Over the last 2000 years, Maidu Indians created bowl-shaped depressions in rocks called mortars, each made for a specific purpose such as grinding acorns or other nuts. Acorns were one of the most important foods for Native Californians because they were readily accessible in the California foothills, could be stored for long periods, and had a high fat content. Before acorns were used for food, they were collected, dried, ground up, and leached with water to remove the bitter tanic acid.