There is no fossil formation on the planet quite as memorable as the Hell Creek Formation, stretching across rocks that make up parts of modern day Wyoming, Montana, North Dakota and South Dakota.

From bison encounters to ancient wilderness, these are the Most Treasured Views in South Dakota. Here’s how to see them.

So, too, in these ancient spiders, whose silvery tapeta still shine in the fossils. “They're so reflective—they clearly stick out at you,” says study coauthor Paul Selden, a paleontologist ...