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A hiker walks along the Apatosaurus dinosaur trackway along the Purgatoire River in the Comanche National Grasslands of southeast Colorado. During the late Jurassic Period about 150 million years ago, dinosaurs congregated along the shores of a vast freshwater lake in what is now southeastern Colorado. They left behind large trackways in the mud which shows up today eroded by river waters as over 1300 dinosaur footprints in limestone along the Purgatoire River. Most of these tracks are by made by herding Apatosaurus or sauropod dinosaurs. Part of the Picket Wire Canyonlands, the tracksite is the largest dinosaur tracksite in North America.