From a poster by Keith Brady & David Perdue (USGS):
Copper plate engraving was used to produce the linework and lettering for practically all USGS topographic maps from 1885 until 1942.
For each map, an original drawing was photographed. From the photographic glass negative, a contact print with a mirror image of the map was made on a zinc plate. From the zinc plate, wax impressions were obtained on celluloid, then burnished onto three copper plates.
The three copper plates were engraved by hand, one for each of the three colors on the printed map - a culture plate for features and names to be printed in black, a relief plate for the contours to be printed in brown, and a drainage plate for water features to be printed in blue. The maps had to be engraved as they appeared on the plates - in reverse or mirror image. Transfers were then made from the copper plates to three lithographic stones, from which the maps were printed.
During the 1940s and 1950s, copper plate engraving gradually was phased out in favor of positive drafting, negative scribing, and photographic methods.