If you were to write a wildlife guide to eastern Oregon as it was, say, 30 million years ago, you'd have to describe bear-dogs, entelodonts, saber-toothed cats, rhinoceros and many other strange creatures. In fact, the book exists, in the form of fossil beds that cover some 10,000sq mi (including more than 670 identified sites) of north central Oregon. Named for a major river that flows through the region, which in turn was named after an obscure fur trapper who passed through Oregon around 1811, the national monument protects a few of the better pages of that great book. Together they tell the story of ancient plants, animals, climate, and the grand workings of geology. What makes these fossil beds significant is that they preserve not just one era but a nearly continuous fossil record from 54 million years ago to about 6 million years ago. During that time, the Cascade Mountains came into being. As they rose, they created a barrier that cut off ocean moisture from the interior, gradually transforming what had been a warm semi-tropical forest soaked by about 100in of rain per year, to the near-desert that exists today. As the climate changed, resident plants and animals also changed, through evolution and migration. Meanwhile the volcanic Cascades belched periodic waves of airborne ash which buried and preserved the various organic remains of that changing scene. In some cases, specimens were washed into ash-filled ponds and streambeds, creating deposits that would one day provide rich ground for paleontologists and a wealth of sites for visitors to explore.