The first thing you notice when looking at the Grand Canyon is the different layers of rocks that are exposed. Each separate layer was deposited millions of years ago, in varying types of environments, and each layer has a name. The diagram at right is a cross section of the different rock layers as you descend into the canyon on the Bright Angel Shale (Thayer, 1990). The following mnemonic sentence provides an easy way to remember the primary rock layers in the Grand Canyon: Know (Kaibab Limestone) The (Toroweap Formation) Canyon's (Coconino Sandstone) History (Hermit Shale), Study (Supai Formation) Rocks (Redwall Limestone) Made (Muav Limestone) By (Bright Angel Shale) Time (Tapeats Sandstone). The only primary layer that is missing here is the Vishnu Schist which you will simply have to remember to add.

•Paleozoic Strata

Kaibab Limestone - This layer averages about 250 million years old and forms the surface of the Kaibab and Coconino Plateaus. It is composed primarily of a sandy limestone with a layer of sandstone below it. In some places sandstone and shale also exists as its upper layer. The color ranges from cream to a grayish-white. When viewed from the rim this layer resembles a bathtub ring and is commonly referred to as the Canyon's bathtub ring. Fossils that can be found in this layer are brachiopods, coral, mollusks, sea lilies, worms and fish teeth.

••Toroweap Formation - This layer averages about 255 million years old and is composed of pretty much the same material as the Kaibab Limestone above. It is darker in color, ranging from yellow to grey, and contains a similar fossil history.

•Coconino Sandstone - This layer averages about 260 million years old and is composed of pure quartz sand, which are basically petrified sand dunes. Wedge-shaped cross bedding can be seen where traverse-type dunes have been petrified. The color of this layer ranges from white to cream colored. No skeletal fossils have yet to be found but numerous invertebrate tracks and fossilized burrows do exist.

•Hermit Shale - This layer averages about 265 million years old and is composed of soft, easily eroded shales which have formed a slope. As the shales erode they undermine the layers sandstone and limestone layers above which causes huge blocks to fall off and into the lower reaches of the Canyon. Many of these blocks end up in the side drainages and down on the Tonto Platform. The color of this layer is a deep, rust-colored red. Fossils to be found in this layer consist of ferns, conifers and other plants, as well as some fossilized tracks of reptiles and amphibians.

•Supai Formation - This layer averages about 285 million years old and is composed primarily of shale that is intermixed with some small amounts of limestone and capped by sandstone. The limestone features become more and more prominent in the western regions of the Canyon, leading one to believe that that region was more marine. The eastern portions where probably a muddy river delta that fed into an ancient sea. The color of this layer varies from red for the shale to tan for the sandstone caps. Numerous fossils of amphibians, reptiles and terrestrial plants exist in the eastern portion which are replaced by marine fossils as you move westward.

•Redwall Limestone - This layer averages about 335 million years old and is composed of marine limestones and dolomites. This is probably the most prominent rock layer in the Canyon as it usually forms a sheer cliff ranging from 400-500 feet in height, which has become a natural barrier between the upper and lower regions of the Canyon. The only way though this barrier is in areas where the rock has faulted and broken apart to form a slope which can be climbed upon. The deep reddish color of this layer is caused by iron oxides leaching out of the layers above it and staining its outward face. Behind the reddish face the rock is a dark brownish color. Numerous marine fossils can be found in the Redwall Limestone including brachiopods, clams, snails, corals, fish and trilobites. Many caves and arches can also be seen in the Redwall.