La Ventana Natural Arch, second largest of New Mexico's accessible natural arches, this massive sandstone formation lies just off of highway 117 in the park's eastern section. A short trail leads to the base of the 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, a missing rock record of over 60 million years.
By Cretaceous time the local climate had shifted dramatically wetter as evidenced by the many fossilized inverebrate burrows. At times the CO2 levels were more than double those of today.
The arch formed 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 stress and failed first. Rocks on either side failed too somewhat symmetrically. The load from the overlying rock and the ensuing cracking apart lessened upwards. Over time this created an arc that climbed as more rock failed and collapsed.
It faces southwest so the lighting can be even more spectacular after noon in the fall, winter, and spring.