You might be thinking "Why didn't we just draw the cross section using the photo. Why did we need to make the map in between". Good question. In some areas like the Grand Canyon, geologic cross sections are almost self evident. The Colorado River has formed a marvelous natural cross section into the Earth (the Grand Canyon in the satellite image below) and allowed us to see the arrangement of rock bodies below the surface of the northern Arizona first hand. [Image courtesy of the USGS]