Arizona is a geological laboratory because the natural forces in play are so visible. Erosion is a general term for the weathering of rocks and the transportation of them.  The forces of erosion are gravity, ice, water and wind.

The San Francisco Mountain in Flagstaff is a strato-volcano in the midst of a volcanic field of over 600 smaller volcanos. This largest volcano is now 12, 633 feet in elevation, but it is estimated it reached 16,000 feet before a sideways eruption or glacial erosion, or both, collapsed or carved out the northeastern side of the mountain.

Imagine lines connecting the two outer slopes of the San Francisco Peaks. That much has eroded away.

You can see green lichen growing on this large basalt boulder at the base of Mount Elden in Flagstaff.  Weathering and erosion from the lichen, rain, snow, and freezing winter temperatures have  and cracked the boulder in half.

Basalt boulder from the Mt. Elden lava dome.

In the mesa shown below, you can see how the soft sedimentary layers at the top have been weathered away by wind and water and gravity has piled it around the bottom.

Eroded mesa in northern Arizona.

Wind is a powerful force in erosion as you can see in the photo of Tsegi Canyon below where it hollows out the sedimentary rock.

To the right in this picture of Tsegi Canyon you can see the wind weathering scooping out the soft rock.

Water helps to carry away the debris weathered from the higher elevations and the grit and sand in the water help to erode more rocks as it passes.

Downpours of rain quickly run off the land, carrying the weathered particles of rock along. Note the truck crossing the stream.

The classic Arizona erosion example is the Grand Canyon of the Colorado River.