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Dunes on the Wind-Swept Prairie: North Dakota’s Eolian Sands

Fred J. Anderson

IntroductionThe notion of sand dunes in North Dakota may at first soundrathermisplaced. Onemay think thatwithbotha climateandever-changing weather conditions like ours - long cold winterswithshort,relativelyhotandhumidsummers-theexistenceofsand dunes on the physical landscapewould seem to be fairlyunlikely.Imagesofmoreclassicaldesertlandscapes,suchastheEureka Dunes in Death Valley, California (fig. 1), or the Saharain northwestern Africa (fig. 2), or perhaps even the gleamingwhite gypsum sands in New Mexico’s White Sands NationalMonumentarea(fig.3),probablycometomind;ormaybeeventhearchitecturallyimpressivesandstonesofZionNationalParkinUtah(fig.4),whicharethelithifiedremainsofJurassic-ageeolian(wind-blown)dunes.ButNorthDakotadoesindeedhaveitsshareofsanddunesandotherwindsweptlandforms.

NorthDakota’sDunesDunesarefoundinnearlyeveryregionofthestatebuttendtobeconcentratedintheeast.Thisisdue,inpart,toglacialprocesses,which reworked and deposited younger, immature sedimentsthathadbeenscouredfromtheunderlyingbedrock.OneofthemorevisuallyimpressivedunesisfoundinthesoutheasternpartofthestateinsouthwesternSargentCounty(fig.5)andisapartoftheRiverdaleRidgeDuneswestofthetownofBrampton.

Thereareover30dunefieldsinNorthDakota(fig.6),thelargest

Figure4. Spectacularsedimentarybeddingstructuresexhibited intheNavajoSandstoneinZionNationalPark,Utah.TheNavajoSandstoneisalithifiedeoliandunedepositofJurassicagethatcropsoutextensivelythroughout southern Utah. The dunes consist of reworked shallowmarine deposits from Jurassic seas further to thewest (National ParkService).

Figure 3. Gypsum dunes with wind ripples in White Sands NationalMonumentinNewMexico(NationalParkService).

Figure1.EurekaDunesinDeathValleyNationalPark,attributedtobethetallestinCaliforniaatnearly700feetinheight(NationalParkService).

Figure2.DunesfromtheSaharadesertinnorthwesternAfrica(MaiengaAgency,WikimediaCommons).

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ofwhich isnearDenbigh, in centralMcHenryCounty (table1).Dunesalsodevelopedonglaciofluvial-lacustrinedepositsoftheSheyenneandPembinaDeltasthatformedinGlacialLakeAgassizbetweenabout13,000and9,000yearsago(Bluemle,2000).

DuneStructureAsonemightimagine,therearemanytypesofdunes,eachbeingformed from the ever-changing interactions of wind and sand.Abasic sanddune (fig.7)has thecrestat itshighestpointandtwosides,generallyonelongerthantheother,whicharenamedthe stoss (windswept) and the lee (slipface) sides respectively.Whenmultiple dunes are present, such as in a dune field, thedistancebetweeneachsuccessivedunecrest,iscalledthedunewavelength.

As sand particles are transported by thewind along the dune,fromthestossorwindwardsidetotheleeside,theyareblown

up and over the crest,whereupon gravity takes over, and theycascadedownwardalongtheslipface.Overtimethesandbuildsupintoindividualbedformsontheleadingedgeoftheduneasitproceedsinadownwinddirection.Theleeordownwindsideofaduneiscommonlysteeperthanthewindwardsidebutgenerallywillnotexceedanangleof34o,whichisthecommonlyacceptedvalue for the angle of repose for loose dry sand (Tarbuck andLutgens,1990).

Bylookingattheorientationofthedunecrestsandindividualdunecharacteristics,suchasheight,wavelength,andparticlesizeand

Figure6.LocationofwindblownsandsanddunesinNorthDakota.

