Thanks to geologic mapping, we understood the story of the Moon well before we actually went there.
Eugene Shoemaker with some of the first geologic maps of the Moon, Flagstaff AZ, mid-1960's
Many people are surprised when they learn that well before the first landing of Apollo in 1969, we already understood the geological history of the Moon. The idea that such a thing was even possible drew considerable skepticism during early preparations for landing on the Moon. The principles for the remote mapping of the geology of the Moon came from several closely related but distinct threads. Eugene M. Shoemaker, a geologist with the U. S. Geological Survey (USGS) who founded the Branch of Astrogeology, laid out the methodology in broad outline from and through the systematic study of lunar surface images in the early 1960s.
One of the basic principles of geology is that younger rocks lie on top of (or intrude into) older rocks. Interestingly, this relationship can be discerned from a photograph. In the case of the Moon, images show the dark smooth plains of the maria (lava) and the rough, cratered highlands. Some craters were found on top of the dark mare plains, while others ...