Ashfall Fossil Beds is a fascinating park showing the results of a volcanic eruption in what is now Idaho some 12 million years ago. The ash entered the lungs of the animals living over a wide area and slowly killed them. Their bones were preserved by the drifting ash as they fell by, and in, a waterhole. The skeletons are quite well preserved even today. This is a site of ongoing research by the University of Nebraska and when you visit you can frequently see students, and sometimes professors, busily studying the fossils and the environment in which they fell. The park is open from 1 May to mid October. Hours vary a bit so you may want to check their website. Admission is $5 per person plus you will need a Nebraska park permit $5 daily.
Fondest memory: Talking to the students and watching them recover the fossils.