NEW YORK (AP) — American sculptor Robert Morris, one of the founders of the minimalist movement that emerged in the 1960s, has died. He was 87.

The Castelli Gallery, which is currently exhibiting his work, confirmed that Morris died Wednesday, November 28, of pneumonia in Kingston, N.Y.

Morris studied art at the Kansas City Art Institute. After a stint in the Army Corps of Engineers, he began making Abstract Expressionist paintings.

In 1956, he moved to New York City where he joined the avant-garde scene and began producing and exhibiting neo-Dada sculptures.

Morris later went on to expand on the minimalist style using a variety of media, from performance to plywood. One of his large-scale works was composed of piles of earth and felt. A common theme was fear of nuclear war.