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River of History

A Historic Resources Study of the Mississippi National River and Recreation Area 557016_Book_r 8/5/03 7:57 AM Page 20

FIGURE 1. What the Glacial River Warren Falls might have looked like in St. Paul 12,000 years ago. Gustav Grunewal, Horseshoe Falls from below High Bank. Fine Arts

Museums of San Francisco. Gift of John Davis Hatch, V,in memory of John Davis Hatch, A.I.A., architect of San Francisco, 1996.52.2. 557016_Book_r 8/5/03 7:57 AM Page 21

Chapter 1 The Geology of the MNRRA Corridor

Thomas Madigan • Hemisphere Field Services, Inc.

revealing a geologic history spanning over 500 million years. Spectacular bedrock bluffs are common along the river between St. Anthony Falls and Hastings. The Crow River, which marks the corridor’s northern boundary, occu- pies an ancient glacial river channel that drained into the his chapter is about foundations. As subse- Mississippi. Between Dayton and Minneapolis, the river quent chapters show, landforms created thou- has developed on thick layers of sediment deposited during sands to millions of years ago shaped the cor- the last glacial era. Glacial sediment borders the river south Tridor’s development. Native American villages, early of Dayton and large deposits of sand and gravel form flat- American settlements, milling sites, locks and dams, rail- lying terraces along both sides of the river south to the con- roads, roads and modern urban expansion overlay or fluence of the Minnesota River. Below Minneapolis, the responded to ancient geologic processes. A cursory glance at Mississippi is cut into flat-lying, 570- to 450-million-year- the present-day Mississippi River valley reveals that the geo- old Paleozoic sedimentary rocks. Throughout the stretch logic processes operating during the valley’s formation were from Dayton to the Minnesota River, the valley is relatively much different from those of today. This chapter summa- narrow and floodplain development limited. rizes the current knowledge regarding the physical history Near Fort Snelling, the Minnesota and Mississippi of the Mississippi River valley. It includes a brief introduc- Rivers join, and consequently the Mississippi valley tion to some of the early studies of the valley’s geology, an becomes much wider. Glacial River Warren, predecessor to overview of the valley’s geologic configuration, and a sum- the Minnesota River, carved out the river’s wide valley, as it mary of geologic events responsible for the valley’s appear- carried the meltwater pouring from glacial Lake Agassiz, ance today. between 11,800 and 9,200 years before the present (B.P.). The Mississippi River winds more than 2,300 miles Since that time, sand, silt, and clay have been filling the val- across the heart of the nation on its course to the Gulf of ley, forming a complex mosaic of landforms across the Mexico. In Minnesota, the river flows over 660 miles from floodplain. its source at Lake Itasca through bogs and spruce forests in Downstream from the confluence, the Mississippi the glaciated northern region, across fertile agricultural heads northeast toward downtown St. Paul, bordered on fields in the central portion of the state, then southeastward through scenic bluff country. Along this course the river’s character varies dramatically, due to the geologic events. The Mississippi River, within the MNRRA corridor (Figure 2),cuts through a sequence of sedimentary rocks,

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RIVER OF HISTORY: A Historic Resources Study of the Mississippi National River and Recreation Area in 2.TheFIGURE Missis en wide and turnssouthward. arc the riverAt thispoint typical ofarecently glaciated landscape.and lakes, hummocky topography containing numerous depressions the however, glacial form deposits abelt of south, Going the braided unlike outwash types upstream. occurring face, ments and are terraces thin developed over the bedrock sur- the bedrock subsurface asseverely. glacial sedi- Therefore, ab bo d ofthethis part river valley filled withsand and gravel l si the valley due to ero- widensCottage dramatically, Grove, the ancient bedrock. As the river southward winds toward ast glacial maximum(the farthest the glaciers advanced), eposits forming abroad forming eposits level surface atanelevation of

on that occurred before the the last glaciation. During the Twin metropolitan Cities area. ov te th sides by glacial terraces more lying than 100feet rs h lopan I hsrah glaciers didnot erode e the floodplain. Inthisreach, J ust past downtown St. Paul, the Mississippi makes the Mississippi a ust past downtown St.Paul,

its preglacial valley sippi National River andRecreation Area located , where spectacula r bluffs exposer bluffs es re that ittook approximately 7,800years for the waterfall to icethe final retreat from Minnesota he estimated asabasis Lake Agassiz, whose outlet stream, glacial River glacial Warren, outletstream, whose Lake Agassiz, co a complete understanding covering ofthemaps the area, reg becauseHowever, ofthe scarcity ofsubsurface information vided clues to the processes that helped shape them. and their form pro- two years million (Quaternary Period), oped response in to continental the last glaciation during Minnesota. Mostofthe surface features the area in devel- and describing themapping surface geology central in Survey. began withthe aid of Warren Winchell, Upham, newly formed Minnesota Geological and Natural History be Th InvestigationsEarly a Minnesota Rivers upstream to itspresent location. its former atthe confluence position and ofthe Mississippi w Riverthe Mississippi were completed atthistime. Winchell in we ex and atonetimemay havethe large river lake downstream, the MNRRA corridor. This area was once ofLake part Pepin, re and smallterrace meandering secondary channels, sloughs, stream for the last 9,500years. Backwater lakes and a delta that has the been Mississippi in down- migrating onthemulating floodplain near the confluence has formed ch alarge forming alluvialfanand diverting the Hastings, bo as the first toas the first address the retreat ofSt. Anthony Falls from ra,afigure that has proven remarkably close to currenttreat, mnants characterize the floodplain atthe southern end of timates. Upham conducted adetailed studyofglacial ve te mp a nte17sudrNwo .Wnhl,atthe the 1870sundergan in Newton H. Winchell, annel to ofthe the Mississippi northeast. Alluvium accu- e first studies intoe first the geologic history of Minnesota re ar ut 120feet above the modern floodplain. nd stigations into the history ofgeologic development of

