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Straddling the Utah-Colorado state line, Dinosaur National Monument protects one of North America's largest dinosaur fossil beds, discovered here in 1909. Though both states' sections are beautiful, Utah has the bones. Don't miss the Quarry Exhibit, which is an enclosed, partially excavated wall of rock with more than 1600 bones protruding – quite the sight to see.
In summer, hours may be extended a little and you will have to take a shuttle to see the quarry; out of season you may be required to wait until a ranger-led caravan of cars is scheduled to drive up. From below the quarry parking lot, follow the Fossil Discovery Trail, a 2.4 mile round-trip, to see a few more giant femurs and such sticking out of the rock. The rangers' interpretive hikes are highly recommended. Plus there's easily accessible Native American rock art to see on the Utah side.
In Colorado, the Canyon Area is at a higher elevation – with some stunning overlooks – but is closed due to snow until late spring. Both sections have numerous hiking trails, interpretive driving tours (brochures for sale), Green or Yampa river access and campgrounds ($8 to $15 per campsite). The Quarry portion of the park is 7 miles north of Jensen, UT, on Hwy 149. The Canyon Area is roughly 30 miles east, outside Dinosaur, CO.