Figure5. Viewtotheeastofawell-exposedduneinsoutheasternSargentCounty. ThisduneisapartoftheRiverdaleRidgeDuneslocatedinsoutheasternNorthDakota.Gentlydippingbedformsarevisibleintheupperportionsofthedunejustbelowthesurfacevegetationthatiscurrentlystabilizingthisdune.Telephonepolesintherightforegroundgiveanindicationofscale.Thisparticularduneisover30feetinheight.

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mineralogy,cluestotheeolianconditionsresponsiblefor dune creation can be determined and used tointerpret theclimaticandgeologicenvironmentsatthe time of dune formation, as well as the source(provenance)ofthesand.Forexample,Muhsetal.(1997) deduced that the northwest-trending dunesoftheDenbighDunes(MinotDuneField)weremostlikelyformedwithinthelast1,200yearsorso,whenthe climatewas similar to thepresent, rather thanunderolderpost-glacialconditions.

DuneFieldsinNorthDakotaA dune field is an area where many dunes haveformed as a result of winds blowing the relativelyfiner fractions of sediment from a source area(commonly along a former river or stream) intolocalized accumulations of dunes (fig. 8). A dunefield commonly contains several individual orcomplex dune types, which in North Dakota maybe interspersed with interdune areas where localvegetationhasbecomeabundant,andmayalsobeboundedbylargeaccumulationsoftabular-likesheetsanddeposits.Forthepurposesofdiscussionhere,adunefieldorduneareaisunderstoodtocontainallofthelocalizedsand-sizeddeposits(bothsheetsandsanddunesands)thathavebeenmovedormodifiedbythewind.

Most of the dunes in North Dakota are either

Figure8. MapofbedrockandsurfacegeologicunitsinthesoutheasternCattailBay-WinonaFlatsareainsouthwesternEmmonsCountyalongtheMissouriRiver. Dunesarepresentonboth thenorthandsouth sidesofCattailCreekandareexamplesofwindsweptlandformssourcedfromthefinerportionsoftheseinitiallywater-depositedsediments.

Figure7.Schematiccross-sectionofapairofdunesillustratingsomeoftheirbasicdescriptivecharacteristics.

Figure9.ViewtothenorthwestintotheMissouriRiverValleyatCattailBay(southofWinonaFlats)fromNDHighway1804acrossthedunefieldsouthofCattailCreekinsouthwesternEmmonsCounty.Thelow-reliefundulatingtopographyofthesegrass-covereddunesischaracteristicofmanyofthewind-sweptareaswherelongitudinaldunesarepresentinNorthDakota.

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crescentic (parabolic) or longitudinal (linear) (fig. 9). Areasdominated by linear dunes tend to be larger in expanse withrelatively low localized relief. Conversely, areas of parabolic orcrescenticdunestendtobeclusteredclosertogetherwithhigherlocalrelief(fig.6).

SedimentaryCharacteristicsDunesand iscommonlywell-sorted,meaning that themajorityof individual sand grains are of a similar size (fig. 10). This isdue, inpart, tothevelocityanddurationofwindsthateroded,transported, and deposited the individual sand grains into thedunesthemselves(Boggs,1995).

Duringthedroughtperiodofthe1930s,whenclimaticconditionsin theUpperMidwestweremuchdrier, several dunefields inNorthDakotawere reactivated. Also,newduneswerecreatedfromfreshlyexposedsedimentsources,andinmanyruralareaswerefoundencroachingonfarmbuildingsandotherstructures(fig.11).

DenbighDunesArguably themost familiar sanddunes inNorthDakotaare theonesnearthetownofDenbighinMcHenryCounty.Alsoreferredtobysome(e.g.Muhsetal.,1997)astheMinotDuneField,theDenbighDunescoveranareaof990squaremiles(2,564km2)orabout28standardtownships.Localreliefisashighas100feet(30m)withdunescommonlyaround5to20feet(1.5-6m)inheight.Dunesarenorthwesttrendingparabolictypes(crescenticshaped)that are currently classified as being inactive because they arestabilizedbyawell-developedcoverofprairievegetation.

