Sentence:  ... they heard the voices of living men shouting, finally. And the lowing of cattle. And the creaking of wheels. But they saw nothing. The symphonious clamor was everywhere, filling the sky ..., it was the great <WSD>processional</WSD> of the Union armies, but of no more substance than an army of ghosts. 
Meaning of the word marked with <WSD> tags:  A group of people or things moving along in an orderly, stately, or solemn manner.