mincraeft:

FOSSILIZED SAND DUNES: Crossbedding in Coconino Sandstone Along the South Kaibab Trail of the Grand Canyon
Photo and text by Wægen

In the early Permian Period (a few hundred million years ago) there was a great desert in the area of the yet to be carved Grand Canyon, a desert that stretched as far north as present day Montana. (This was during the time of the last supercontinent, Pangea - at the time when the northern Appalachian Mountains were pushed higher than the present day Himalayan Mountains.) Northerly winds deposited sands, creating sand dunes. The dunes would shift and move as they do today in the Sahara Desert, at times building, at times cutting down. The sand was sorted by the wind, and the quality of the sand would vary. The remains of dunes were buried in succession until a layer 65 feet deep had been laid down in the region of the present day Grand Canyon, several hundred feet deep in areas to the north. Over time, these sedimentary remains were transformed into stone. The result is the fossilized sand dunes shown above.