The record of the earth's history in the walls of the Grand Canyon has been deciphered through hard work by many people during the past 100 years. Much still remains unsolved. John Wesley Powell's contributions were of a pioneering type, though he was not the first to discuss the rocks of the Grand Canyon. Far more important than his own observations and deductions in the field of stratigraphic geology was his tremendous influence upon the work of his associates and his successors. This chapter traces the evolution of thought concerning the stratified rocks of the Grand Canyon and summarizes present concepts of stratigraphic history as recorded in the walls of the canyon. A brief summary of available data and conclusions on each of the principal sedimentary units is presented.