The Morrison Formation extends for 600,000 square miles throughout the Western United States, though most of it is covered by other rock layers. Where it is exposed, its red and green rock layers preserve large amounts of highly detailed fossils that excite paleontologists and geologists and help researchers gain a clearer understanding of what the world looked like during the Jurassic Period. This area was a vast floodplain in a semi-arid environment 155 million years ago, where frequent flash floods covered the banks of streams in muddy silt while pools in the streams collected coarser sands. Over the course of 10 million years, these streams continued to stack sediments on top of each other, and those sediments were compressed into 280 feet of mudstone and sandstone.