This is a world class arch! It faces southwest so the lighting can be even more spectacular before local sunrise and after noon in the fall, winter, and spring. Pull off the highway for an easy view of New Mexico's second tallest arch. Enjoy a panoramic vista of the contact between Jurassic desert sand dunes of the Zuni Sandstone and the overlaying Cretaceous Dakota Sandstone. A visible white stripe near the 200 foot cliff top separates the two units and marks an unconformity, absent rock for a period of over 60 million years.
By Cretaceous time the local climate had shifted to receeding and transgressing shoreline of a salt water sea as evidenced by the many fossilized invertebrate burrows. At times the CO2 levels were more than double to - some studies say - triple those of today.
Curious how the arch formed? Thinking freeze-thaw? You're getting warm. It's from the daily temperature swings of over 50F on the rock's surface throughout the year. The sandstone expands in the day and cracks apart from the still cool rock hidden behind. The rock at the base and in the center was under the greatest load stress and cracked then failed first. Rocks on either side failed soon after somewhat symmetrically. The load from the overlying rock and the ensuing cracking apart lessened upwards. Over time this created an arc that grew from the base as more rock failed and collapsed.
Can you picture it happening? Hint: Try standing at the trail at the base of the arch for the best view. Can you find what may have once been an earlier arch, that has since collapsed? Arch formation is a very slow process, so there's still time to view the spectacle. Passing motorists occasionally stop for the vault toilets or to picnic. Fewer walk the 100 yard paved then gravel trail up to the base of the arch. Possible wildlife sightings, birdlife, wildflowers, a photo spot of the arch - which is revealed as a natural bridge - and interpretive signs await those that venture closer.