Geology of Mesa Verde National Park
Mesa Verde National Park is a beautiful example of how geology affects landscapes and people. While primarily known for the cliff palaces, incredible houses built directly into the rock walls, the natural history is just as fascinating.
This park is situated on the Colorado Plateau, a region of the Southwestern United States in the Four Corners region (where Arizona, New Mexico, Colorado, and Utah meet), just south of the San Juan Mountains of Colorado. Other national parks in this region include Grand Canyon, Bryce Canyon, Arches, and Zion. Mesa Verde shares a similar geologic history with many of the parks in this region, but the alcoves within the sedimentary formations here are unique for their well-preserved records of human history. Despite its name, Mesa Verde is actually more of a cuesta (a gradual slope that dips in one direction) than a mesa (isolated, flat-topped highlands surrounded by steep slopes). The cuesta has been dissected over time by rivers and streams, creating a series of mesas and canyons.
Mesa Verde’s rocks reveal a rich geologic history that scientists interpret by understanding the basic principles that shape Earth’s surface. The rocks in Mesa Verde are primarily sedimentary, and reveal a lot about the environment of the past. Now, the area is semi-arid (mostly dry), surrounded by deserts and mountains. More recent processes created the current topography, and gradual processes continuously shape the sedimentary structures and key features. Mesa Verde National Park is still a very dynamic place today. Waterways are still carving paths through mesas, eroding away the inner walls of alcoves.
Geologic History
Geologists use stratigraphy, the study of rock layers, to understand how environments change over time. A stratigraphic column is a representation of the layers of rock in a specific place, as viewed as a vertical cross section with the oldest rocks at the bottom and the youngest rocks at the top. In general, each rock layer is named for where it was first described, when it was deposited, or other defining characteristics.
Four Cretaceous-aged formations are exposed in Mesa Verde National Park. From oldest to youngest, they are the Mancos Shale, Point Lookout Sandstone, Menefee Formation, and Cliff House Sandstone. During the early Cretaceous, this part of the continent was deep underwater, as evidenced by the fine-grained, dark gray to black Mancos Shale.
Above the Mancos Shale is the Point Lookout Sandstone, indicative of shallower seas. The lowest part of this formation is interbedded layers of fine-grained brown sandstone and gray fossiliferous shale, suggesting a transition from deep to shallow water, known as a marine regression. The upper layers are yellow-brown sandstones deposited as barrier beaches and near-shore sands after the water receded.
Next, the Menefee Formation lies conformably above the Point Lookout Formation. This means that there is no missing time between the two. Throughout geologic history there are periods of nondeposition, in which no new sediments are deposited or where sediment was eroded away, leaving a gap in the record. These are called unconformities. When two layers are conformable, there is a continuous record, providing more information about the past. The Menefee Formation contains rocks deposited in even shallower environments than the Point Lookout Formation, revealing the continual retreat of the sea. These are gray and gray-orange cross bedded sandstones and dark carbonaceous shales, coals, and bentonite clay beds deposited in a coastal plain. The high organic carbon in the shale, coal, and clay beds are indicative of high vegetation, and the area was above sea level at that point.
Cliff House Sandstone (named for the dwellings built from and in these rocks) is the uppermost and youngest of the formations found in Mesa Verde National Park. These beds are very thick (~90 feet) and consist of gray to orange-yellow, fine-grained sandstones. During the late Cretaceous the sea was making a resurgence, resulting in a northward transgression (inland movement of water). Deposition occurred in a shallow, near-shore marine environment. These rocks are susceptible to chemical weathering from weak acids in percolating water, forming the alcoves that eventually became the homes for the people who lived here.
Although most of the rocks in Mesa Verde are sedimentary, there are also some intrusive, igneous rocks as well. Intrusive igneous rocks are the cooled and solidified rocks formed when magma intrudes (squeezes into) existing rocks. Two types of intrusive bodies (or shapes) of igneous rocks can be found within Mesa Verde National park – dikes and laccoliths. Dikes are intrusive magma bodies that cut across other layers, such as those exposed in the Menefee Formation and Cliff House Sandstone. Laccoliths are intrusive magma bodies that push upward through overlying rock and cool into a mushroom or domed shape. Every rock layer an intrusion cuts through is older than the intrusion itself. In fact, this period of volcanism was relatively recent, formed around 30 million years ago. This may not seem recent from our perspective, but the Cretaceous period (when the sedimentary rocks were deposited) was over 60 million years ago.
Uplift
How did the land become the arid cuesta that we see today if it was once a marine environment? The topography and climate have changed vastly over time. Following the last periods of volcanism, there was a period of large-scale tectonic uplift, in which colliding land forced the area upward. The uplifted land was exposed to weathering and erosion, resulting in some missing sections of the geologic record. The uplift and erosion began around 30 million years ago, slowly forming the landscape we see today. As the land was lifted, different rock layers were eroded more easily than others, and water cut through formations as it flowed from higher to lower elevations. Continuous cutting and erosion have formed the cuesta we see today.
Weathering, alcoves, and desert varnish
The park is best known for the cliff dwellings, impressive homes built within the rocks that provided protection from the elements. These dwellings are found in alcoves, formed by chemical and physical weathering over a long time period. Alcoves are similar to caves, but are recesses into the cliff face, whereas caves are underground. Alcove formation mostly depends on a physical weathering process called ice wedging. Water seeps into cracks in the cliff face and freezes when the temperature is low enough. As water freezes, it expands, and cracks the rock. As the ice in the crack melts, rock debris is carried along as it drips down due to gravity.
As discussed above, the rocks in Mesa Verde National Park are mostly sandstone with a base of shale. Sandstone is permeable, allowing water to travel through pores, whereas shale is not (impermeable). As water travels downward through the rock layers, it is blocked by the impermeable shale and is forced to move horizontally. Where the horizontal layer of water reaches the cliff face a seep spring is formed. These fresh water springs were used and by the Ancient Puebloans and were carefully protected with an intricate network of tunnels and wooden ladders. Chemical weathering also alters the sandstone by preferentially removing calcium carbonate (CaCO3) grains that are easier to dissolve than the silica (SiO2). This both weakens the rock and changes the physical shape over time. The end result is that the alcoves are C-shaped as the walls are gradually eroded away. These C-shaped overhangs created protective “roofs” for the Ancient Puebloans to build the impressive kivas (round-shaped rooms), gathering spaces, and grain storage areas.
Desert varnish is a common feature in Mesa Verde National Park, and looks similar to large dark brown to black brushstrokes down the cliff faces. The varnish is actually created by a natural process, attributed to a combination of dripping water and bacterial fixation of manganese onto the rock faces.
