Mesozoic Colorado

At the beginning of the Triassic period, the land that would become Walnut Hills (oh, that's too many words. Let's just call it "Walnut Hills") was at longitude -31.6506, latitude 8.2783. Today, that's about halfway between Venezuela and Northwest Africa, and it's definitely tropical. The global landmass was Pangaea and dinosaurs were well established as the dominant life form.

 Colorado was beachfront property with the ocean to the north and west. It was situated on the western side of a large peninsula on northern Pangaea. 

This was 250 million years ago and the coastal areas and inland deserts were covered with fine, red, windblown sand that would later pack down into a fine shale. Pangaea was about to break apart.

The North America drifted northward during Jurassic times. Early on, the sea was to the west but land rose there and inland seas flowed in to cover most of what would become Colorado. Sands were washed in that would become the Morrison formation.

At the end of the Jurassic period, the North American craton had drifted north and Walnut Hills was 35 degrees above the equator. There were mountains to the west but sea flowed in from the northwest.

225 million years passed as the Ancestral Rockies wore down and the sea covered the lands to the east and receded repeatedly laying down sediments that would become the Dakota sandstone to the west and the Pierre Shale, underlying most of the Denver region. Then during Cretaceous times, the land was buckled again, probably by the Pacific plate crashing into the western edge of the continent, beginning the Colorado uplift and the Laramie orogeny ("orogeny" means "mountain building event"), the creation of today's Rocky Mountains. As soon as the uplift began, erosion started tearing it down. 

As counterpoint to the upward motion of the land to the west, the Denver area sank to form the Denver Basin and sediments began pouring in. The Rocky Mountains literally buried themselves. The Great Plains formed a gradual ramp from the east all the way to the summit of the mountains. The mountains were not near their full heights but Walnut Hills was on the ramp at a considerably higher elevation.

At the end of the Cretaceous, the outlines of North America would have been recognizable to any modern inhabitant and Denver was around -80 degrees longitude, and 47 degrees latitude, east of where it is today but a little further north.

And suddenly, something happened that wiped out half of all life on Earth, including all the dinosaurs (well, chickens and crocodilians are still around). Big animals were doomed, clearing the way for smaller creatures, including mammals, to establish dominance. One would evolve into humans.

Most geologists today think that an asteroid slammed into the area that is today the Yucatan peninsula, about 66 million years ago, throwing enough material into the atmosphere to block the sun for years, casting the Earth into what seemed like eternal winter.

The first phase of uplift that began to create the Rockies in the Cretaceous period continued for 32 million years, well into the Eocene period. Walnut Hills was on a gently sloping plain that rose gradually to the west.