SandfromtheDenbighDunescanandhasbeendescribedasaquartzcommon,yellow-brown(onoutcrop),well-sorted,fine-tomedium-grained sand,with dominantly angular to subroundedgrains,thatisgenerallydevoidofcarbonates(Lemke,1960;Lord,1988.)However,recentanalyticalworkhassuggestedthatthese

Figure10.DistributionofsedimentgrainsizesofselecteddunesandsfromNorthDakota.Theseeoliansedimentscanbeclassifiedaswellsorted,fine-grainedsands.Grainsizedistributiondiagramslikethiscanbeusedtodeterminetheamountofsandinasamplethatisofaparticulargrainsize.

Figure11.SanddunesencroachingonthesideofabarninKidderCountyduringthe1930’sdrought.Sandripplescanbeseenonthesurfaceoftheduneindicatingthatthedirectionofsedimenttransportwasparalleltothefaceoftheroof(SHSND,ca.1930).

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Figure12:(a)Aerialobliqueviewtothesoutheastofahigh-reliefportionoftheDenbighDunesjustsouthofDenbighalongNDHighway2,McHenryCounty (modified fromBluemle,1987). (b) Viewtothenorthofgrass-covered,high-reliefdunes located just to thenorthwestofDenbigh. (c)Photomicrographofdunesandcollectedfromaroad-cutinthenorthernDenbighDunesarea(25Xmagnification).

sandsmayhavehigheroverallacidsolubilities(around17%)thanothersandsfoundinNorthDakotabyafactoroftwo(Anderson,unpub. data), which could be explained by glaciolacustrineprocesses associated with the existence of Glacial Lake Souris(Lord,1988).

NorthDakota’sdiverseeolianlandscapesareveryscenicwithaninterestingassemblageoffloraand localwildlife thatcanmakeevenashortvisittotheseareasbothinterestingandenjoyable.

SelectedReferencesBluemle, J.P., 1987, Dunes: North Dakota Geological Survey

Newsletter,December1987,p.36-41.Bluemle, J.P., 2000, The Face of North Dakota: North Dakota

GeologicalSurveyEducationalSeries26,210p.Boggs,S.,Jr.,1995,PrinciplesofSedimentologyandStratigraphy

(2nded):Prentice-Hall,774p.

Lemke, R.W., 1960, Geology of the Souris River Area, NorthDakota:U.S.GeologicalSurveyProfessionalPaper325,138p.

Lord,M.L.,1988,SedimentologyandstratigraphyofGlacialLakeSouris,NorthDakota;effectsofaglacial-lakeoutburst:GrandForks,UniversityofNorthDakota,PhDdissertation,221p.

Muhs,D.R., Stafford,T.W., Jr.,Been, J.,Mahan, S.A.,Burdett, J.,Skipp,G., andMuhs,R.Z. 1997,Holoceneeolian activity intheMinot dune field, North Dakota: Canadian Journal ofEarthSciences,v.34,no.11,p.1442-1459.

Tarbuck,E.J.,andLutgens,F.K.,1990,Earth-AnIntroductiontoPhysicalGeology(3rded):Princeton,N.C.,MerrillPublishing,330p.

FurtherReadingMcKee,E.D.,1979,AStudyofGlobalSandSeas:U.S.Geological

SurveyProfessionalPaper1052,429p.

(a) (b) (c)

Kickoff to Summer Reading Day, 2011When was the last time you were able to sit down with a good book? This is just the thing that the annual Kickoff to Summer Reading day is trying to encourage in children of all ages. The theme this year was “One World, Many Stories” and participants were encouraged to use that topic to relate to children and inspire them to read. The North Dakota Geological Survey was one of more than 20 sponsors of the event.

The event is held annually on the capital grounds, in the Heritage Center, and at the State Library, and attracts many area residents. This year more than 3,000 people walked through the doors of the Heritage Center during the event.

The North Dakota Geological Survey paleontology department has participated in these activities for many years and we show off various fossils pulled from the North Dakota State Fossil Collection. The paleontology department had a table set up and answered many questions about the fossils of North Dakota, and

encouraged people to think of their own stories about the fossils. A “pin-the-tail-on-the-dinosaur” activity was available, as well as the recently published NDGS coloring book by Hoganson and Gould (ND Ed series 32, 2011).