Over 150 million years ago, during the Jurassic period, portions of this
area resembled the Sahara Desert of today. The sand dunes were formed from
wind blown, rounded grains of sand. Try to visualize dinosaurs coming to
take advantage of the deep pools of clear water that were probably scattered
throughout the area.
The sand dunes were eventually buried by sediment. The weight turned the dunes to stone (Zuni sandstone). As time passed the earth experienced some growing pains, causing the massive Zuni sandstone to crack. As water passed along the cracks, the cementing material holding the grains of sand together dissolved, widening the cracks. Look at the vertical and horizontal cracks that allows the water to seep into the sandstone. As the water froze, the cracks widened, eventually causing the sandstone to break away, forming the arch you see.