Geological Survey Bulletin 1271—E Pecos National Monument New Mexico: Its Geologic Setting

The building stones used for the construction of the pueblos by the Indians of Pecos were obtained directly from the sandstones and conglomerates of the Sangre de Cristo Formation, which underlie most of the area in the vicinity of the national monument. These rocks split naturally into convenient sizes and shapes for building stones, and consequently very little, if any, trimming was necessary. Locally, flagstone that was quarried from the Sangre de Cristo Formation could be easily adapted for use in walkways, steps, and shelves by the Spanish priests.

Although soil was used to chink the stone used in the pueblo buildings, adobe brick as such was not used by the Indians. The technique of adobe brickmaking was introduced by the Spanish and was often used by the missionaries in construction. The brick was undoubtedly made from the deep red soil in the valley of the Pecos River and Glorieta Creek. Selenite was used as panes in a few windows and served the purpose very well. Selenite is a crystalline variety of gypsum (calcium sulfate) that can be split into very thin transparent plates. Here, the plates were very large and could be used as windowpanes. The selenite crystals were probably found in some large gypsum deposits not very far away—southwest of Canoncito. Isinglass, which is a white mica that can be split into thin transparent sheets, was also probably used for windowpanes. The mica was found in large booklike crystals in the ancient Precambrian rocks of the mountains to the north.

Gypsum, from nearby small deposits or from large deposits southwest of Canoncito, was probably used as plaster of paris to cover the interior walls of the mission and some of the convent rooms.

New Mexico affords many interesting geological productions, of which the most useful to the natives is yeso or gypsum, which abounds in many places. Being found in foliated blocks, composed of laminae, which are easily separated with a knife into sheets from the thickness of paper to that of window-glass, and almost as transparent as the latter, it is used to a great extent in the ranches and villages for window-lights, for which indeed it is a tolerable substitute. [Gregg, 1844.]