Although theAlthough details ofMinnesota’s geologic history To di o ul nw,perhaps someofthe mostimportant not fullyknown, le ed ng glacial stratigraphy and lack ofadequate base x glacial history was never fullyrealized. da

up to St.Paul. y the Vermillion River at the Mississippi joins 1 U sing 22 Chapter 1 • THE GEOLOGY OF THE MNRRA CORRIDOR from eraging ally av action constantly reworked action constantly ern Minnesota, wherein the ast e ng the river, loc uth So where wave wave where , eology e high bluffs alo nt that the Mississippi River floodplain is a dynamic floodplain Mississippi the nt that River More recently, the Army Corps of Engineers,Army Corps the recently, More Paul St. Th re me compacted and cemented to form sandstone, form to cemented and shale, me compacted from the remains of biologic organisms in coral reefs and reefs in organisms coral of biologic remains the from pa vironment with an ever-changing set of resources. set with an ever-changing vironment

njunction with cultural resource investigations. The investigations. resource with cultural njunction ailability during time of occupation, the geomorphic and ocesses operatingocesses on the floodplain influence the poten- unding region. Sand accumulated along the shoreline in shoreline the along accumulated unding Sand region. NRRA corridor lies, corridor NRRA dipping sedi- of gently is composed on. Limestone and dolomite units are strong and usually and strong units are dolomite and on. Limestone as exposed the bedrock in the valley walls. The rock for- rock The walls. in valley the bedrock the as exposed alls. However, deep incision by the river below the falls the below river the incision by alls. However, deep imestone, dolomite. and ment of the Mississippi River valley. Mississippi of the River ment investigations geomorphological District sponsored has in valley Mississippi of the portions River various along co mapping of surficial land- on detailed focused have studies in an effort to environment within floodplain forms the On the sites. archaeological of buried location the predict studies, these basis of information from become it has ap en distribution site of resource is a function Archaeological av pr preservation. site tial for General G • Geology Bedrock M Mississippi River The a plateau. form that rocks mentary extensively. this plateau eroded its tributaries have and bedrock, the buried deposits have glacial out- few Because Anthony St. above Mississippi the along River appear crops F h during Cambrian the and deposited mations were B.P.),when (570-438 million periods years Ordovician sur- the and Minnesota southeastern covered seas shallow ro and bars beaches it. Silt and clay formed in mud flats or settled out of rela- out in settled mud flats or formed clay it. Silt and accumulat- Calcium carbonate offshore. water quiet tively ed eventually sediments The floor. sea on the layers as large beca l 100 to 400 feet in ero- and relief, 400 feet 100 to weathering resisted have si His cal- 6 e work completed during this time completed e work Th 4 Leverett first recognized that multiple gla- multiple that first recognized Leverett 3 7 Sardeson also reevaluated Winchell’s original Winchell’s also reevaluated Sardeson ng. 5 upies the sand plain. sand the upies Cooper evaluated the sequence of glaciation in of glaciation sequence the evaluated Cooper c

2 S. oc e Minnesota Geological and Natural History Survey History Natural and Geological e Minnesota

dati His work detailed the origin of the Anoka Sand Plain Sand Anoka origin the of the detailed His work om the early 1950s to the 1980s, the 1950s to early om the of many studies y, 8 In addition to his work in glacial geology, in Sardeson glacial his work to In addition W. Th Fr a profound effect on the Mississippi River valley’s shape valley’s Mississippi on the River effect a profound tion of 8,000 years is even closer than Winchell’s to the to Winchell’s than closer is even tion of 8,000 years le nage.

ast central Minnesota,ast central an advancing when formed which iod was a major step forward in recognizing the complexi- the in recognizing forward step a major was iod imate of retreat for St. Anthony Falls by considering the considering by Anthony Falls St. for of retreat imate e ht on the complexities of the glacial sequence in sequence glacial of the complexities ht on the la ai ometry and thickness of the limestone cap rock. cap limestone of the thickness and ometry id

ntral Minnesota and its relation to the formation of the formation the to its relation and Minnesota ntral t g istinct landforms. istinct urrent estimate of 10,000 years, estimate urrent on radio- is based which 23 es ge per history. glacial Midwest’s Upper ty of the out- in bedrock the fossils contained described and mapped in southeastern valley Mississippi the along River cropping Minnesota. officially ended in the early 1900s following the retirement the 1900s following early in the officially ended thereafter, Leverett, Shortly Upham. and Frank Winchell of Sardeson, of Frederick aid with the studying gla- the began much used Sardeson and Leverett of Minnesota. cial history of Minnesota’s reevaluation in their work earlier of Upham’s geology. surface had publication of a huge the to led His work configuration. and and development Agassiz’s Lake detailing monograph dr cu c carbon cial advances formed the region’s glacial deposits, glacial region’s the formed each and cial advances deposits, numerous created containing advance subsequent d in A southward. flow Mississippi the River’s diverted lobe ice and Dayton corridor, MNRRA of the portion between Fr develop- to relates sequence glacial the how and Minnesota ce postglacial and Wisconsin during Late Mississippi River time. Minnesota’s glacial geologic history were conducted. Herb conducted. were history geologic glacial Minnesota’s Wright, Jr., his colleagues, of University at the students and new study shed Each most of them. completed Minnesota li 557016_Book_r 8/5/03 7:57 AM Page 23 Page AM 7:57 8/5/03 557016_Book_r 557016_Book_r 8/5/037:57AMPage24

RIVER OF HISTORY: A Historic Resources Study of the Mississippi National River and Recreation Area area around butitmay be asmuch as80to 90feet the in unknown, sandstone. Total thickness ofthe Jordan Sandstone is w portion ofthe MNRRA corridor. The unitisagenerally Lake nearalong Spring Nininger Township the southern in te ar floodplain developmentstone. ismoreAlso, extensive in bou is the oldest bedrock unitoutcropping the park within be formations outcropping the MNRRA in corridor ispresented Group and Galena Decorah Shale, Platteville Limestone, Shale, Glenwood St.Peter Sandstone, duChienPrairie Group, yo units andoccupyare glacial outwash terraces generally occur ontop ofcarbonate fl d v followed out carving topographic low area the bedrock, in removed more gentle slopes along the valley sides. Glacial processes and poorly cemented forming sandstone are easily eroded, form steep adjacent cliffs stream to tributary valleys. Shale le stream flow has more effectivelyunits occur, eroded the val- the valley isnarrow. Where poorly cemented sandstone W large co and the the location ofglacial terraces, the valley width, pe and deeper. After the glacial meltwater slowed and disap- de alleys advance during across the area. Stream erosion and eposition alsoplayedeposition amajor role. Glacial meltwater, ow y, ie asv owl-edd commonly massive cross-bedded to well-bedded, hite, r e urse of the Mississippi River.urse ofthe Mississippi Valley width iscontrolled ar lo her ri net h erc nt r:the Jordan Sandstone, the bedrock unitsare: ungest, istics setting from itapart adjacent units.From oldest to as that have greater valley width. nd re ve w. ed i ly by the sedimentary properties ofthe bedrock. Each bedrock set unithas adistinct ofphysical charac- Bedrock has been animportant factor determining in gfo h eraigiemse,cut the valleysng from the wider retreating ice masses, e the river intersects more resistant carbonate units, sulting in amuch in sulting greater late width. Consequently, d from the erosion ofupland surfaces.

ari ,t (Figure 3)

