The Mandans relied heavily on cultivated plants, but they were also skilled hunters, and bison hunting was as important as farming because bison were vital to the Mandan diet. Before the arrival of horses, bison were stalked individually or driven in large numbers over cliffs or into corrals to be killed. Bison that had drowned in the fall and winter were taken from the river during the spring ice break-up, when the frozen carcasses came downriver and were pulled out and processed. The importance of bison in Mandan life was expressed in material culture as well as in religious practices and ceremonies. The Mandan people also gathered many wild plants, including juneberries, chokecherries, plums, grapes, buffalo berries, strawberries, and prairie turnips, which were important food sources. Wood from the river's bottomlands was gathered for lodge construction, while willows along rivers and creeks were used to make baskets, mats, and lodges, and grass was used to insulate lodges and cache pits. Green ash served as a hard wood for mortars and pestles and for handles for hoes and other tools. Plants were also gathered for medicinal and ritual purposes. Knowledge of plants accumulated over several generations through experience and observation, was held by few people in the community, and was transferred gradually to apprentices over long periods of time.