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This vast area was once covered by extensive sand dunes. Some 200 million years ago, winds from the northwest carried tons of fine-grained sand into this area, creating an immense desert. Over time, the sand drifts were covered by other layers of sediment, compressed, and cemented together by quartz and calcite into Navajo Sandstone. Erosion has since washed away the overlying layers, leaving the "petrified" dunes. These ancient dunes are often erroneously referred to as 'petrified', implying that they were once organic, like wood. Here this term simply means sand compressed into rock.