In April, my husband and I visited several of the sites participating in Doors Open Denver, the Denver Architectural Foundation’s annual open house.  Our favorite stop was Denver Water’s Three Stone Buildings.  The knowledgeable guide told us that these three small buildings were built in 1880, 1881 and 1905, all of Castle Rock rhyolite.

The photo below is from the Denver Architectural Foundation’s Facebook page, and shows you two of the three buildings as they appeared in 1896.  While the history and use of the buildings are interesting, I was most interested in the stone.

Castle Rock rhyolite, known geologically as Wall Mountain tuff, was formed during a volcanic explosion near Mt. Princeton about 36 million years ago.  The volcano spewed an immense amount of hot ash and pumice into the air.  When it landed, the hot ash and pumice compressed and cooled to form either tuff or rhyolite.  Tuff is a soft pumice stone that erodes easily and is not a durable building stone. (The ancestral Puebloan people carved caveates out of tuff.  Many are now protected as part of Bandelier National Monument in New Mexico.)  Rhyolite, however, was better compressed and is much more durable as a building stone.  According to the geology website Geocaching:

“This rhyolite is ideal for a building stone because of its uniformity, durability, and light weight. The glassy matrix does not have cracks that weaken many types of rock. When it does break, it breaks in conchoidal fractures similar to patterns in broken glass or other non-crystalline volcanic rock like obsidian.”

Fortunately for us, a thick band of fine-grained rhyolite about 15 to 30 feet thick was deposited in the Castle Rock area, just south of Denver.  Most of the Castle Rock rhyolite is quite durable, but inclusions in the stone like iron cause staining and deterioration of the stone.  Castle Rock rhyolite began to be quarried in the late-nineteenth century, and many historic buildings along the Front Range were constructed of this stone. (A nice collection of historic photographs of Castle Rock and its environs, including one of a rhyolite quarry, can be found here.)

The rhyolite used to erect the Three Stone Buildings is very fine grained and is in excellent condition despite being over a century old.  Take a look at the original tooling marks banding the corner stones and the irregular, chipped appearance (conchoidal fractures) typical of rusticated rhyolite units on the 1905 building, below.

You can also see small iron inclusions that are rusting and staining the stone’s surface, especially at lower right in the photo above, and the two units in the photo below.

The bedding pattern of the hot ash and pumice is also evident on some of the stones in the above photograph, which was taken of one of the two older buildings.  The quality of the stone is much less regular on these two older buildings, as is the carving of the stones.  Still, the condition of these buildings is quite impressive.  Clearly Denver Water has been a good steward of these historic and important buildings.