    • When lake levels reached the elevation of the moraine dam at Browns Valley, an outlet drained lake water through River Warren, a torrent of water that flowed through the present Minnesota River Valley.
      • potholes were eroded into the granite bedrock of the area
        • potholes 1
        • potholes 2
    • When lower outlets were uncovered by the retreating ice, River Warren ceased to flow, and the present Minnesota River was developed, with a head at the divide near Browns Valley
    • Beach ridges of individual shorelines of Lake Agassiz rise in elevation to the north. They were original horizontal for any one lake level, so this means that the ground must have been tilted after glaciation. This is the result of rebound of the crust due to unloading of the surface by melting of the ice. Rebound is greatest where ice was thickest, hence areas to the north rose more, also uplifting beach ridges in this area
  • in northeast Minnesota, Lakes Aitkin and Upham ponded at the front of the retreating St. Louis sublobe, and Glacial Lake Duluth formed between the edge of the Superior Basin and the Superior Lobe about 12,000 years ago
    • preceded the development of Lake Superior
    • drained southward through the St. Croix River Valley into the Mississippi
      • The Dalles of the St. Croix River were carved by this sudden outburst flood of great magnitude
      • Potholes were also eroded into the basalt bedrock of the St. Croix Valley, and are well-exposed in Interstate State Park at Taylors Falls.
    • further ice retreat uncovered lower outlets to the east and ancestral Lake Superior began to drain toward the St. Lawrence Seaway
    • the lake at this point was about 200' lowere than today, but glacial rebound has resulted in its rise to its present position.
  • The formation of the Great Lakes is the result of the advances and retreats of the Pleistocene ice.
    • Lobes of glacial ice deepened already-existing lowlands where the Great Lakes are now located.
      • distrubution of lobes 14,000 years ago
      • distrbution of lobes 10,500 years ago
    • The history of development of the lakes is a history of rising and falling water levels related to fluctuations of the ice sheet.
      • 18,000 years BP
      • 14,000 years BP
      • 12,000 years BP
      • 10,000 years BP
      • 8,000 years BP
• River Terraces
  • The discharge or the volume of water flowing through a river in a given interval of time changes because of variables that are independent of the river itself.
    • For example, more runoff from storms in the drainage basin will cause discharge to increase.
    • On a larger scale, climate change will cause changes in discharge. For example, if climate changes from a glacial period to an interglacial period, discharge of river systems draining the ice will increase because of large volumes of meltwater available to the streams.
  • When discharge increases, a river has greater power to cut down through sediments formerly deposited on the floodplain. And so a river begins to erode or cut down into its channel and floodplain rather than deposit sediment in the channel and floodplain. This downcutting will leave the former floodplain standing high above the river and terraces will be created
  • River terraces of the Mississippi at Pine Bend between St. Paul and Hastings
    • terraces at 825 and 875' above sea level. Modern floodplain at 705 feet above sea level
    • sand and gravel deposits underlie the terraces
    • borings into the deposits reveal that the river has downcut and filled and downcut and filled its valley several times, indicating change in the discharge through glacial and interglacial periods
    • gravel of the highest terrace traceable to the St. Croix moraine of the Superior Lobe. Rock fragments in terrace gravels similar to those in the till
    • retreat of Superior Lobe resulted in higher discharge in the Mississippi River and downcutting occurred (stage 3 of accompanying diagram).
    • Renewed activity in the Des Moines Lobe brought new outwash fill into the valley (stage 3 of the accompanying diagram). It was at this time when drainage was blocked, resulting in formation of Lake Grantsburg.
    • retreat of the Des Moines Lobe 14,000 years b.p. , resulted in more downcutting, forming the Des Moines Lobe outwash terrace (stage 4 in the accompanying diagram).
    • draining of Lake Agassiz and development of Glacial River Warren then cut down into the pile of fill to create a deep gorge (stage 4 in the accompanying diagram).
    • By 9,000 yrs. b.p. Lake Agassiz was drained, and the sediment brought into the Mississippi valley could not be carried away by the now lowered discharges. So, the river began to deposit valley fill (alluvium), resulting in the present course of the river (stage 5, accompanying diagram)
  • Ponding along the Mississippi River to form lakes such as Lake Pepin
    • where tributaries entered the Mississippi in post-glacial time, small deltas were deposited, ponding the river and creating lakes upstream
      • Aerial photo 1 showing delta of Chippewa River
      • Aerial photo 2 showing delta of Chippewa River
      • Islands developed on delta of Chippewa River at south end of Lake Pepin
      • South end of Lake Pepin and beginning of delta
      • Lake Pepin, dammed by the delta of the Chippewa River
    • Lake Pepin once extended upstream as far as St. Paul because of the large amount of sediment deposited in the delta of the Chippewa River near Wabasha. Downstream advance of a delta at the confluence of the Minnesota and Mississippi Rivers near St. Paul has shortened the length of Lake Pepin. Eventually, the lake will be completely filled in.