Complex sentence: After World War I, she moved to New York, where she worked for the sculptor Gutzon Borglum, who was then producing statuary for the Cathedral of Saint John the Divine and who would later sculpt the memorial at Mount Rushmore.
Split into simpler sentences: After World War I, she moved to New York, where she worked for the sculptor Gutzon Borglum. He was creating statues for the Cathedral of Saint John the Divine and would later carve the memorial at Mount Rushmore.