Published by the American Geophysical Union as part of the Field Trip Guidebooks Series, Volume 363. The Appalachians are one of the Earth"s classical orogenic systems. These mountains gave rise to the concept of the geosyncline in the writings of James Hall (1859) and James Dwight Dana (1873). During the ensuing years the concept evolved quite differently in Europe and America, a story that has been recounted by Hsü (1973). The geosynclinal concept was never an adequate theory to explain orogenic processes, but as long as the architecture and correlation of the metamorphosed and deformed internal zones of orogens remained obscure, it provided a framework for categorizing observational data. The concept reached its zenith in 1951 when Marshall Kay classified North American geosynclines into eight subdivisions.