Th he much ofthe bedrock the in Twin Cities area. Ice e e Jordan years Sandstone (515-505million B.P.) . xoue r e,hwvr and however, occurs. Exposures only are few,

v Cott alleys withsediment gradually began filling . descriptionA brief ofthe major bedrock age Grove.age as where er 10 osion cutawosion ay thesand- ihyrssatntr,the limestone serves asacaprock resistanthighly that nature, dolomitic limestone. Because ofits bedded, thinly tan, buff m isthe Platteville Limestone River, the (455-454 Mississippi one that forms relatively flat-topped benches and mesas along Limestone. St.Peterlying Sandstone and overlying Platteville small seam weathered ofhighly bedrock between the under- iseasily identified asa when present, Glenwood Shale, someplaces and in isentirely absent. feetThe thickness, in ye sandy shale called the Glenwood Formation (455million bedded, River thinly the Mississippi isagreenish gray, to obtained ofinformation On the basis from wells the area, in the sandstone iseasilyits poorly eroded. cemented nature, to M ar years(458-455 million B.P.). Exposures ofthisrock type the River upper Mississippi valley isthe St.Peter Sandstone ap duChienPrairie years Group (505-458million which B.P.), 100 to 300feet throughout the MNRRA corridor. Oneota dolomite. Total thickness ofthe group ranges from member dolomite isabluishgray nature in similar to the d fine-grained New Richmond gray, member isalight which consists oftwoShakopee members. Formation, The lay thinly- medium-grained, to gray tan, Dolomite buff isalight sedimentary the Oneota properties. At the baseofthe group, is divided into two formations ofvariations onthe basis in plateaulying across the upland areas. The duChien Prairie P olomitic to gl to olomitic aul, and in the bluffs at Hastings, forming anextensive forming flat- atHastings, the and bluffs in aul, illion yearsillion B.P.). the Platteville to gray isalight Ingeneral, e common in bluffs throughoute common bluffs in the northern half ofthe t NRRA corridor. The St. Peter Sandstone consists ofwhite

ar pe ye al thickness ofthe St.Peter isapproximately 150feet. er s B.P.). Onav ar llow, medium-grained, friable quartz sand. quartz Because friable of medium-grained, llow, ed P Th One ofthe mostextensively exposed bedrock units in Overlying the Jordan Sandstone isthe Ordovician ossibly the most recognized andossibly bedrock formation, s along the Mississippi Rivers along the Mississippi channel near South St.

to e thinnest bedrock unitoutcropping of the bluffs in

massive dolomite. Lying above the Oneota isthe uoii adtn,and the Willow Riverauconitic sandstone, erage, the Glenwood the r erage, anges from 3to 5 11 24 Chapter 1 • THE GEOLOGY OF THE MNRRA CORRIDOR 9 stratigraphy of the upper Mississippi River valley in southeastern Minnesota. Redrawn from Hobbs. from Redrawn Minnesota. in southeastern valley River Mississippi the upper of stratigraphy FIGURE 3. Generalized bedrock bedrock FIGURE 3. Generalized 25 557016_Book_r 8/5/03 7:57 AM Page 25 Page AM 7:57 8/5/03 557016_Book_r 557016_Book_r 8/5/037:57AMPage26

RIVER OF HISTORY: A Historic Resources Study of the Mississippi National River and Recreation Area d and have amucky consistency.saturated, Soils formed from organi and clay silt ping brought by in or wind activity. fluvial trap- faces. Plant litter continually collects atthe surface, d grai and morphological expression onthe landscape.tial location, spa- Each deposit has environment adistinct ofdeposition, and colluvium. river alluvium, organic sediments; types: across Minnesota. Ge Surficial ex southeastern(in Minnesota the entire Galena Group forms an isexposedof the Cummingsville the MNRRA within corridor Fo s ri Picke in be i m lo responsibleis partially for controlling the rate ofretreat and we d ph along the river floodplain response in to the present geomor- s to al categories oftheir onthe relation basis to the geologic his- the MNRRAwithin corridor can be separated into two gener- ng thin limestoneng thin interbeds. The lateral extent ofthis sigo he ebr:CmigvleFrain Prosser Formation; Cummingsville ofthreeisting members: ists ofsediments that are upland accumulating in areas and pstdb tem nrvres lopan,and alluvial floodplains, eposited by streams onriverbeds, or onupland sur- epressions occupying the floodplain, pst eae ogaito,consists ofsediments that relatedeposits to glaciation, do ca linyasBP) reihga,calcareous yearsillion agreenish B.P.), shale gray, contain- yo h ra Tefrtgop ogaildpst,con- nonglacial deposits, ry ofthe area. The group, first

te dr mto,and Stewartville Formation. Onlythe lowerrmation, part ic agents operati bluffs along the Mississippi River along thebluffs Mississippi St.Paul in above re tion ofwaterfalls the MNRRA within corridor. e eieti luh,lks and poorly drained lakes, ned sediment sloughs, in nsive plateau across the uplands). r isthe Galena years Group (454-450million con- B.P.), oc

luimi h cuuaino ad it and clay silt, Alluvium isthe accumulation ofsand, consistOrganic deposits ofplant material and fine- consist deposits Nonglacial surficial ofthree main Th Ly rel Lake. rel d eposited the advance during and retreat ofglaciers ntars h rai iie,butitdoes outcrop k unitacross the area islimited, i e uppermost bedrock unitexposed the MNRRA in cor- ng above the Platteville isthe Decorah Shale (454 ology • ng onthelandscape. The second group, Surficial geologicSurficial occurring deposits c deposits are da c deposits kclrd water rk colored, lo on the surface throughout geo- time.Manyofthe surficial M front ofanice sheet. terr across the surface. Manyoutwash consist deposits of broad g of the Outwash deposits. typically consists ofsand and be o and numer- terraces, natural levees, cutbanks, bars, point Individual landforms created by stream processes include er fans. The often deposits exhibit complex sedimentary prop- advances are known to have to takeadvances known are from two years million to 10,000years B.P. Four major ice spans knownEpochs. asthe Great The Ice Pleistocene, Age, the Pleistocenetwo and Holocene formal geologic periods: Quaternary Period. The Quaternary Period isdivided into series ofgeological events ofthe since the beginning Th Glacial History River of the Mississippi reso wh ro d pe the Great Ice Age. Glacial tillisthe unsorted mixture of consists ofoutwash and tilldeposited during glaciation, fi masses ofbedrock slumped the valley off wall of amatrix in and adownslope unitcontaining large from upland areas, co d slopes and cutbanks along the river valley. Colluvial rav us backwater features. eposited directly from glacial ice. The of compositions anupslope unit generallyeposits consist oftwo units: ne-grained sediment. gi ties and variable display ahighly internal stratification. NRRA corridor reflects the geologic processes operating ck dr nsisting ofsmallbouldersnsisting ofsediment amatrix in eroded be,cbls and boulders ofsediment amatrix in cobbles, bbles, e upper Mississippi Rivere upper Mississippi valley has experienced acomplex ich had adirect onthe impact development ofnatural aces that were once large braided streams the draining c features formed response in to continental glaciation, urces that are ofthe part present-day landscape. el

