New Mexico Mineral Symposium — Abstracts

In 1973, the Congdon Family donated a sum of money to the Arizona-Sonora Desert Museum, enough to establish the Earth Sciences Department and begin construction of the Stephen House Congdon Earth Sciences Center, in honor and memory of Steve who passed away in 1972. Steve was a geologist, practicing his trade in the Southwest, especially Arizona. How appropriate to remember him through the Desert Museum’s nod to the geologic development of the Sonoran Desert.
Desert Museum co-founder Bill Carr long hoped for a vehicle to interpret the geology of the Sonoran Desert region of Arizona, the Baja Peninsula, Sea of Cortez and islands within and the state of Sonora, Mexico. Desert Museum director in the early 1970s, Merv Larson, decided that vehicle would be limestone caves.

Limestone caves are found throughout the Sonoran Desert region. The limestone rock they occur in is hundreds of millions of years old, formed from carbonate muds that settled to the bottom of the sea floors that covered what is now Arizona. The slow process of dissolving limestone with naturally-occurring weak carbonic acids to open underground passages and chambers takes millions of years in itself. That dissolved limestone is transported to other parts of the cavern system and re-deposited in the form of countless cave formations called speleothems. And that takes additional millions of years. But once started, the Desert Museum’s caves took a mere two and a half to three years to take shape on the Museum grounds.

Ground breaking was in 1974. A place had to be made to accommodate four 2,000 square foot buildings for the offices and work space, wet and dry caves, and Earth history and mineral halls. That took some blasting and digging into layers of caliche bordering King Canyon on the western edge of the Museum grounds.

Limestone rock was recreated using cement and gunite that was applied through high pressure hoses over a framework of rebar, pencil rod, chicken wire, and metal lathe. It was then hand textured and painted, and finally sandblasted for that 300 million-year-old weathered look, and that was just the outside.

Inside, the wet cave was meant to look and feel like a “real” cave. Running water flowed in streams and waterfalls over cave formations of cement, gunite, and polyurethane foam. The cave formations were hand-carved, textured, painted and coated with an epoxy resin for a “wet” look, resulting in formations that look like the real thing. In some cases, plastic soda straws and pipe cleaners were dipped in epoxy resin to simulate soda straw stalactites and twisted helectites. Commercial and wild caves were visited and explored so that the Desert Museum’s artificial caves would look and feel as natural as a real cave.

The dry cave was designed to show Museum guests the importance of caves to animals and humans alike. Exhibits draw attention to fossils of ancient marine life in the limestone, Ice Age ground sloths, and a 10,000 year old packrat nest, a midden. Finally, the human factor is brought into play with a Hohokam grotto fashioned after an actual find in a cave not too far from Tucson.

Here in the Desert Museum caves, guests are encouraged to get “close up and personal” with this simulated underground environment. Why? So that they can carry that experience with them when they visit a real cave. “I know what that feels and looks like because the Desert Museum let me experience it first hand in their artificial caves.”

Finally, one exits the cave complex through a “time tunnel.” 4.6 billion years of Earth history is condensed into a 67-foot-long time line, with each brick comprising about 33 million years and each group of bricks about 500 million years. The last half inch of the time line represents the presence of humans in this complex accounting of life on Earth.

The Desert Museum’s caves are 40 years “young” this year, 2017. If you visited when they first opened decades ago, visit again. You’ll understand why some guests, even today, say “how fortunate for the Desert Museum that they had these caves here for us to visit.”

The Desert Museum makes a distinction between minerals found in caves and minerals found in mines. The caves and all they contain naturally are left as found. In some cases, during the course of mining for ore, caves are encountered. In such cases, since cave minerals have no measurable economic value, they are not “mined,” but left intact. Two cases in point, Crystal Cave in the Southwest Mine in Bisbee remains as it was found. The cave is a showcase of crystalline calcite and aragonite. And, in a mine in Santa Eulalia, Chihuahua, Mexico, a cave whose walls were lined with fluorite crystals and whose floor was littered with selenite crystals up to three meters in length, was encountered and remains as it was found more than 30 years ago.

Many minerals extracted from mines tend to take on the look of cave formations called speleothems. Chalcanthite and malachite helictites, stalactites and stalagmites, goethite soda straw stalactites, selenite rams horns, post-mining azurite and malachite cave pearls sitting in a calcite bird’s nest and azurite and hemimorphite crystallized in the horizontal and vertical lines of boxwork are all common formations that occur in working mines. These specimens, rather than minerals that have been robbed from caves, are all a part of the Desert Museum’s Permanent Mineral Collection. The message of cave conservation and preservation is paramount to the Desert Museum’s geologic interpretation. A mantra repeated by cave enthusiasts the world over is simple—take only pictures, leave only footprints, kill only time.

From the permanent mineral collection:

References:

1. Anthony, J.W., Williams, S.A., Bideaux, R.A., and Grant, R.W., Mineralogy of Arizona, Third Edition, The University of
   Arizona Press, 1995.
2. Arizona-Sonora Desert Museum Permanent Mineral Collection, 2017.
3. Moore, G.W. and Sullivan, G.N., 1978, Speleology: The Study of Caves.