This bedrock mortar was discovered circa 2016 next to Harry and Olive Carey's ~1932 adobe home at Tesoro Adobe Historic Park in lower San Francisquito Canyon (29350 Avenida Rancho Tesoro) when a landscaping contractor pruned a large cactus plant that was covering it.
The mortar cavity measures approximately 8 inches deep and 7 inches wide. It is surrounded by numerous smaller circular indentations (cupules?).
Tataviam Indians were present in San Francisquito Canyon. An ancient village, Tacyumaman, was located a bit to the north on the same (west) side of San Francisquito Creek, at what became the Ruiz Ranch. The mortar feature could be associated with the village.
It should be noted that while the Careys employed Navajo Indians from Arizona to work at their trading post in the 1920s, they had nothing to do with the mortar feature (unless they happened to use it); it took several hundred years of grinding seeds and acorns to reach its ultimate depth. Also, even if the floodwaters of the 1928 St. Francis Dam Disaster had been forceful enough to move this giant boulder, they did not reach this elevation.
Tesoro Adobe Historic Park is a unit of the Los Angeles County Department of Parks and Recreation. The county acquired the property in June 2005 from Montalvo Properties LLC, developer of the surrounding Tesoro del Valle residential community.
LW3679: Download original images here. Photos by Leon Worden, January 15, 2020.