In this talk, we present results from a geophysical investigation at the Chimney Rock Great House using magnetics, electromagnetics, and DC resistivity. Our data is focused on a grid southwest of the Great House, where we use geophysics to detect potential buried walls. These walls may be covered by 1 to 3 m of fill as a result of nearby excavations in the 1920s, and should be approximately 0.5 m thick. Using geophysics, we were able to identify several potential targets in the multiple datasets, which are consistent with sketches from earlier archaeological digs. Through our investigations, we have likewise gained a better understanding of the geophysical responses of buried walls at Chimney Rock.
The Chimney Rock area, which can easily be recognized by the iconic shape of the towering sandstone formations, is located about twenty miles west of Pagosa Springs, CO. The area has at least seven Native American settlements but our prime interest is the Great House located at one of the highest points. Surrounded by cliffs on every side, the Great House offers protection and views of the entire valley. Much of the Great House has been excavated throughout the 20th century and a 1920s map of the settlement depicts the structure. However, some of the walls indicated in this map are not currently visible at the site (see Figure 1). Chimney Rock was inhabited by the ancestors of the modern Puebloan tribes during the late 11th century to the early 12th century. These ancestral Puebloans were characterized by their subsistence farming regimen, construction of dwellings, production of tools and crafts, use of storage rooms, and integration of cultural structures. This particular settlement is connected to the practice of astronomy. The two pinnacles, Chimney and Companion Rocks, were most likely used for the observation of lunar standstills. Once every 18.6 years, the moon is seen to rise directly between the two rocks (Richardson, 2006). In 2009, researchers at the University of Colorado at Boulder reduced fill in advance of stabilization and conducted limited tests to determine depth to the sandstone bedrock in two rooms of the Great House. The geology of the Chimney Rock area is part of a larger area called the Mesaverde Group (Chronic, 2001). The rock that composes Chimney Rock is a Cretaceous shoreline deposit, an artifact of the ancient sea that used to cover most of Colorado. The sedimentary cap at Chimney Rock is Pictured Cliffs Sandstone (about 70-100 million years old) while the dark gray sediments below are Lewis Shale (CRIA, 2009). When looking at the Chimney Rock area, it becomes clear that the sandstone, of which the two pinnacles are comprised, is more resistant to erosion than the underlying shale. Over time, these structures will also erode away. The Native American structures at Chimney Rock were built using local material. We will show that geophysics can be used to distinguish between the local rock used to build houses and the soil that fills and covers the structures.
