As you finish your long traverse in the red layers of the Supai Group, you’ll come to a very steep part of the trail, where switchbacks wind down through the next layer, the Redwall Limestone. It doesn’t matter which trail you are on – you’ll feel as though you arrived at the edge of the earth as you step off this 400 to 500-foot thick cliff! The Redwall Limestone forms a sheer cliff that looks rose-colored on the outside, but if you see a fresh surface of it, is actually a lovely silver-gray. Fossils of creatures similar to those found in the Kaibab Formation tell us that this layer formed in another warm and shallow tropical sea, one that stretched all the way across the continent 330 million years ago. The only things sticking up out of this ocean were the Antler Mountains in Nevada (long since eroded) and the beginning of the Appalachian Mountains to the east. So you could have strapped on a snorkel in Nevada and snorkeled almost all the way to the east coast in the same ocean! If you’ve gone to Mammoth Cave National Park in Kentucky, you’ve been in limestone from the same ocean. Kentucky, Indiana, Wyoming, southern Arizona all have limestones that are essentially the same as the Redwall, just called different names. They may be a little older or younger, but they were formed in the same ocean. Another interesting feature of the Redwall is that it is filled with caves and underground passages eroded by flowing groundwater many hundreds of millions of years ago. Most of these waterways have run dry, but some are still flowing and where that water intersects the canyon, it pours out in a waterfall, spring or seep. Most of the perennial side streams in Grand Canyon have their origin as springs in t he Redwall Limestone, or the underlying Muav Limestone.
You’ll be glad to finish your steep switchback descent through the Redwall and as you do, you will have crossed over millions of years to the next layer. In the main corridor, there is one thin formation under the Redwall that is hard to pick out as you go through it, but it is interesting and if you look carefully, you can find it. Just at the base of the Redwall cliff, another thinner purplish-red cliff sticks out like a buttress. This is the Temple Butte Formation, another limestone that is much older than the Redwall – over 400 million years old. The Temple Butte changes dramatically from one end of the canyon to the other. In the eastern Grand Canyon, the Temple Butte exists only as lens-shaped deposits in between the Redwall and the Muav Limestones. These purplish smile-shaped layers have been interpreted as ancient river or estuary (tidal) channels. The Temple Butte becomes a thin but continuous layer by the time it reaches the Phantom Ranch area, and becomes thicker as you head to the west. At the western edge of the canyon, the Temple Butte Formation is thicker than the Redwall! What this probably represents is the estuary channels of the east emptying into the ocean, which becomes progressively deeper as you go to the west, offshore in this ancient ocean.
Below the Temple Butte Formation, the trail will soften a little as you head into the next layer, a mixture of yellowish-green slopes and cliffs called the Muav Limestone. As you step down into this new limestone, you cross over into a time so ancient that there was no life on land, only in the oceans. The Muav, and the formations just underlying it, the Bright Angel Shale (shown right) and the Tapeats Sandstone, were all formed in an ocean that is half a billion years old – 530 million years! These three formations really need to be discussed together, since they were all formed in different parts of the same ocean, as it advanced across the land. The Bright Angel Shale is a soft purple and green banded layer that erodes easily to form gentle slopes and rounded hills. As this soft layer eroded back away from the edge of the Inner Gorge, it formed a wide plateau known as the Tonto Plateau. If you walk out to Plateau Point, or sit at Tipoff, you are on the Tonto Plateau.
Underneath the Bright Angel Shale lie the thin brown ledges of the Tapeats Sandstone, which forms a cap on the Inner Gorge like icing on a cake. In order to understand these formations, you need to start with the oldest, the Tapeats, and work your way up to the Muav. About half a billion years ago, the ocean began to move in across the land from the west. As the sea advanced the waves ground up the underlying rock into a sandy beach, which we see as the Tapeats. This sediment was all laid down near the shore, in the beach and slightly offshore zone. As the water moved eastward, and the sea became deeper in the Grand Canyon region, muds and clays began to settle out in the calm, deeper, offshore waters away from the beach. This we see as the Bright Angel Shale, complete with worm burrows, trilobites and trilobite tracks in the mud. As many as 40 different kinds of trilobites have been found in the Bright Angel Shale, they’re just not as nicely preserved as the ones you find in rock shops! Finally, as the sea reached its furthest point to the east, the water in the Grand Canyon region was far enough offshore that no more sand or mud reached the sea floor, only creatures with limey shells that lived and died and became the Muav Limestone. This illustration of an ancient ocean advancing onto the land is so perfect in Grand Canyon that it is illustrated in geology textbooks worldwide.
So now you’ve come down through all the “young” layers of Grand Canyon. You are at just below Indian Gardens on the Bright Angel Trail, or just below Tipoff on the Kaibab. Hang onto your hats, you’re about to go WAY back in time with your next step!