oc types found the tillprovide in clues about the source Th Colluvium isthe unsorted mixture ofweathered Th

la namti fsn,sl,and clay the hill flanking silt, ofsand, amatrix k in e complexity landscape ofthe surficial the within eodgopo ufca eois thoserelated to e second deposits, group ofsurficial id

do wn by glacial meltwater streams flowing n place the Great during Ice 26 Chapter 1 • THE GEOLOGY OF THE MNRRA CORRIDOR 15 The early of the Mississippi the position Presumably each major glacial phase glacial major each Presumably of ice recession has been has recession of ice 16 e prominent St. Croix Moraine, St. Croix e prominent a massive clear where where clear Th 18 . This advance,This of phase . St. Croix as the known . ng ice margin. Glacial outwash graded to terrace to graded margin.ng ice outwash Glacial e St. Croix Moraine forms a northeastward trending, a northeastward forms Moraine e St. Croix i most of which joined the present course south of St. south course present the joined most of which Little is known about the nature of the advance; of the nature the about is known Little how- During the late Wisconsin maximum,Wisconsin During late the Superior the Th nc r, 17 umented. upies a prominent gap eroded through the St. Croix the through gap eroded a prominent upies dscape within the Twin Cities area resulted from resulted area Cities Twin within the dscape va isconsin Glaciation (35,000-10,000 B.P.) • B.P.) (35,000-10,000 Glaciation isconsin c urse of the river north of St. Paul changed repeatedly dur- repeatedly changed of St. Paul north river urse of the ashington County lends support to this hypothesis. to support lends County ashington n as followed by the establishment of a new course for the for course of a new establishment the by as followed on by streams also occurred during the early Pleistocene. during early the also occurred streams on by ve as been obscured by late Wisconsin glacial events. The events. glacial Wisconsin late by obscured as been aul. Previously formed bedrock valleys were subsequently were valleys bedrock formed aul. Previously eposits along the Mississippi River in southern Mississippieposits the along River oc illed with glacial sediment derived from the Superior Lobe Superior the from derived sediment with glacial illed ng the Pleistocene. ng the ever, a detailed record d w ri Researchers investigating stream valleys of the Driftless of the valleys stream investigating Researchers inci- valley deep suggest Wisconsin in southwestern Area si W Paul St. above Mississippi of the River history Pleistocene h co i accumulation of glacial sediment extending from the Twin the from extending sediment of glacial accumulation Falls, Little to of northwestward marks terminus the Cities the lobe. It is un P f the dotting lakes numerous The Sublobe. Grantsburg and la River was at this time. The Mississippi River presently Mississippi River The at this time. was River oc meltout of glacial ice blocks buried in the bedrock valleys in bedrock the buried blocks ice of glacial meltout 4) (Figure basin Superior Lake axis of the the down advanced Lobe Minneapolis position its terminal near to southeastward and Rainy and Lobes St. Paul,and Wadena the while Minnesota north-central across advanced Sublobe Brainerd 5a) (Figure Lobe, Superior the 15,500 years approximately culminated B.P. Moraine. Most likely the river maintained its current posi- its current maintained river the Most likely Moraine. the underneath flow continued by St. Paul tion below ad d W hills and numerous containing of landforms belt rugged In Minnesota, gla- early during pre-Wisconsin Near the Mississippi River valley the valley Mississippi the River Near een established 12 e presence of glacial till in southeastern of glacial e presence b Th ep cutting must have occurred during early the occurred must have ep cutting 13 e e course of the upper Mississippi River along the along Mississippi upper River of the e course ,d Th c development of the river in the MNRRA corridor. in MNRRA the river of the c development e, begin- Pleistocene of an early theory the supports e. For simplicity,the the discussion ignores e. For following ce gi 14 It is uncertain when the upper Mississippi River valley Mississippi upper River the when It is uncertain Most of the present-day landforms developed during developed landforms present-day Most of the nc nc en r eastward from central Iowa to its present position. its present to Iowa central from r eastward ut 35,000 to 10,000 years B.P., dramatically altered the B.P., dramatically altered 10,000 years ut 35,000 to ation of characteristic glacial deposits associated with the deposits associated glacial ation of characteristic e-glacial and post-glacial events. However, exposures of However, exposures events. post-glacial and e-glacial va va id itic (branch-like) pattern across the region, the across pattern itic (branch-like) drains the isconsin Glaciation. In Minnesota the sequence of glacia- sequence the In Minnesota isconsin Glaciation. rridor. Therefore, an overview of the glacial history of history glacial Therefore, of the an overview rridor. isconsin for glacial geologic events occurring prior to events geologic glacial isconsin for olo sents the last 10,000 years of geologic time. of geologic 10,000 years last the sents t ve rst three named glacial advances and uses the term pre- term the uses and advances glacial named rst three bo nitially formed. However, on the basis of present geologic basis However, on the of present nitially formed. upland landscape consists of gently rolling hills topped by a by rolling hills topped of gently consists landscape upland residuum. bedrock drift of glacial weathered or thin veneer networks, stream forming a den- of well-integrated A system dr a consid- exposed has valleys stream along Erosion uplands. erable amount of bedrock. i ev Pleistocene. margin of the Driftless Area of southeastern Minnesota is Minnesota of southeastern Area margin Driftless of the to have believed the displaced west the from advance a glacial time when ri Minnesota, glacial during a pre-Wisconsin deposited ad ning. Age, and each has been given a name based on the geographic Age, on the a name based given been has each and loc ad during the occurred that episodes glacial multiple the W MNRRA of the development impact on the a direct tion had co discussing for a context provide to is necessary Minnesota ge • History Glacial Pre-Wisconsin by obscured been largely have within valley the cial events la fi W Glaciation,Wisconsin The spanning from B.P. 35,000 years a landscape of Minnesota. Holocene,The or Epoch, Recent rep- re and Washington in drift surface at the occur pre-Wisconsin Counties. Dakota 27 557016_Book_r 8/5/03 7:57 AM Page 27 Page AM 7:57 8/5/03 557016_Book_r 557016_Book_r 8/5/03 7:57 AM Page 28

FIGURE 4. Map of preglacial bedrock valleys (solid lines) in the Twin Cites area, showing the location of present-day lakes developed by meltout of buried

glacial ice. Discharge of glacial meltwater and waterfall retreat are responsible for development of present valleys (dashed lines).

associated depressions. Glacial sediment deposited during for the glacial meltwater. Outwash deposits filled both val- this advance consists of reddish-brown sandy till, outwash leys between an elevation of 870 and 920 feet. Meltwater sand and gravel, and ice-contact sands and gravel. streams subsequently excavated the outwash deposits dur- As the Superior Lobe retreated from the area, the ing a later glacial advance. Mississippi and St. Croix Rivers acted as the major course Numerous readvances, possibly surges, accompanied the retreat of the Superior Lobe from the St. Croix Moraine.19 Numerous features associated with the retreat- ing ice, including moraines and associated meltwater chan- nels, developed behind the moraine (Figure 5b). The Mississippi River, in the central portion of the state, flowed

