The rim of the canyon is formed of a layer about 300 feet thick called the Kaibab Formation. This creamy yellow limestone has fossils in it: sharks, fish, corals, brachiopods, bryozoans, crinoids and sponges, that tell us it formed in a shallow, warm, Caribbean-like ocean. 245 million years ago, Arizona lay near the Equator and a clear, warm sea no more than a couple hundred feet deep covered this region. This sea teemed with ancient life forms, many of who became extinct just after the Kaibab Formation was formed.
The coastline of this ancient sea lay near New Mexico; westward the sea remained shallow until the sea floor dropped off to deep water somewhere around central Nevada. To the east, beyond New Mexico, all the continents were assembled into one giant land mass called Pangea; Arizona was the edge of this massive supercontinent. You would have had to walk a long way to get to the ocean again on the other side of the continent!
As you continue down the trail, the next layer is the Toroweap Formation. Limestone, mixed with a little sand and mud, make this layer not quite as hard a cliff. Instead the Toroweap will be a cliff in part, but with slopes in between these cliffs. The Toroweap was also formed in the ocean, but much nearer the shoreline. This explains the mud and sand mixed in with the lime; mud and sand were able to wash off the land and mix in with the ocean sediments to form an impure limestone. On the Kaibab and Hermit Trails, you can see “teepee” shaped features in the walls that form when excess water is pushed out of the sediment by the weight of overlying sediment, like when you go to the beach and stomp on the sand to bring water up. This tells us that the Toroweap was buried by Kaibab sediment while it was still soft and gooey, and the water pushing up through it deformed the still soft sediment.
Below the Toroweap, the trail will change dramatically as you enter the third formation down: the Coconino Sandstone. This sheer, white 350-foot cliff is a major barrier to canyon travel, and it is usually only possible to build a trail through it if movement along a fault has broken it. Even so, the Coconino creates a steep trail, usually marked by numerous switchbacks. The Coconino Sandstone is a pure sand made of quartz grains, so it is a gleaming white where a fresh surface has been exposed by a rock fall. Features in the Coconino, including large swooping lines that look like pulled taffy, tell us that 260 million years ago this part of Arizona was covered with windblown desert/coastal dunes. The angled, swooping lines you see in the Coconino are the internal structure of wind blown dunes, called crossbeds. The lines angle in the direction that the wind blows. In the case of the Coconino, the wind blew constantly towards the southwest. The tracks of reptiles, spiders and scorpions have been found wandering the dunes of the Coconino. Perhaps they were looking for water; it does seem to have been a very dry environment. The edge of this ancient dune field lay to the south, in the Sedona area. Remember, at this time, there were no dinosaurs, no mammals, no birds. Only reptiles and amphibians ruled the land as vertebrates.
Below the Coconino, your trail will turn red and soft as you move down into the Hermit Shale, a gentle slope covered with trees and blocks of sandstone broken off from the overlying Coconino cliff. In some places, you can’t even really see the Hermit at all—it’s so covered up. Imagine a river system like the Mississippi, flooding out over its banks and covering the land with rich mud, and you’ve got the environment the Hermit was laid down in, 280 million years ago. At that time, a big river system was winding its sluggish way across the lowlands of the region, flooding and shifting channels back and forth. Amphibians and reptiles left their tracks in the mud, and the impressions of ferns and tree bark tell us that the banks of these channels had plants growing along them. The rich red color of the Hermit Shale is a sure indication that the sediment was formed in an environment with lots of oxygen to rust the iron in the mud. Running water is one of the best ways to provide this oxygen to the sediment, so rivers, floodplains and tidal flats often deposit sediment that will turn that rich red color.
Moving further back in time, your trail takes you into the 800-foot thick stack of sandstones and shales collectively called the Supai Group. Around 300 million years ago, great masses of sand and mud were washing down rivers from mountains to the east, in western Colorado and eastern Utah. For millions of years, these sediments accumulated in the rivers, floodplains, dune fields and near shore environments of the Supai Group. Made up of four formations (top to bottom, these are the Esplanade, Wescogame, Manakacha, and Watahomigie), the Supai appears in the canyon as a series of ledges and slopes, ledges and slopes, bright red in color.
The sandstone ledges represent portions of the ancient environment where faster-moving water or wind could pick up and deposit sand; the muddy slopes represent lower energy water, sluggishly carrying mud and silt. Fossils in the Supai include reptile and amphibian tracks and numerous kinds of plants.
At this same time back east, massive forests were forming along the rivers draining off the newly uplifted Appalachian Mountains. These forests would ultimately form coal in thick deposits throughout the east. We did not have the same thick forests in this region, but the banks of these rivers had some vegetation growing along them, with places for newly-evolved amphibians to hang out and look for food, mate and lay their eggs.