His first field work in the Marathon region was on Permian marine strata that form a sequence about 7,000 feet (2,100 m) thick on the northern side of the Glass Mountains.
Instead of an orderly sequence, the strata of the Glass Mountains were a disorderly array of discontinuous bodies of carbonate rocks, shale, and sandstone.
During King's field work there, he gave much attention to the Capitan Limestone, which stands in lofty white cliffs at the summit of the mountains.
In 1940–44, the wartime search for strategic minerals by the U.S. Geological Survey afforded King an opportunity to investigate the Appalachian Mountains in Virginia and Tennessee where he unraveled and interpreted the massive folds and low-angle thrusts of that region.
As he did these earlier investigations, he was aware of their broader significance and developed regional syntheses that resulted in his publications Evolution of North America (1959), the Tectonic Map of the United States (1944; 2nd ed.