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 moreeasily eroded
shale below.
Notice in the Coconino Sandstone dramitic
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
