outwash plains - Michigan State University

Outwash plains made up of outwash deposits are characteristically flat and consist of layers of sand and other fine sediments. Such plains with their sandy ... OUTWASHPLAINS Thelargequantitiesofwaterthatflowedfromthemeltingicedepositedvariouskindsof materials,themostimportantofwhichiscalledglacialoutwash.Outwashplainsmadeup ofoutwashdepositsarecharacteristicallyflatandconsistoflayersofsandandother finesediments.Suchplainswiththeirsandysoilsareoftenusedforspecializedkindsof agriculture,suchasthepotatoproductioninMontcalmCounty.    Asthemarginsoftheicelobesretreated,glacialmeltwaterand precipitationdrainedsouthwardtotheGulfofMexicobecausehigherlandorglacialice blockedflowinotherdirections.Ice-marginallakes(orproglaciallakes)formwhenthe landinfrontoftheicemarginslopestowardtheice,allowingmeltwatertopond directlyincontactwiththeice,asshownbelow. However,whenthelandslopedawayfromtheice,aswasthecaseinmostof extremesouthernMichigan,andincertainlocationselsewhere,watercouldnotpondin frontofit. Instead,meltwaterstreamsformed,washingsedimentout,awayfromthe ice. Sandandgravel,whichthesestreamscould notcarry,waslaiddownasoutwash,inbroad,flatoutwashplains(seethediagram above). Rapidmeltingoftheiceasitstoodatendmorainesproducedrapidly-flowing,broad sheetsofwateralongtheicefront.Movingswiftlyoverthemorainicmaterial,thewater workeditover,sortedit,carrieditoutfromthemoraine,andasthecurrentsslackened, droppedthesorteddebris,leavingthecoarsecobbles,gravelandsand–spreadin thatorderinfrontofthemoraine.Thesestratifiedgravelly,gentlyundulatingtoflat plainsarecalledoutwashplains—theycanbebestseenastheflatlandsof Kalamazoo,Cass,andRoscommoncountiesandthosearoundBaldwin,Cadillac,andGrayling. Althoughresemblingeachother,outwashandtillplainsarereadilydifferentiated.Atill plainisbehind(towardsthedirectionofretreat)ofamoraine,andtheoutwashplainis infrontofthemoraine.Atillplainiscomposedofunsortedmaterial(till)ofallsizes withmuchclay,anoutwashplainismainlystratified(layeredandsorted)graveland sand.Thetillplainhasagentlyundulatingtohillysurface;theoutwashisflatorvery gentlyundulatingwhereitisathinveneerontheunderlyingtill.Asaruleflourishing farmsareonthetillplains;themajorityofabandonedfarmsinthenorthernpartofthe LowerPeninsulaareonsandyoutwashplains.Hardwoodforestsgrowontillplains; conifersontheoutwashplains.    Outwashplainswerebuiltoverthetopoflandscapesthathadbeenonly recentlydeglaciated,andhencesomeiceblocksfromthatpreviousiceadvanceoftenstill layonthelandscape.Someoftheseiceblocksbecameburiedbyoutwashsandas sedimentationcontinuedandtheirmeltingwasdelayedforhundredsofyears.Eventually thedebris-coveredicemeltedandleftbasinswhichhavesincebeenfilledwithwaterand form"pit"lakes.HigginsLake,Gull,Cadillac,andmanyotherlakes,arein depressionsof"pittedoutwash."Probablyallsuchpitswereoncefilledwith waterbutmanyofthemarenowswamps,othersaredrybasins,andothersarerepresented bythepatchesofblacksoilontilloroutwash. Themapaboveshowsthat,asofabout14,800yearsago,theicemarginswerepositioned suchthatmeltwatersflowedawayfromtheice(werenotpondedinfrontofit).  Thus,largeoutwashplainsformed,andcanbeseentodayasflat,sandyplainsinmany partsofCass,St.Joseph,andHillsdaleCounties. Cutoverforestlandsarefrequentlyfoundonoutwashplains.Fromthiswemaygetthe impressionthatalloutwashplainsarewasteland.However,broadstretchesofoutwash makeupthefarmlandsbetweentheTekonsha,Kalamazoo,Valparaisomorainesandothersin thesouthwesternpartoftheState.InthesouthwesternpartoftheStatetheearly settlersfoundlargeunforested,veryfertile,areaswhichtheycalled"oak openings"orprairie.Theyaresituatedmainlyonoutwashplains.Ifirrigated,these outwashplainsprovideveryfertilefarmland. Theimagebelowisofasandy, irrigatedoutwashplain,withalargeendmoraineinthe background.    Inplaces,glacialmeltwaterflowingfromtheicewasconfinedto channels,andcutspillwaysthroughthemorainesorwasforcedtoflowinanarrowchannel alongtheicefront.Meltwater,sorestricted,builtvalleytrainsinthedrainageways ---depositsofstratifiedsandandgravelwhichextendedoutfromandalongtheborders oftheice.Theyaresimilartotheoutwashinmaterialandstructure,butusuallyquite limitedinextent.Outwashplainsarewideandformedinplaceswherethewaterflowedout inarelativelyunrestricedmanner;valleytrainsarelongandnarrowandformedin restrictedvalleys.OtsegoLake,inOtsegoCounty,isinawidevalleytrain.Manyold pre-glacialvalleysinthewesternhalfoftheNorthernPeninsulawereeitherwhollyorin partfilledwithvalleytraindeposits. Today,outwashplainsareoneofMichigan'smainsourcesofaggregate--sandandgravel. Themapbelowshowshowsandand graveloperationsarelocatednearlargepopulationcenters.  Note,intheimagebelow,thelargeproportionofgravelinthisoutwashsediment. Inotherareas,outwashismostlysand(below),andhenceoflessvalue. Inall instances,however,theoutwashsedimenthasbeenstratified(sortedintodistinctlayers) bytherunningglacialmeltwater. Thismaterialhasbeencompiledforeducationaluseonly,andmay notbereproducedwithoutpermission. Onecopymaybeprintedforpersonal use. PleasecontactRandallSchaetzl([email protected]) formoreinformationorpermissions.