Abstract: Weathering of Rock Art at Pictograph Caves State Park, Near Billings, Montana

Jon Reiten, John Wheaton, Richard B. Berg

Physical and chemical weathering has reduced the visibility of rock art images in Pictograph Cave. Observations by archaeologists and the general public, in addition to historic photographs of the rock art, indicate that the weathering has accelerated over the past 20 to 30 years. Damage to the art can be attributed to: mineral crusts obscuring the images; loss and extensive threat of images due to slaking sandstone; and physical damage to the images caused by graffiti, bullets and other forms of vandalism.

Natural seepage of water through fractures in the massive sandstone overlying the caves probably played a major roll in cave formation. Accelerated seepage of water since the mid 1960's appears to be directly related to the construction of a stock pond overlying the cave. This additional water apparently provided the right conditions to accelerate slaking, and to promote precipitation of gypsum which covers the rock art. Prior to an archeological excavation of the site in the late 1930s the cave floor was marked with a line painted on the exposed sandstone. Gypsum crusts overlie this paint line, indicating fairly recent deposition. Processes causing the precipitation of gypsum are not clear; but, speculation suggests dissolution and transport of sulfate from the overlying sandstone or possibly from airborne sulfates.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana