A dietary staple for most indigenous populations in interior California was acorns, which were dried, shelled, ground to flour, soaked in water to leach out their tannin, and cooked. The grinding holes worn into large rocks over centuries of use are still visible in many rocks today. The ground and leached acorn flour was usually cooked into a nutritious mush, eaten daily with other traditional foods. Acorn preparation was a very labor-intensive process nearly always done by women. There are estimates that some indigenous populations might have eaten as much as one ton of acorns in one year. Families tended productive oak and tanoak groves for generations. Acorns were gathered in large quantities, and could be stored for a reliable winter food source.