The bluffs along many Minnesota rivers are sandstone. This sedimentary rock is excellent at purifying seeping water. The process also creates caves. They tend to be moist and cool. During the 19th century, they were often extended with pickaxes and then used for storing perishables. Cave tours are available in the Twin Cities. Until recently, you could experience a 30 minute tour of the Stillwater Caves. It was used as a trading post beginning in 1838. In the 1880s, Joseph Wolf bought it to store beer from his adjacent brewery.