The Grand Canyon's top five rock layers are clearly visible on the opposite cliff. Notice that some rock layers form sheer vertical walls, while others form slopes. Shale and siltstone erode easily and crumble into slopes. Sandstone and limestone resist erosion in this arid climate, but shear off vertically as they are undermined by the more easily eroded shale below.
The top layer is Kaibab Limestone from a shallow inland sea 250 million years ago. It is composed primarily of a sandy limestone with a layer of sandstone below it. In some places, sandstone and shale also exist as its upper layer. The color ranges from cream to a grayish-white. Fossils found here are brachiopods, coral, mollusks, sea lilies, and worms. The next layer below is Toroweap Formation - a layer of sandstone, limestone, and gypsum from a shallow inland sea 260 million years ago. It is darker in color, ranging from yellow to gray. This layer contains a similar fossil history. The third layer is Coconino Sandstone from windblown sands about 270 million years ago. It's composed of pure quartz sand and is basically a petrified sand dune. The color of this layer ranges from white to cream colored. No skeletal fossils have been found yet but numerous invertebrate tracks and fossilized burrows do exist. The fourth layer is Hermit Shale from swamps and lagoons 280 million years ago. It's composed of soft, easily eroded shales which have formed a slope. As the shales erode, they undermine the layers of sandstone and limestone above which causes huge blocks to fall off and into the lower reaches of the Canyon. The color of this layer is a deep rust-colored red. Fossils found in this layer included ferns, conifers, and other plants, as well as fossilized tracks of reptiles and amphibians. The fifth and bottom layer is the Supai Group - sandstone, limestone, and shale from lagoons and tidal flats 300 million years ago. This layer is composed primarily of shale intermixed with small amounts of limestone and capped by sandstone. The limestone becomes more prominent in the western regions, leading one to believe that that area was more marine. The eastern portions probably had a muddy river delta that led into an ancient sea. The color of this layer varies from red for the shale to tan for the sandstone caps. Numerous fossils of reptiles, amphibians, and terrestrial plants exist in the eastern portion which are replaced by marine fossils as you move westward.