28 Chapter 1 • THE GEOLOGY OF THE MNRRA CORRIDOR b) development of tunnel valleys of tunnel b) development d) the Automba phase Automba d) the 16 ases of glaciation in Minnesota. Taken from Wright. from Taken in glaciation Minnesota. ases of a) the St. Croix phase St. Croix a) the FIGURE 5. Ph 29 c) deposition of eskers in tunnel valleys in tunnel c) deposition of eskers 557016_Book_r 8/5/03 7:58 AM Page 29 Page AM 7:58 8/5/03 557016_Book_r 557016_Book_r 8/5/037:58AMPage30

RIVER OF HISTORY: A Historic Resources Study of the Mississippi National River and Recreation Area Au t lo ther into the Lake resulted Superior basin of deposition in oped underneath the retreating ice lobe. Retreat ofice far- fed by tunnel valleys (discrete meltwater channels) devel- being along the western ofthe St.Croix Moraine, margin in e in which bounds the western edge ofMilleMoraine, LacsLake 5d) in Wisconsin. Th Highland Moraine along the north shore ofLake Superior. clay settled out of the melting ice mass, forming acontinu- forming clay settled out ofthe melting ice mass, higher topography to the southwest. and Fine-grained silt which sediments were deposited alarge body in ofopen water, glacial lake the formation Glacial Superior in lake basin. gin into th Moraine down to valley the Mississippi below St.Paul. still channeled along the ofthe outer St.Croix margin itismostlikely that however, meltwater wasmovements; River channel the area in was destroyed by subsequent ice plain. Any evidence for the location ofthe Mississippi d la developed al Sublobe across the area erased anyshoreline features that weste which presumably drained along the Iand UphamAitkin I, resulting the formation in ofglacial lakes of the ice lobe, meltwater ponded along the northwesternMoraine, margin River River into the Mississippi valley. from the Superior Lobe discharged through the St.Croix unnel valleys ast asthe and CromwellWright Moraines and then asthe eposited by the St.Louis Sublobe after overriding the lake ke ng, sinuous ridges sinuous ofsand and gravel (eskers) the within ng, to

southeastern Crow extending Wing to the County, north- e Automba phase iscorrelated withthe Tiger Catadvance t . The extent ofthisadvance ismarked by the Mille Lacs mapae ismarked by advance ofthe Superior Lobe omba phase, speevdi hn e n ry stone-poor red till and gray, s ispreserved athin, in

the Mille Lacsregion ofeast central Minnesota Th Th W rn end ofthe iceThe margin. advance ofthe St.Louis formed between the retreating Superior Lobe and hile the Superior Lobe stood atthe Mille Lacs etmjravneo h ueirLb,the e next major advance ofthe Superior Lobe, e Superior Lobe retreated from the Automba ice mar- e Superior lowland, initiating the stage first initiating of e Superior lowland, ong these lakes. evidence However, for these 20 (Figure 5c) uigteTgrCtavne meltwater the During Tiger Catadvance, . (Figure o ov advanced from thefrom southwest the Des Moines Lobe, d sand and forming gravel portions ofthe St.Croix Moraine, outwash channels were thisadvance, During cut through of Des the Moines terminal marks ofthe ice position lobe. Iowa about 14,000years B.P. An end moraine near the city which reached itsmaximumextent centralCity phase, in of the Des Moines Lobe from the northwest the Pine during Croix Moraine by the Des Moines Lobe Moraine and the overriding ofthe central portion ofthe St. the readvancemarks ofthe Superior Lobe to the Cloquet previously formed deposits. The Rock-Pine Cityphase Split layer athin depositing ofreddish clay across Rock phase, of a P do blocked thesouthward drai 820 feet along the valley. advance ofthe lobe Second, series terraces offlat-lying between elevations of800and wo RiverMississippi valley withsand and gravel. The deposits Fi ment River ofthe Mississippi valley two in important ways. advance was re r dr Grantsburg Sublobe retreated meltwater to the southwest, Wisconsin. RiverMississippi valleyAs the atPrescott, meltwater en w south flowed into glacial Lake Grantsburg. A large delta meltwaterMinnesota and west draining central Wisconsin, i ng in the formationng in ofglacial Lake Grantsburg. eworking eworking bo us blanket ofsediment onthe lake floor. The next advance eposits that reachedeposits River the Mississippi near Hastings. u,rahn t emnsna rnsug icni,by Wisconsin, reaching itsterminus near Grantsburg, aul, sfre erSonr icni,assediment-laden Wisconsin, as formed near Spooner, s,outwash the coming off advancing lobe filled the rst, ained around the outer (northeast) ofthe ice lobe, margin er nteS.CoxRvr eventuallywn the St.Croix reaching River, the th uld later be entrenched by glacial meltwater a forming ut 13,500years B.P. ri e Th Retreat ofthe Superior Lobe was followed by advance W di Su hile the Grantsburg Sublobe occupied east central rnsugSboe anoffshoot ofice developed e Grantsburg Sublobe, ng the St.Croix Moraine between St.Cloud and St. perior Lobe overrode the lakebed during the Split the overrode lakebed Lobe during the perior the former lake bed the and forming Anoka Sand tered the head head the tered sponsible for alteringsponsible the geologic develop- (Figure 6) nage of the lake. The lake drained fteMsispi result- of theMississippi, . This short-lived Fgr e 5f) (Figure 5e, . 30 Chapter 1 • THE GEOLOGY OF THE MNRRA CORRIDOR of glacial Lake Duluth nage h.) drai f.) formation of the Anoka Sand Plain Sand Anoka of the f.) formation 16 Phases of glaciation in Minnesota (continued). Taken from Wright. from Taken (continued). in of glaciation Minnesota Phases FIGURE 5. e) Split City phase Rock-Pine g.) Nickerson-Alborn phase g.) Nickerson-Alborn 31 557016_Book_r 8/5/03 7:58 AM Page 31 Page AM 7:58 8/5/03 557016_Book_r 557016_Book_r 8/5/037:58AMPage32

RIVER OF HISTORY: A Historic Resources Study of the Mississippi National River and Recreation Area FIGURE 6.FIGURE Advance of lobes. Redrawn from Meyer et al. et Meyer from lobes. Redrawn andforming glacial Lake Grantsburg. Drainage channelsMississippi River, show paths takenby meltwater coming off theGrantsbu the Grantsburg Su the Grantsburg 21 lb,anoffshoot blobe,

of the Des Moines L of theDesMoines obe, overriding th overriding obe, e St.Croix Morai ne blocki ng southward dr rg and Des Moines icerg andDesMoines ai nage ofthe 32 Chapter 1 • THE GEOLOGY OF THE MNRRA CORRIDOR a- rel ainage affected the affected er the Platteville ov ainage rking the dr wo

acial Lake Agassiz exis- into came acial Lake glacial lake dr River Warren plunged plunged Warren River nages, gl of sediment-free meltwater from glacial Lakes from meltwater of sediment-free from the Nickerson ice margin ice Superior the into Nickerson the from where the where of River Warren was more than adequate to carry the carry to adequate than more was Warren of River

