According to the geographer John R. Borchert, American cities have undergone five major epochs, or periods, of development shaped by the dominant forms of transportation and communication at the time. These include sail-wagon epoch (1790-1830), iron horse epoch (1830-1870), steel rail epoch (1870-1920), auto-air-amenity epoch (1920-1970), and satellite-electronic-jet propulsion and high-technology epoch (1970-present).