Obsidian, or volcanic glass, has the chemical ability to absorb small amounts of water over time. The longer the time, the more water it absorbs. This water forms a thin surface rind (the hydration rim) which is visible under a microscope. The longer a tool or flake has been exposed to the elements, the thicker will be the hydration rim. These rims are measured in microns -one micron equals 0.000039 of an inch. At Crooks Canyon, an artifact with a hydration rim of 1.0 micron corresponds to about A.D. 1850, 2.0 microns to A.D. 1400, and 3.0 microns to A.D. 600.
Hundreds of obsidian tools and flakes were recovered from the interiors of the houses at Crooks Canyon and subjected to obsidian hydration analysis. When the resulting rim values are plotted on a graph for each house, a distinct pattern emerges: most of the rim readings cluster between 0.8 and 1.5 microns. This confirms that the excavated houses documented in the canyon were occupied only within the last 300 to 400 years, and that many were still in use when the U.S. Army entered the area.
Obsidian hydration profiles from Native American house structures in Crooks Canyon. Note the large number of readings between 0.8 and 1.5 microns indicating occupation at the very end of the Terminal Prehistoric Period, as well as during the Historic Period.