Glacial ice,Glacial continental the again across advancing Meltwater from Lake Agassiz drained down the River the Agassiz down drained Lake from Meltwater Drainage ge e. In northeastern Minnesota,e. In northeastern Lobe Superior the rren, St. Croix, the Mississippi the val- presumably and rren into the Mississippi River valley. River Warren was Warren River valley. Mississippi the into River rren nts associated with nts associated nts within the Mississippi River valley. A number of A number valley. nts within Mississippi the River med after G. K. Warren, St. G. K. of the after firstmed commander the ar e e ys approximately 11,700 years B.P. Glacial Lakes Agassiz Lakes Glacial B.P. 11,700 years ys approximately as named River Warren Falls in honor of the mighty in of the honor river Falls Warren River as named ivide, within River the up of sediment a build caused aul District, St. Paul,Above of Engineers. Corps the utwash sediment was rapidly eroded from the preglacial the from eroded rapidly was sediment utwash le. neteotahwscridaa,a waterfall away, carried was outwash the Once alley. alley of the Mississippi River that was filled with outwash filled was that Mississippi of the River alley v formed waterfall The valley. bedrock preglacial the into Limestone w its formation. for responsible was that d Wa le ice as the B.P. 11,500 years after reformed Superior and theagain continental beyond retreated drainage divide. a fairly established lakes of the out of meltwater Discharge until approximate- lasted that of downcutting period active B.P. ly 10,800 years morphology of the upper Mississippi system. upper of the morphology Wa na P Platteville of the on top flowing was Mississippi River Below force. Limestone, erosive river’s the resisted which bedrock a preglacial intercepted St. Paul,Warren River the v dis- The Limestone. Platteville of the elevation the up to ch it; to supplied load sediment therefore, unconsolidated the o tionships of these lakes and their impact upon the land- impact upon the their and lakes tionships of these the how and works is a summary of these Below scape. ev north of the divide that separates the Hudson Bay and Bay Hudson the separates that divide of the north Mississippi drai tenc retreated Lowland, Duluth. Lake initiating of glacial formation the in downcutting multiple Agassiz resulted Duluth and ev in active been have geologists son Moraine . . The Nicker The . (Figure 5f) (Figure sces into a fairly well defined a fairly into well sces (Figure 5g,(Figure 5h) the Nickerson-Thomson Moraines in north- Moraines Nickerson-Thomson the

to where it coale where

back into Minnesota to join the Des Moines Lobe, Moines join Des the to Minnesota into back ed as retreating up the Minnesota River valley. A large valley. River Minnesota up the as retreating e last major glacial advance in Minnesota occurred in Minnesota advance glacial major e last rther south,rther was Lobe Moines Des of the retreat d north central Minnesota, central d north forming Culver the nc d the surface of Minnesota were in full retreat. The in full retreat. were of Minnesota surface d the Th II, Upham Lake into Aitkin drained II most likely Lake Fa By 12,000 years B.P., all ice lobes that had previously had that B.P., lobes all ice By 12,000 years re de e to form the Big Stone Moraine in Moraine west-central Big Stone the form e to va s Aitkin and Upham II to develop, ponded between the develop, II to between Aitkin Upham and s ponded ich eventually drained down the St. Louis and St. Louis the down drained eventually ich annel along the Kettle River. The Kettle channel drained channel Kettle The River. Kettle the along annel ve va treated further, Mississippi of the Minnesota and treated level the nc ad ke iscontinuous moraines in northern Iowa and southern and Iowa in northern moraines iscontinuous astern Minnesota astern braided meltwater stream developed along the retreating the along developed stream meltwater braided margin, ice Grantsburg of blanket forming a continuous Mississippi of the course present the along gravel and sand As ice River. Minnesota with the its confluence above River re in 810 feet of about at an elevation established was Rivers area. metropolitan Cities Twin the St. Louis the when phase during Nickerson-Alborn the Sublobe, Lobe, Moines Des of the extension eastward an in glacial allowed Sublobe St. Louis of the Retreat Moraine. la moraine. marginice Culver the and wh same time,At the Mississippi Rivers. Lobe Superior the re which w punctuated by a number of readvances, a number by of formingseries a punctuated d B.P., Sublobe Grantsburg the By 12,300 years Minnesota. retreated is marked by a belt of hummocky topography along the along topography of hummocky a belt by is marked Holyoke. and Lake Moose lineCarlton-Pine County between Moraine off Nickerson the plain extends outwash A broad southward e ch the to down then and River St. Croix the into meltwater Mississippi. co up the northward rapidly retreating was Lobe Moines Des dis- a short readvanced then Ice valley. River Minnesota ta retreated ice the After B.P. 11,900 years about Minnesota Plain in east-central Minnesota Minnesota Plain in east-central 33 557016_Book_r 8/5/03 7:58 AM Page 33 Page AM 7:58 8/5/03 557016_Book_r 557016_Book_r 8/5/037:58AMPage34

RIVER OF HISTORY: A Historic Resources Study of the Mississippi National River and Recreation Area in dr Early H valley.Mississippi fl altering the physiography ofthe floodplain. de ca from thanthe itstributaries the Mississippi bigriver could fairly active period ofalluviation.More sediment entered resulting a soon in transported River, into the Mississippi Sedimentsthe Mississippi. stored valleys tributary in were ta feet) deep. upto 50meters possibly (about 163 its present-day level, (Figure 4) the softSt.Peter Sandstone that underlay limestone caprock be the valley.of alluviationwithin The River Warren Falls water ev the River upper Mississippi system northThese ofIllinois. the last timethat meltwater from glacial lakes flowed down be c utaries began to establish itself by 8,000 years B.P. Bythis ex moving downstream. This delta and atHastings begins P bo to Zumberge proposed that Lake Pepin once extended upstream Lake Pepin. forming tively River, the dams Mississippi P Bridge St.Paul. in aused renewed downcutting valley the within Mississippi epin has builtadeltaepin that isslowly the Mississippi within pnCut,Wisconsin. County, epin The formation ofthe delta effec- ue ry streams cut initially their channels to reach the level of

