El Morro National Monument
Figure 1. Visitors examine inscriptions at the base of the Zuni Sandstone cliff at El Morro National Monument.
Location
El Morro National Monument is located about 42 miles southwest of Grants along New Mexico Highway 53. El Morro is approximately 15 miles west of El Malpais National Monument.
Facilities
Geology Within the Monument
El Morro, which means "bluff" or "headland" in Spanish, is an imposing cliff made of Middle to Late Jurassic (155 to 165 million years old) Zuni Sandstone capped by Late Cretaceous Dakota (~95 to 96 million years old) sandstone and shale. Travelers, including Native Americans, Spaniards, and citizens of the United States, have carved their symbols, names, messages, and dates of their passage into the soft cross-bedded Zuni Sandstone for centuries (Figures 1 and 2). El Morro lies on the southern flank of the Zuni Mountains and the rocks dip gently (3°) toward the southwest (Figure 3; Anderson and Maxwell, 1991).
Figure 2. Inscription on large-scale cross-bed in Zuni Sandstone.
Figure 3. Geologic map of El Morro National Monument (Anderson and Maxwell, 1991). Click here for a .pdf version of the map and the legend.
The quartz grains than make up the yellow-gray to tan Zuni Sandstone are well-rounded and the sandstone is well sorted (Anderson and Maxwell, 1991), which means that the sand grains are all about the same size. Large scale cross-bedding is common in the Zuni Sandstone. These features are characteristic of sand deposited in large wind-blown dunes that can form in an arid (dry) environment. These sand dunes were part of a dune field that covered much of northwestern New Mexico, as well as northeastern Arizona, southeastern Utah, and southwestern Colorado about 150 million years ago. The Zuni Sandstone is stratigraphically equivalent to the Entrada Sandstone and the overlying Bluff (Cow Springs) Sandstone to the northeast (Lucas et al., 2003, p.6).
The top of the Zuni Sandstone beneath the Dakota Sandstone is bleached to white several feet below the contact (Figure 4). The contact represents a time gap on the order of 60 million years and the bleached horizon is a paleo-weathering surface. In places, pieces of the underlying Zuni Sandstone have been incorporated into overlying sandy to gravelly stream deposits (Figure 5) of uncertain age (Late Cretaceous (?), Anderson and Maxwell, 1991).
Figure 4. Brown Cretaceous Dakota Sandstone stream channels cut into the top of the Jurassic Zuni Sandstone. Note the white paleo-weathering horizon at the top of the Zuni Sandstone.
Figure 5. Blocks of Zuni Sandstone incorporated into the base of the overlying stream deposits. Black pocket knife ~4 inches long shown for scale.
The lower and middle part of the main body of the Late Cretaceous Dakota Sandstone is preserved along the trail within the monument. The lower part is cross-bedded sandstone with lenses of conglomerate containing pebbles of chert and quartzite. The sandstone is overlain by a gray mudstone and shale. The pueblo ruins are built on the gray mudstone to shale unit. This deposit represents streams flowing across the coastal plain along the shores of the Western Interior Seaway in Late Cretaceous time.
Basalt flows from the Zuni-Bandera volcanic field to the east underlie the low country to the north, northwest, and northeast of the sandstone cliffs. These basalt flows are ~79,000 years to 1.38 million years old (Anderson and Maxwell, 1991). An apron of younger Quaternary colluvium (rocks eroded from the cliffs), alluvium (water-laid deposits in arroyos), and wind-blown silt surround the sandstone cliffs.
References
Anderson, O.J., and Maxwell, C.H., 1991, Geology of El Morro quadrangle, Cibola County, New Mexico, New Mexico Bureau of Mines and Mineral Resources Geologic Map 72, scale 1:24,000.
Lucas, S.G., Semken, S.C., Heckert, A.B., Berglof, W.R., Hoffman, G., Kues, B.S., Crumpler, L.S., and Aubele, J.C., 2003, First-day road log, New Mexico Geological Society Guidebook 54, p. 1-34.
Additional References to the Geology of El Morro
The Water Supply of El Morro National Monument
New Mexico Bureau of Geology & Mineral Resources
New Mexico Institute of Mining & Technology
801 Leroy Place
Socorro, NM 87801-4796
information: (575) 835-5420
fax: (575) 835-6333
http://geoinfo.nmt.edu
The New Mexico Bureau of Geology & Mineral Resources is a research and service division of:
