In prehistoric times, when people wanted to commemorate something or someone, they erected a monument—even if it was nothing more than an upright slab or heap of stones. Elaborating on those origins, traditional architecture would come to embrace an array of monumental forms readily recognizable for their symbolic import. As a result, the Washington Monument is an obelisk, the nearby Lincoln and Jefferson Memorials are temples, Eero Saarinen's Gateway Arch in St. Louis is a modernist rendition of a triumphal arch and Felix de Weldon's Marine Corps War Memorial in Arlington, Va., a realist sculptural composition. All mine...