The Geology of Chaco Canyon

National Historical Park National Park Service Chaco Culture U.S. Department of the Interior

THE GEOLOGY OF CHACO CANYON

THE LANDSCAPE The landscape of Chaco Canyon is lar place to enjoy the natural beauty mentary layers that have eroded hauntingly beautiful. Sunlight and and to marvel at the grandeur of into plateaus, mesas, buttes, and shadows play on towering rock the ancient sandstone buildings canyons. The rocks exposed in walls where petroglyphs keep secret that remain. Chaco Canyon record an interval in the history of an ancient people. the Earth's history during the Late Chaco Canyon lies near the center The canyon floor still conceals the Cretaceous Period, approximately of the San Juan Basin of New buried remains of Pueblo 75 to 80 million years ago. During Mexico, which is near the south­ dwellings. Those which have been this time, Chaco was part of the eastern edge of the much larger excavated are as mysterious as they migrating coastline of an ancient Colorado Plateau. This region has are revealing. Visitors come from inland sea. broad exposures of horizontal sedi- all over the world to this spectacu­

THE CRETACEOUS SETTING The name "Cretaceous" is derived lines of this epicontinental seaway from the Latin word for chalk were oriented generally north-south (creta) which is a characteristic and repeatedly shifted position to rock type of this period for many east and west in response to land masses in the northern hemi­ changes in global sea level. The sphere. Throughout much of the Cretaceous rocks visible today in Cretaceous period, sea level was both Chaco Canyon and Mesa higher than the present, and por­ Verde were deposited in alternating tions of many continents were in­ marine and nonmarine environ­ undated by shallow seas. In the ments as the Western Interior area of the southern Rocky Seaway repeatedly inundated parts Mountains, mountain building ac­ of the Four Corners area and then ash, and birch. The landscape of tivity also produced an adjacent receded. late cretaceous Chaco Canyon re­ broad area of subsidence known as sembled modern day coastal areas, In the Late Cretaceous, much of the Western Interior Basin. This such as those found on the south­ the Southern Rocky Mountain basin was flooded by seas from eastern Atlantic and Gulf coast of Region was characterized by a sub­ both the Arctic and Gulf Coast re­ the United States. These mainland tropical climate which supported gions. By the Late Cretaceous, the beach, coastal plain, and barrier is­ lowland forests of conifer, eucalyp­ Western Interior Seaway was hun­ land environments consist of tus, ebony palm, cypress, and mag­ dreds of miles wide and had divid­ marshes, swamps, river deltas and nolia. Upland areas supported ed North America into two lagoons adjacent to beach and rela­ hardwood forests of oak, walnut, separate land masses. The shore­ tively shallow coastal waters.

CHACO GEOLOGY The majority of the exposed fea­ visible in Chaco, while all three are tures in Chaco Canyon belong to a exposed at Mesa Verde. An addi­ suite of rocks known as the Mesa tional two younger units, the Lewis Verde group. The further subdivi­ Shale and the Picture Cliffs sions of the unit are, from oldest to Sandstone, are generally exposed youngest, the Point Lookout only near the northern boundary of Sandstone, the Menefee Formation, the park. and the Cliff House Sandstone. Of these three formations, only two, the Menefee and Cliff House are the canyon, with a number of ex­ ded shales which were deposited in MENEFEE FORMATION cellent exposures visible on Fajada deeper water when the entire area The Menefee Formation is the old­ Butte. was covered by the Western est exposed unit of the Mesa Verde Interior Seaway. This unit is less re­ Formation at Chaco and is com­ sistant to erosion than the lower posed primarily of siltstone and CUFF HOUSE SANDSTONE sandstone and forms gentle slopes mudstone interbedded with sand­ rather than cliffs. Fossils found in The Cliff House Sandstone is a stone as well as carbonaceous shale this unit are similar to those of the complex sequence of marine sand­ and thin coal beds. The Menefee lower sandstone. stones with locally interbedded Formation was formed from sedi­ shales which overlies the Menefee The uppermost unit exposed in ments deposited by rivers flowing Formation. There are three princi­ Chaco Canyon is the Upper north and east across New Mexico ple Cliff House units visible within Sandstone. It is similar in composi­ toward a retreating Interior Seaway. Chaco Canyon. The massive lower tion to the lower sandstone unit At the edge of the seaway, the unit forms the 80-100 foot promi­ and also forms prominent cliffs streams meandered through a wide, nent cliffs throughout the canyon. and ledges. This unit represents a flat coastal plain with deltas, shal­ An abundance of ripple marks and beach and barrier island environ­ low swamps and lagoons, accumu­ a wide variety of lating plant material which would fossils are visible eventually form thin coal beds. in this unit. Common vertebrate fossils include Fossils include an abundance of turtle, fish and shells and casts crocodile as well as fragmentary ev­ from clams, am­ idence of larger creatures such as monites, snails, hadrosaur dinosaurs and giant ma­ shark's teeth, and rine lizards known as mosasaurs. the knobby casts Plant fossils found in the Menefee of burrows include leaf impressions of palm known as and conifer as well as specimens Ophimorpha Fajada Butte closely resembling modern laurel, ("dwelling place") Nodosa ("nodu­ ment similar to that of the lower witch hazel and camelia, suggesting lar"). These casts are thought to be sandstone unit. It was deposited in a warm, moist, subtropical environ­ the fossilized remains of burrows shallower water than the intermedi­ ment. Today, the Menefee in left by a small shrimp-like crus­ ate shale unit and reflects a lower­ Chaco is visible as a slope forming tacean known as Callianasa major. ing of the local sea level as the unit that underlies the steep mesa Wester Interior Seaway receded for Immediately above the lower layer walls of the more resistant Cliff the last time. House sandstone. It is especially is an intermediate sandstone/shale prominent on the southern side of unit. Although primarily a marine sandstone, it has locally interbed­

EROSION AND CLIFF FORMATION The Menefee Formation is less re­ "Threatening Rock". When the an­ "Threatening Rock" collapsed tak­ sistant to erosion than the Cliff cient builders were constructing ing several rooms of Pueblo Bonito House Formation and often com­ Pueblo Bonito, "Threatening with it. pletely erodes from beneath the Rock" rested in a precarious posi­ It is probably safe to say the only younger sandstone. The unsupport­ tion just behind it. Aware of the constant, geologically and other­ ed sandstone will then break away danger that it posed, the Chacoans wise, is change. Soil and rock are in large slabs and boulders as the built an earth and masonry retain­ always on the move through undercutting reaches joints and ing wall beneath this massive rock weathering, erosion, gravity, and local weaknesses. This step-wise slab. The slab was first described in the lateral movement of the earth's erosion is responsible for produc­ 1901 and was referred to as the continents and ocean floors. The ing both the prominent cliff faces "Elephant", the Navajos called it landscape is reshaped by these and the debris mounds or talus "Braced-up Cliff, and the Park forces over hundreds, perhaps slopes piled against them. When Service named it "Threatening thousands, of years. Rapid occur­ this erosional process continues, it Rock". In an attempt to predict the rences, like rockfalls and earth­ may actually "sever' a landform fall of "Threatening Rock", the quakes, also do their share of into separate free standing rock Park Service took on the job of redistributing soil and rock. masses. monitoring its movement. However, there was very little that One of the most dramatic exam­ could be done to prevent its fall , ples of such erosion is the im­ and on January 22, 1941, mense slab of sandstone known as

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