rry away. anumber builtfan oftributaries As aresult, e ainage ainage te tween 9,900and 9,500years B.P. This episode final is gan retreating aswater valley, upthe Mississippi eroded tsit h issip ie,deflecting itscourse and ltas River, into the Mississippi ri t al based onthe existence ofclay found deposits in St. Paul, d nts played nc ischarge through River the Mississippi following the n ngs taken the construction during ofthe Robert Street A One final advanceOne final ofice blocked eastern outlets and Equilibrium betweenEquilibrium River the Mississippi and itstrib- st h edo aePpn south ofRed Wing.ds to the head ofLake Pepin, e of the Mississippi Rivere ofthe Mississippi withthe Chippewa River in through northern and eastern outlets initiated astage good example of a tributary delta occurs at the con- the at occurs delta tributary a of example good olocene olocene of glaciallakes andsubseque . Below St.Paul the valley had been below cut far 22 nrsos otecag nbs ee,tribu- In response to the change baselevel, in a vital r (9,500-7,000 years B.P.) • 23 Sediment entering the river above Lake ole in theHoloce ole in nt rerouting ofmelt- ne evolution ofthe The decrease The Late H si active lateral channel and inci- migration Inresponse, runoff. in Cooler temperatu wo dr o loc the late Holoc fl the present-day surface morphology River ofthe Mississippi in in while lateral channel and migration (built up)ofsediment, Th ab Middle H (Figure 4) and retreatedSnelling upthe valley ofthe Mississippi oped atthe confluence ofthe Minnesota River near Fort the preglacial died out. Mississippi St. Anthony Falls devel- valley of buried a intersected it where two miles, additional F River valley, the River Warren Fallstime, had reached the Minnesota ley v re or charge ri by Holocene. Ve Riveration along the Mississippi continued into the middle alls continued to retreat upthe Minnesota River valley an us portions ofthe while valley, lateral channel migration, oo on dominated processes fluvial the valley. in acting ve ut the appearance offloodplain features the val- within sult, t h einlciae which may have increased initially ate the regional climate, cision into previouslycision deposited sediment were occurring

iving forceiving for geomorphic processes. The regional cli- le

do c e upper reaches were characterized by vertical accretion uld againalter the processes the within valley. acting ation along the valley. Vertical accretion dominated vari- hstm. hrfr,the change upper in midwesternthis time. Therefore, tadfl eune,dominated other parts. As a sequences, ut and fill rs was mostlikely related effects to onriver climatic dis- dplain isthe result activityoccurringduring offluvial

re,dep aries, along the entire stretch ofthe River. upper Mississippi As During thelate During Geomorphic processes the in valley acting were vari- wnstream reaches. olocene olocene rather than changes in vegetation. changes in than rather th . olocene olocene e mi where getation was well establ e endi dd ne. However, fluvial processes fluvial ne. However, varied with (3,500 years B.P. - A.D. 1850)• le Holocene progressed, climatic changes climatic progressed, Holocene le re (7,000-3,500 years B.P.) • s andincreased precipitation began to dom- ng onlocation. it split into two into it split parts. The River Warren ooee cl Holocene, imate was amajor still ished onupl 24 Slow alluvi- and areas Much of 34 Chapter 1 • THE GEOLOGY OF THE MNRRA CORRIDOR left their er drainage Some farmers glacial meltwat

prominent terraces occur along occur terraces prominent Three Accumulations of up to two meters of meters two of up to Accumulations 26 nd no-till planting. ,a as adjacent to the upper Mississippi valley and the and Mississippi upper valley the to as adjacent ith the increased awareness of soil erosion along the along erosion of soil awareness increased ith the e opment of modern agricultural equipment, agricultural of modern opment farmers Humans have changed the landscape of the valley and valley of the landscape the changed Humans have W Inset below the Richfield Terrace is the Langdon is the Terrace Richfield the below Inset ar l

ace, of distribution range through- widest the has which ation from 890 feet above mean sea level (amsl) in the level sea mean above 890 feet ation from ds fallow to increase the soil’s nutrient capacity. These capacity. nutrient soil’s the increase to ds fallow ds entering the Mississippi the River. ds entering the southeast corner. The city of Minneapolis is built The corner. southeast the e Mississippi River valley is a significant natural feature in feature is a significant natural valley e Mississippi River tion. This will become clear in each subsequent chapter. subsequent in each clear will become This tion. ve gan to use improved farming techniques. By the 1930s By the farming techniques. use improved gan to

a rgely upon this terrace surface. upon this terrace rgely nd ev on tillage el ut the corridor. Elevation of the Langdon Terrace ranges Terrace Langdon of the Elevation corridor. ut the efforts greatly reduced topsoil erosion, topsoil reduced sediment efforts greatly decreasing lo ways, in other Mississippi of the River flow the some as pro- Overall, glaciers. as the foundly however, humans have around adapted to and the developed river’s geologic foun- da fi de be plowing, contour practiced farmers increasingly conserva- ti post-settlement alluvium may occur on the floodplain in floodplain on the occur alluvium may post-settlement corridor. MNRRA of the reaches southern the la along the major rivers in the MNRRA corridor. The corridor. in MNRRA the rivers major the along surface, terrace highest is the Terrace ranging in Richfield el amsl feet 840 to corridor MNRRA of the corner northwest in la Terr o corri- MNRRA of the corner 850 amsl innorthwest from the Th found features of individual right. a number However, its own are corridor MNRRA the through valley river the along below. is presented locality of each A brief description notable. • Terraces Glacial corridor. MNRRA in the Mississippi of the River course the of late as a result formed Each of major tributary streams. Review of Mississippi River Review tributaryof major streams. that changes of the evidence Commission mapsprovides occurred. have Natural Features Prominent oler temperatures and oler temperatures co

rd without absolute chronological Land clearing efforts for the devel- the efforts for clearing Land his realization is important for several for is important his realization ns of the valley near the confluence of confluence the near valley ns of the olocene, T 25 ng and the burial of previously developed surfaces. developed burial the of previously ng and i most likely well developed on landforms not subject to subject not on landforms developed well most likely It is difficult to assess the major changes that occurred that changes major the It is difficult assess to Geomorphic studies conducted in various portions of portions in various conducted studies Geomorphic rking of previously deposited sediment. Erosional sediment. deposited rking of previously

s. Many of the changes occurring within the area were occurring within area the changes s. Many of the ood ct influence on geomorphic processes. Vertical accretion Vertical processes. on geomorphic ct influence ly subjected to lateral channel migration, channel lateral to ly subjected in resulting re backwater areas on the floodplain. on the areas backwater ated to shifts in regional climatic shifts in regional to patterns,ated a had which fl minated portio minated te y position of the river channel changed little during the changed channel y position river of the creased precipitation. By this time, precipitation. soils and creased vegetation

