Almost everything about the geology of Eastern Oregon originated with volcanic or geo-thermal activity of some kind, somewhere in the Western part of the USA during the past 50 million years. Of course, some of this activity is still taking place, to one extent or another, in Yellowstone and at various places throughout the Cascade Mountain Range. Sporadically during the not-so-distant past, the last 50 million years or so, this activity would blossom into very large events that could cover the landscape for hundreds of miles around with layers of ash-fall 50 feet or more deep; or rivers of red-hot flowing basalt that would cover several thousand square miles with a 200 foot layer of solid rock after it cooled; or volcano caused mud-flows that would pour down and cover everything in their paths for hundreds of miles around. These were events far larger than anything we've witnessed in our lifetimes... Mt. St. Helens was a very small event on the scale when compared to this early geologic activity.
During the same historic time period, the last 50 million years, mammals came into existence and were developing, competing, adapting, and evolving. Because of the explosive nature -- the quickness and speed -- of the volcanic activity, often these early animals were unable to escape and were frozen in time within a matrix of material that was perfect for preserving their bodies along with the plants of the time. This combination of large scale events that covered the landscape with thick layers of material and large numbers of rapidly developing and evolving animal and plant life came together to create the most complete and highest quality fossil beds in North America, and possible the world.
The colors of the rocks and hillsides are dramatic and quite variable -- due to chemicals and minerals contained in the original material, and the natural processing of those materials that took place over many years, the pressures of the layers above, etc.
There are greens, (that to our surprise are not the result of copper), reds, (which are due to high iron content), pinks, yellow, light blue, white, and more. Of course, the oldest layers are at the bottom.
If someone has any interest in natural history, paleontology, or geology, this is a wonderful place to explore. Frankly, I wasn't expecting very much from this exploration. But it turned our to be a real gem.
John Day Fossil Beds National Monument