If you've ever wondered why so many of the dinosaurs we know about date from oddly specific time periods--say, 70 to 65 million years ago--the answer lies in formations like Hell Creek, whose sediments date from the late Cretaceous period, right before the dinosaurs went extinct. Discovered in 1902 by the famous paleontologist Barnum Brown, Hell Creek has yielded dozens of genuses of dinosaurs, of all varieties, including ceratopsians, hadrosaurs, ankylosaurs, ornithomimids and pachycephalosaurs.
What did Hell Creek look like 70 million years ago? Based on the relatively small dinosaurs that lived there (this part of North America wasn't exactly a hotbed of sauropods or tyrannosaurs), it seems likely that the area was thickly forested, which would have imposed natural size limits on its inhabitants. This conclusion is supported by the non-dinosaur fossils that have been found in Hell Creek, including the remains of tortoises, monitor lizards, and tree-dwelling mammals.
Hell Creek is famous for a reason besides its fossils: the iridium deposits in its uppermost layer of sediment offer firm support for the hypothesis that a meteor impact 65 million years ago caused the extinction of the dinosaurs. Some paleontologists have interpreted the rock evidence as showing that some hardy dinosaurs survived for a couple of million years after the impact, but this theory doesn't have wide support.
