A bedrock mortar (BRM) is an anthropogenic circular depression in a rock outcrop or naturally occurring slab, used by people in the past for grinding of grain, acorns or other food products. There are often a cluster of a considerable number of such holes in proximity indicating that people gathered in groups to conduct food grinding in prehistoric cultures. Correspondingly the alternative name gossip stone is sometimes applied, indicating the social context of the food grinding activity. Typical dimensions of the circular indentations are approximately 12 centimeters in diameter by 10 centimeters deep, although a considerable range of depths of the cavities have been documented . The bedrock mortar has been identified in a number of world regions, but has been particularly intensely documented in the Americas. An alternative term for the bedrock mortar site is bedrock milling station.
Example locations of occurrence
- Southern Arizona: In the Santa Catalina, Santa Rita, Rincon, Sierrita, and Tucson Mountains, and also in rock outcroppings in the valleys
- Along the north banks of the middle reaches of the Merced River in Mariposa County, California, USA 
- In northern Mexico within the Sierra Tarahumara of Southern Chihuahua
- The Upper Cumberland Plateau of Tennessee, USA
- Inside many rock shelters in Menifee County, Kentucky, USA
- The Cueva de los Corrales region of northwestern Argentina 
Maidu bedrock mortars in northern California.
Bedrock mortars at the Anza-Borrego Desert State Park in southern California.
Line note references
- Archaeology online glossary
- Discovering Prehistoric Sites: Objective and Subjective Survey Techniques, Daniel G. Foster, Brian D. Dillon, and Linda C. Sandelin, California Department of Forestry and Fire Protection, June 13, 2005
- "Cultural Resources of the Santa Rita Experimental Range". John H. Madsen. Retrieved 4/12/2013.
- G. Deghi, J. Buxton et al., Earth Metrics, Environmental Impact Report for the Saxon Creek Water Project, Mariposa County, California, published by the County of Mariposa Planning Department and Mariposa County Water Agency, Report 10298, May, 1990
- C. Melvin Aikens, Cultural Continuity in the Sierra Tarahumara of Southern Chihuahua, Mexico, June 2003
- M. Del Pilar Babot, María C. Apella, Maize and Bone: Residues of Grinding in Northwestern Argentina, Archaeometry, Volume 45 Issue 1 Page 121-132, February 2003