Exit Tunnel
Like the Connecting Tunnel, the Exit Tunnel is mostly human-made, but it does display a few distinctive natural features. Just past the airlock door there are two “clay pockets” that hint at past cave systems. Beyond these larger spaces, closer to the exit, rusty iron oxide deposits and fast-growing cave formations adorn the walls.
Geology
Room Development
The Exit Tunnel was completed in February 1931 using 7.5 inch drills and dynamite (Muirhead). The process was challenging because of the array of rock types the work crews encountered. Marble, of course, is the most common but chert, metabasalt, and loose sediment can be found throughout the Exit Tunnel (Roth 47).
Due to the complexity of the Klamath-Siskiyou regional geology, the rocks of the Oregon Caves National Monument do not stack neatly atop one another like in other regions. Some places, like the Grand Canyon, exhibit layers of rock that are sometimes referred to as a layer-cake, because of the way they are stacked. Here, the rocks are arranged more like scrambled eggs than a layer-cake. The Exit Tunnel, by clearing a straight section through this geo-mess, presents a cross-section view of the mountain, which is truly a unique perspective of the rocks.
During the blasting process, several rooms were uncovered that were not connected to the rest of the cave system. Because of the amount of mud that kept filling in during excavation, these spaces were dubbed the “clay pockets” and were shored up with cedar planks to prevent collapses (Muirhead).
In the second clay pocket, the largest space in the Exit Tunnel, the steeply slanted room shape is notable. The tilted room owes its orientation and size to a fault that was enlarged by dissolution. There is also a contact between metabasalt and marble that aided the expansion of the room: the interface acts as a water funnel since argillite does not readily permit water to flow through (Roth 47; Palmer 79-80, 235-236).
Cave Formations
Under a coating of yellow-tan silt, thick flowstone arcs about 15 feet overhead in the 
second clay pocket, indicating that there was once airflow through this room (Hill and Forti 691). There is airflow again now but the connection to the outside is artificial. Whether artificially or naturally sourced, airflow still influences cave formation growth, which is demonstrated by the cave’s newest formations (Hill and Forti 691).
In the second clay pocket, a thick layer of flowstone was removed to study climate change. Based on the radiometric date, the thick flowstone mostly formed between 117,000 and 120,000 years ago (Ersek, Vacco, Turgeon).
The clay pockets are rich in sediment, seen smeared over the walls, but the blasted sections of tunnel show fresh rock surfaces. A variety of rock types can be seen including dark, iron-rich metabasalt and dark, fine-grained argillite that breaks along bedding planes. As well as knobby, gray, and erosion-resistant chert and marble banded in gray and black. In the metabasalt, about a third of the way up the tunnel, an expanse of iron oxide deposits can be seen. Basically, this blaze of red-orange limonite is the same as rust. It is a weathering product from pyrite or fool’s gold (Ralph and Chau; Hill and Forti 511).
Little nubbins of future speleothems and moonmilk have been growing since the 1930s, depositing at a significantly higher rate than other parts of the cave. The average speleothem growth rate in the Exit Tunnel is about 1 inch per 100 years, compared to 1 inch per 1000 years throughout the cave (Vacco; Turgeon et al). Most likely, this fast accumulation is due to the proximity to the exit, adequate water flow into the tunnel, and residual marble dust from the blasting (Roth 48).
Bates, Robert and Julia Jackson, ed. Glossary of Geology. Alexandria: American Geological Institute, 1987. 343.
Hill, Carol and Paolo Forti. “Minerals in Caves.” Encyclopedia of Caves and Karst Science. John Gunn, ed. New York: Fitzroy Dearborn, 2004. 511-514.
Hill, Carol and Paolo Forti. “Speleothems: Carbonate.” Encyclopedia of Caves and Karst Science. John Gunn, ed. New York: Fitzroy Dearborn, 2004. 691-692.
Palmer, Arthur. Cave Geology. Dayton: Cave Books, 2007. 79-80, 235-236.
Ralph, Jolyon and Ida Chau. “Limonite.” Mindat.org. Mineralogy Directory: 2011. Retrieved from http://www.mindat.org/min-2402.html
Roth, John. “Interpretive Manual for the Monument’s Showcave”. Cave Junction: Oregon Caves National Monument, 2011. 47, 48.
Turgeon, Steven, et. al. “Growth dynamics, petrography and stable isotopes as paleoclimatic indictors in cave calcite from southwestern Oregon.” Abstracts with Programs - Geological Society of America. 2009. 31(7): 153-154.
Vacco, David A. Developing climate records from speleothems, Oregon Caves National Monument, Oregon. M.S. Thesis. Corvallis: Oregon State University, 2003.