The Geology of El Morro National Monument, New Mexico

El Morro National Monument protects Inscription Rock, with inscriptions and petroglyphs made over many centuries, and ancestral Puebloan sites on top of the El Morro bluff. Established on December 8, 1906, it was the second national monument created under the Antiquities Act and the first to protect cultural resources. The National Park Service's Geodiversity Atlas, drawn from its Geologic Resources Inventory, summarizes the monument's geology — which here is inseparable from its history.

Where geology and history meet

Geology touches human history everywhere, but at El Morro the approximately 2,000 inscriptions, petroglyphs and pictographs are cut directly into it. El Morro ("the headland") was a landmark for ancestral Puebloans, Spanish explorers and settlers, and European American travelers. The permanent pool at the foot of Inscription Rock supplied Puebloans living in the area and overland travelers from the Spanish period until 1881, when the transcontinental railway was completed through a pass north of El Morro.

One of El Morro's inscriptions. The extremely friable Zuni Sandstone made an ideal surface. NPS photo.

Setting

• Colorado Plateau. The monument lies near the plateau's southern edge — one of 25 physiographic provinces in the contiguous United States, with high overall elevation, thick continental crust and relatively little deformation.
• Zuni Uplift. It sits on the uplift's southern flank, where a belt of Jurassic and Cretaceous sedimentary rocks is exposed; the Zuni Mountains rose in the Laramide Orogeny, between about 75 and 50 million years ago.
• Zuni-Bandera Volcanic Field. Near the field's western end, this is a monogenetic field of lava flows and vents, many of them cinder cones, active over the last 700,000 years. El Malpais National Monument, about 40 miles (65 km) east, protects its younger lava terrain.

The Colorado Plateau, with National Park System units in green. Graphic by Philip Reiker, NPS Geologic Resources Division.

The rocks

The bluff is an erosional remnant of the Colorado Plateau's late Cenozoic wearing-down. Inscription Rock lies on the northern scarp of a cuesta — an asymmetric ridge with a gentle slope on one side and a steep or cliff-like face on the other — held up by resistant Dakota Sandstone over Zuni Sandstone, with a regional dip of a few degrees to the southwest.

The volcanic field erupted in three phases: the Old Basalt Flows, beginning approximately 700,000 years ago; the Chain of Craters, mostly west of El Malpais, approximately 150,000 years ago; and the El Malpais phase, from 60,000 to 3,900 years ago, when five main flows erupted in today's El Malpais National Monument from vents including cinder cones and a low shield volcano. El Morro's flow belongs to the first.

The unconformity between the Dakota Sandstone and the Zuni Sandstone below, with a bleached zone near the contact. NPS photo.

Inscription Rock: easy to carve, hard to keep

Inscription Rock is the stretch of the bluff near the Historic Pool where most of the inscriptions, petroglyphs and pictographs are found — an open-air record left by ancestral Puebloans, Spanish explorers, surveyors, pioneers and military expeditions through the 1800s. The Zuni Sandstone crumbles grain by grain, so it was easy to carve with crisp edges that lasted decades to centuries; some carvers smoothed the surface first and rubbed off loose grains.

The same softness dooms them. Rockfall, exfoliation, granular disintegration and alveolar erosion all take their toll, most of all in wet conditions, from rain or snowmelt, or through cycles of wetting and drying. Losses depend on the rock's varying cement and permeability, joints, rock varnish, which way a slope faces and more; inscriptions can vanish in a rockfall or slab failure, or bit by bit to crumbling, lichen or boring insects. Rockfall drives the retreat of the escarpment, probably quickly given how weakly cemented the sandstone is.

• Joints. The main joint set runs east-northeast, and part of Inscription Rock's face follows it; a second set crosses it at about right angles. Joints let water in and give rockfalls planes to break along.
• Tafoni. Alveolar erosion, or salt weathering — repeated wetting and drying that dissolves, precipitates and expands salts such as calcium carbonate and gypsum — carves honeycomb hollows called tafoni, common on sandstone across the plateau. It is one of the main causes of damage to the petroglyphs and pictographs.
• Desert varnish. Thin coatings of rock varnish, iron and manganese skins and silica glaze streak many surfaces, especially where water runs down the cliffs.

Satellite view of El Morro showing its two main joint systems. Graphic by Jason Kenworthy, NPS Geologic Resources Division.

Water

• The Historic Pool, the monument's only permanent water, is fed by runoff from the bluff and, at times, shallow perched aquifers nearby; it matters to wildlife as well as history. The Park Service dammed it in the 1920s, uncovering evidence of much earlier human changes. A rockfall destroyed the dam in 1942, and a new dam raised the pool by 13 feet (4 m), greatly enlarging it.
• Tinajas (Spanish for "bowl" or "jar") are hollows in the Zuni and Dakota sandstone that hold rain and snowmelt. Ancestral Puebloans used them, and they host specially adapted life such as fairy shrimp.
• Box Canyon, in the monument's southwest, is a vertical-walled canyon aligned with the main joint set; groundwater seeping out of the cliff, causing spalling and alcoves, drives its headward erosion.

Hazards and other resources

Rockfall threatens both the inscriptions and visitors: besides the 1942 fall, one struck the Inscription Trail in 1999, another damaged the Inscription Loop Trail in July 2013, and talus from a fall older than 1629, dated by nearby inscriptions, still lies below the bluff. Seismic hazard is moderate: USGS's 2014 map gives a 2% chance in 50 years of shaking above 10–12 %g, roughly intensity VI, and 4 to 10 damaging shaking events per 10,000 years. No fossils, karst or abandoned mine sites are known in the monument, though fossils might turn up, likelier in the Dakota than in the fossil-poor Zuni Sandstone.

Damage from the July 2013 rockfall on the Inscription Loop Trail. NPS photo.

The monument's Geologic Resources Inventory geologic map. NPS.

Travelers resting and drinking at the pool carved their names into the soft sandstone. NPS photo.

Sources

Rewritten from the National Park Service article "NPS Geodiversity Atlas—El Morro National Monument, New Mexico" (public domain), in hubnx's own words. The article gives the inscriptions' span as "nearly 1,000 years" in one place and "at least a thousand years" in another, and Inscription Rock's length as about 0.3 mile (0.5 km) in its text and about 500 m either side of the pool in a caption; neither figure of either pair is given here. A caption also calls the Dakota Sandstone Jurassic, which the page does not repeat. Photographs credited only to a named photographer, one marked copyright, and the report cover are not reproduced.