For centuries, acorns were a major food source for many California Indian groups. Nutritious and easy to store, processed acorns could be served as soup, mixed with fruit or meat, or baked into a cake. In autumn, Indian people filled their carrying baskets with acorns from beneath the oak trees, then cracked and shelled the dried acorns and pounded the acorn meat into meal using bedrock mortars, holes worn into slabs of rock. The meal was transferred to a shallow, sandy basin lined with leaves or grass, where water was gently poured over it to wash out the bitter tannic acid. Finally, the meal was mixed with water in large baskets to form a soup or mush and cooked by placing heated rocks into the baskets. In good harvest years, surplus acorns were traded to other Indian groups in exchange for useful goods such as shells, skins, stone, and pine nuts that were abundant in other regions.