Existing comment:
Five Rock Layers
Grand Canyon's top five rock layers are clearly visible on the opposite cliff. Together they represent 50 million years of ancient environments: swamps, seas, and deserts.
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.
Notice in the Coconino Sandstone dramatic evidence of ongoing erosion. In 1991 a large section of the Coconino cliff collapsed and cascaded onto layers below.
A wide range of fossils have been found in these rocks, including these reptile tracks from the Coconino Sandstone.
Kaibab Limestone -- Shallow inland sea -- 250 million years ago
Toroweap Formation -- Sandstone, limestone, and gypsum -- Shallow inland sea -- 260 million years ago
Coconino Sandstone -- Windblown sands -- 270 million years ago
Hermit Shale -- Swamps and lagoon -- 280 million years ago
Supai Group -- Sandstone, limestone, and shale -- Lagoon and tidal flat -- 300 million years ago |