Sketches made by G. K. Gilbert and based on telescopic observations of the Moon look amazingly similar to photographs obtained 75 yr later by spacecraft. He was very successful in correlating lunar surface features with counterparts on Earth. His observations and experiments led him to the conclusion that most lunar craters are the product of impact. After establishing this, he studied the Coon (Meteor) Crater of Arizona. He did not have as much success applying what he had learned from the Moon to the terrestrial case. He conducted a topographic study of the crater to check whether there was an added volume due to the incoming projectile. An overestimation of the size of the meteorite and neglect of the possibility of its fusion, evaporation, and ejection forced him to rule out an impact origin for this crater.

In his observations on lunar features, Gilbert had expressed the basic elements of a lunar stratigraphic system. His discussion of crater rays, and particularly of the “sculpture” that surrounds the Imbrium basin, greatly influenced the thinking of lunar geologists of our day. Coupled with his recognition of the importance of crater density and overlap relationships, he can be easily considered the father of lunar stratigraphy. Today there is a crater on the Moon bearing the name of Gilbert in commemoration of his many contributions to geology.