Mesa Verde Mesa Verde is a flat deeply dissected upland (fig. 2) ranging from 7000 to 8,500 feet in elevation. The tableland dips gently southwest and is underlain by ~3000 feet of Cretaceous-age shales and sandstones. Tributaries of the Mancos River have carved the plateau into numerous elongate mesas, such as Chapman Mesa, Wetherhill Mesa, and Long Mesa (fig. 2). Both the surfaces of the mesas and the walls of the canyons were occupied by the ancestral Pueblo (formerly Anasazi) Indians. Although they inhabited Mesa Verde from around 600 AD through 1300 AD, they only built and lived in cliffhouses during the last 80 years of this occupation, between 1200 and 1280 AD. Approximately 600 cliff dwellings are located in the canyons of Mesa Verde. Tours of some of the larger dwellings are available to the public, however most are inaccessible. To reach Mesa Verde drive north 6 miles from Cortez, CO, located in the Montezuma Valley at the foot of the Mesa. From there it's 25 miles to the visitors' center along a road that switchbacks up the 2000-foot steep face of the North Rim. The drive offers spectacular views of the verdant valley below, and a close look at the stratigraphy of shales and sandstone deposited in the Interior Seaway that flooded the western craton during the Cretaceous Period. The soft shales and thin-bedded coal and sandstones create a very unstable slope, as observed from the numerous landslide scars and slope-stabilization structures along the road.
Marine, coastal, and terrestrial (land-based) sedimentary rocks comprise the ~3000 feet of Cretaceous strata underlying the plateau. Most of these were deposited in or along the margins of the Western Interior Seaway that flooded the continent during the Cretaceous Period. From the bottom to the top, the geologic formations composing the tableland are the Mancos Formation (marine shale), Point Lookout Formation (coastal sandstone), the Menefee Formation (terrestrial shale, sandstone and coal deposits) and the Cliff House Formation (coastal sandstone). The latter three formations are part of a larger geologic unit known as the Mesa Verde Group, which is noted for its coal deposits. The Cliff House Sandstone, and locally the Point Lookout Formation (fig. 4), comprise the mesa's protective caprock. Once eroded the weak Menefee and Mancos formations rapidly disintegrate. Uplift and tilting within the last 20 million years has created the present upland and initiated incision by south-flowing streams (fig. 1). The cliff dwellings occupy the walls of these narrow stream-carved canyons. Despite the mesa's high elevation its southerly orientation offers a longer growing season than the broad valley (Montezuma Valley) to the north. This proved to be an important advantage to the Indians growing crops on its surface.
View image from the Paleontology Portal. It may seem hard to imagine how the ocean could flood the western craton (from Alaska to the Gulf of Mexico) to produce an inland sea. Two events made the west susceptible to flooding during the Cretaceous Period. First sea level was higher. Pangea was breaking up and rates of sea-floor spreading and subduction were quite high. When sea-floor spreading is fast ocean basins are occupied by warmer, more buoyant oceanic crust. The sea floor is therefore higher and water is displaced onto low-lying continental regions. Also, high rates of tectonic activity mean increased volcanism, and emission of magmatic CO2 that warms the atmosphere--hence no glaciers to retain water on land. The second factor promoting flooding was the collision and consequent over-thrusting of island-arcs onto the western edge of the continent. Loading of the continental margin produced a subsiding trough, as the crust east of the collision flex downward (subsided) under the load. (Press a finger into your arm and observe the trough that forms around it.) These events produced an inland sea that transgressed and regressed with changing conditions, depositing stacked layers of shale, sandstone, and coal. The warm moist conditions of the time also resulted in high biological productivity and the accumulation of coal-producing organic matter.
The remains of ancestral Pueblo Indian communities lie scattered across the mesa surface. However, the most amazing structures are the remains of complex cliff dwellings tucked away in natural alcoves along the walls of the deep canyons cutting through the mesa. The Ancient Pueblo People (Anasazi or Hisatsinom) began building in alcoves around 1200 A.D. and then mysteriously abandoned around 1280 A.D. Considering the short period of time these cliff dwellings were occupied, the amount of effort that went into building them was tremendous. There are approximately 600 cliff dwellings on Mesa Verde. The largest is Cliff Palace (fig. 12) which housed over 200 people had over 150 rooms and 23 kivas. Other cliff houses in the Park include Spruce Tree house (fig. 13) and Balcony House (figs. 7-11) Why the ancestral Pueblo Indians built the cliff dwellings and then abandoned them soon after is slowly being revealed (Kohler, 2008). During a time of extreme drought Mesa Verde became attractive to Indians living in the Four Corners region. It's higher elevation offered slightly cooler and wetter conditions. An increasing population density may have resulted in the occupation of the less desirable canyon walls, and as resources became more scarce the dwellings would offered greater protection from a decreasingly stable society. After decimating the local deer population the ancestral Pueblo Indians relied on raising turkeys to satisfy their protein demands. Because maize was the principle food source for both the people and their turkeys the collapse of agriculture, driven by drought, forced the ancestral Pueblo people to migrate to south and west. Although there is currently no indication of violence on Mesa Verde, evidence of warfare and cannibalism at the nearby Castle Rock Pueblo (see The Final Days) emphasizes the harsh conditions that must have existed at the time and the need to move on. Most of the cliff dwellings are located in natural alcoves etched by weathering (fig. 10) in the Cliff House Sandstone (fig. 6). The sandstone forms the protective upper part of the mesa and forms buff-colored cliffs along the canyon walls. Spaced throughout the Cliff House Formation are thin layers of shale that trap and deflect the flow of water, thereby aiding in the formation alcoves (fig. 6 and 10). Alcoves Formation and Occupation The cliff dwellers of Mesa Verde National Park, Motezuma's Castle National Monument, and Canyon De Chelly National Monument took advantage of the unique conditions afforded them by the horizontal strata, deep canyons, and hydrology of the Colorado Plateau. Sandstone is permeable, which means that groundwater flows through it readily. Shales on the other hand are impermeable. Water percolating vertically through thick porous sandstone is deflected laterally upon reaching an underlying shale bed (fig. 3) until it escapes through fractures or where the bedding intersects the cliff face. Contact springs (figs. 10 and 11) or seeps develop. The moisture slowly dissolves the cement binding the sandstone forming a hollow around the seep. It's here where weathering and mass-wasting processes are most active and where alcoves develop. Once a hollow is formed unloading stresses take over and exfoliation (fig. 7) becomes the dominant process in shaping the alcove. The ancient Puebloans expanded the alcoves by removing loose debris and then built walls and dwellings with slabs of the local sandstone. Limy mud was used as mortar. The alcoves not only provided protective sites for housing, but were often supplied with a source of water.
Other sites to
explore and references
On
to Section E : Death Valley National Park
Lindley Hanson/Department of Geological Sciences/Salem State College/QkRef |
||||||||||||||||||||||||||||||||||
The
Archeology Channel: 