re n. Second, landforms within the floodplain away from n. Second, away within floodplain the landforms source records. Third, topographically are records. that source landforms asons. First, active fluvial processes would be confined to asons. First, confined be would fluvial processes active ke ut and fillut and cycles, valley. of the portions these dominated ewo nundation by floodwaters. Lateral channel migration, channel Lateral floodwaters. or by nundation opment of agriculture began during mid-1800s within the began of agriculture opment of topsoil Erosion corridor. MNRRA the to adjacent and into influx the of sediment increased fields exposed from Mississippithe confluence River, the near in areas especially A.D. 1850 - Present • A.D. r chan- active the near dominant been have would processes active been have nel, would processes constructional while in tributary streams. Areas away from tributaries were most tributaries were from away Areas tributary streams. li of sediment and forward movement of alluvial fans/deltas movement forward and of sediment do within the floodplain of the upper Mississippi River valley Mississippi upper River of the within floodplain the during H the late da rel di i c present- the that indicate valley Mississippi upper River the da late Holocene. in we a limited channel area. As a result, area. erosion channel for a limited potential the mar- channel active the near greatest be would of landforms gi of accretion vertical to subject be would mainthe channel has This features. of natural preservation and sediment cultural and environmental the implications both for re prone less been have margin would valley the along higher to re mate continued its trend towa its trend continued mate 35 557016_Book_r 8/5/03 7:58 AM Page 35 Page AM 7:58 8/5/03 557016_Book_r 557016_Book_r 8/5/037:58AMPage36

RIVER OF HISTORY: A Historic Resources Study of the Mississippi National River and Recreation Area nhn al,Minnehaha Falls the offers observer an Anthony Falls, Minnehaha Falls • s Decorah Shale the in Twin Cities. A variety ofinvertebrate fos- and Platteville Limestone, Glenwood Shale, Sandstone, e isoneofthe best St.Paul, in the east bankofthe Mississippi, Shadow Falls • and alock and system. dam co tion. The natural state ofthe fallshas been modified by the Minnesota Rivers butmigrated upstream to itspresent loca- once located atthe confluence and ofthe Mississippi face. Longrevered for the itsnatural waterfall beauty, was the power processes offluvial the Platteville Limestone atSt. Anthony Falls exemplifies St. Anthony Falls • al remnant ofthe once higher Langdon Terrace surface. s Chien Group. However, atGrey Cloud Island the terrace con- co it formed partially over the terrace bedrock. At Newport, va Cloud Terrace occurs onlysouth ofSt.Paul and ranges ele- in Riveralong the Mississippi the MNRRA in corridor. The Grey Grove are builtlargely uponthisterrace surface. sand and gravel. The cities ofSouth St.Paul and Cottage the terrace consists of100feet orof the more bedrock, of the rest ofthearea ments are onlyafew feet to tens offeet thick. Throughout t Anthony Falls and Daytons the terrace Bluff developed on sedimentary characteristics along the valley. Between St. Te do op ofthe underlying Platteville Limestone. Terrace sedi- l,icuigcndnsadtioie,can be collected conodonts including here. and trilobites, ils, ists ofathick sequence ofsand and gravel left asanerosion- xposed and easily accessible outcrops ofthe St.Peter nsists ofsedimentsnsists afew feet thick ontop ofthe du Prairie tion from 750to 700feet Like the amsl. Langdon Terrace, nstruction of milling and hydroelectricnstruction ofmilling power structures rrace exhibits atremendous its in ofvariability amount r to 800feet the southeast in amsl corner. The Langdon The The Grey Cloud Terrace isthe lowest terrace present At Shadow Falls, a small tributary valley asmalltributary on At Shadow Falls, , where preg Fo Th rmed to amanner St. similar in e Mississippi Rivere Mississippi cascading over operati lacial erosion removed much ng uponthelandsur- Sandstone is exposed the valley within ofBattle Creek Park. Battle CreekPark • c t on Native lie American inhabitants some2,000years ago, we isanapron ofcolluviumbluffs derived from sediment late-WisconsinLobe during glaciation. At the baseofthe coveris athin ofglacial sediment deposited by the Superior along Warner Road Daytons in Overlying Bluff. the bedrock and Platteville LimestoneGlenwood are exposed Shale, Daytons Park Bluff/Mounds • this location for clay used the manufacture in ofbricks. dilige bo to the Twin CityClay Pit/Lilydale Regional Park. Inaddition FormationCummingsville excellent offer at hunting fossil and lower Decorah Shale, the Platteville Limestone, Tw ural riches contained two in very different river valleys. her tw pe River the confluence Warren, has been place agathering for ch and MinnesotaMississippi rivers converge. Created by the dis- v downstream. mile Steep bedrock covered bluffs witha is one en Mississippi-Minnesota Rivers • Confluence to River. thesmall tributary Mississippi access to explore the bedrock geology ofthe Twin Cities a in loc opportunity to view awaterfall itsnatural in state. The op of the bluff. op ofthe bluff. This location anexcellent offers example of ombined natural and cultural resources. ari

ople throughout several millennia. Pike Island separates the ar ce th the Superior and Des Moines Lobes can be found upon o be teigadeoigo erc.Sxmud,builtby andathering eroding ofbedrock. Sixmounds, ation of the falls within Minnehahaation ofthe Park fallswithin provides ready e. The confluence the offers opportunity to explore the nat- in ab ge ri , dr as theMdewakant

nt search. The Decorah Shale was formerly mined at ve City Clay Pit/Lilydale Park Regional • le

of meltwater from glacial Lake downAgassiz glacial oc

rs thickness ofglacial sediment characterize the valley elg,exposuresk geology, ofglacial tilldeposited by

where the valleys join, and the physical confluence physical the and valleys join, the where Th on Dakota c e uppermost 50feet ofthe St.Peter Th l t is all it, e St.Peter Sandstone, where the d-e or conflu- Bdo-te, Exposures of 36 Chapter 1 • THE GEOLOGY OF THE MNRRA CORRIDOR a variety of a variety as a haven for as a haven

A terrace remnant related to related remnant A terrace ke now serves ke now Pigs Eye Lake was a naturally occurring a naturally was Lake Pigs Eye e extended to St. Paul during early the St. Paul to e extended hav

k that escaped being filled with glacial sediments with glacial being filled escaped k that to

oc ed e-Wisconsin time. e-Wisconsin dr t glacial trenching of the Mississippi of the River, Grey trenching glacial Lower dplain dwellers. ev

ups constructed by the river valley’s early inhabitants. inhabitants. early valley’s river the by ups constructed be la li

o te eposits overlain by fine sand. The island has many mound has island The fine by sand. eposits overlain It is one of the best examples of a preglacial valley developed valley of a preglacial examples best It is one of the in in Pigs Eye • Lake Mississippi. of the within floodplain the of water body open Pepin, part of Lake been have At one time it may is which be wildlife, including birds, fox, beaver, raccoon, similar and floo • Island Cloud Grey Lower la gravel and sand of stratified is composed Island Cloud d gr Holocene. The la The Holocene. 37 557016_Book_r 8/5/03 7:58 AM Page 37 Page AM 7:58 8/5/03 557016_Book_r