Grand Canyon attracts the attention of the world for many reasons, but perhaps its greatest significance lies in the geologic record that is so beautifully preserved and exposed here. The rocks at Grand Canyon are not
inherently unique; similar rocks are found throughout the world. What is unique about the geologic record at Grand Canyon is the great variety of rocks present, the clarity with which they're exposed, and the complex geologic story they tell.

There are really two separate geologic stories at Grand Canyon:

1.The First geologic story:

The older story is the one revealed in the thick sequence of rocks exposed in the walls of the Canyon.
These rocks provide a remarkable (but incomplete) record of the Paleozoic Era (550-250 million years ago),
as well as scattered remnants of Precambrian rocks as old as 2000 million years. The story these rocks tell is far older than the canyon itself. Mesozoic and Cenozoic rocks (250 million years old to the present) are largely
missing at Grand Canyon (they've either been worn away or were never deposited).

2.The second geologic story at Grand Canyon concerns the origin of the canyon itself:

When and how did it come to be?
On one level the answer is simple: Grand Canyon is an erosional feature that owes its existence to the Colorado River (which is largely responsible for the depth of the canyon). Of equal importance are the forces of erosion
that have shaped it and continue to shape it today—mainly running water from rain, snowmelt, and tributary streams which enter the canyon throughout its length. The climate at Grand Canyon is classified as semi-arid
(the South Rim receives 15 inches (38 cm) of precipitation each year; only 8 inches (20 cm) each year reach the canyon bottom). But what rain received comes suddenly in violent storms, particularly in the late summer of
each year, and the power of erosion is therefore more evident here than in places which receive more rain.