The rock outcropping of El Morro is composed of yellowish-gray white sandstone from the Jurassic Period (200-145 million years ago) known as Zuni Sandstone. The quartz grains that make up this sandstone are the same size and are characteristic of wind-blown dunes that form in arid lands; El Morro was part of a sand dune field that stretched across northwestern New Mexico, northeastern Arizona, southwestern Colorado, and southeastern Utah approximately 150 million years ago. This rock formation was never buried deep enough for the sand grains to be tightly squeezed together. As a result, it is a perfect medium for the finely detailed inscription carvings we see on the rock face today. The dark, vertical streaks found in numerous places on the bluff face are from rainwater that has trickled down from the top of the bluff and left behind a patina of minerals such as iron and manganese.
The darker rocks capping the pale Zuni Sandstone at the top of the bluff is known as Dakota Sandstone and is from the Late Cretaceous time period (145-65 million years ago). Dakota sandstone is made up of a number of components including beach and lagoonal sandstones, shale, and conglomeratic (large-grained) sandstone layers that were deposited in the area as a shallow sea advanced through New Mexico. The sandstone is tan to yellow brown in tone and is interbedded with dark gray.