, ▁trees ▁would ▁later ▁leave ▁behind ▁wood ▁to ▁pet r ify . ▁Le aves ▁are ▁also ▁among ▁the ▁state ' s ▁Pale oc ene ▁foss ils . ▁E oc ene ▁plant ▁foss ils ▁are ▁known ▁along ▁the ▁Kn ife ▁River . ▁F resh water ▁cl ams ▁and ▁sn ails ▁are ▁known ▁in ▁the ▁areas ▁north ▁and ▁east ▁of ▁the ▁Missouri ▁River . ▁The ▁C annon ball ▁depos its ▁near ▁Min ot ▁are ▁the ▁last ▁marine ▁depos its ▁to ▁have ▁been ▁left ▁in ▁the ▁North ▁American ▁interior . ▁As ▁the ▁C en oz o ic ▁progress ed , ▁North ▁Dak ota ' s ▁sea ▁would ▁give ▁way ▁to ▁subt rop ical ▁sw amps . ▁Rich ▁North ▁Dak ot an ▁for ests ▁included ▁plants ▁like ▁cy c ads , ▁dawn ▁red wood s , ▁f ern s , ▁fig s , ▁and ▁pal ms . ▁The ▁local ▁veget ation ▁left ▁behind ▁great ▁co als ▁depos its ▁in ▁the ▁rock ▁record . ▁Through out ▁the ▁remainder ▁of ▁the ▁C en oz o ic , ▁North ▁Dak ota ' s ▁climate ▁co o led ▁and ▁d ried . ▁The ▁state ' s ▁sw amps ▁van ished ▁and ▁their ▁inhabitants ▁became ▁ext inct . ▁In ▁their ▁place ▁wood lands ▁formed . ▁The ▁climate ▁got ▁col der ▁still ▁and ▁eventually ▁gla cial ▁activity ▁res h aped ▁the ▁state ' s ▁landscape . ▁At ▁this ▁time ▁North ▁Dak ota ▁was ▁home ▁to ▁m amm oth s ▁and ▁m ast od ons . ▁ ▁History ▁ ▁Ind igen ous ▁interpret ations