A major theme in Beuys's work was renewal. This sculpture developed out of his environmental concerns, particularly a plan to plant 7,000 oaks in the city of Kassel, Germany. Next to each newly planted tree would be placed blocks of basalt rock. Here, the basalt itself becomes a symbol of potential growth. A cone has been cut out of each rock, allowing the cavity to be lined with clay and felt. Embedded in dead matter, these materials suggest the possibility of new life emerging at the end of a dark century.