See the context of this sign.

Six Rock Layers

Grand Canyon's top six rock layers are clearly
visible on the opposite cliff. Together they
represent 80 million years of ancient envornments:
swamps, seas, and deserts.

Notice that some 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 Supai Group,
composed of interbedded shale and sandstone,
forms alternating slopes and cliffs.

A wide range of fossils has been
found in these rocks, such as
this marine brachioped from the
Kaibab Limestone.

Don't miss the rest of our virtual tour of Grand Canyon National Park in 10803 images.