geographic study of a place or region at a specific time or period in the past, or the study of geographic change in a place or region over a period of time. The writings of Herodotus in the 5th century BC, particularly his discussion of how the Nile River delta formed, probably provides the earliest example of what would be called historical geography today. Historical geography, as the study of past geographies, remained a relatively undeveloped field of study until the 17th century, when Philipp Clüver (Clüver, Philipp), considered the founder of historical geography, published a historical geography of Germany, combining knowledge of the classics with knowledge of the land. In the 19th century the importance of geography as the basis for understanding history was taught in many universities, particularly in Great Britain. Geography as the basis for understanding history changed to the geographic influence upon historical events in the early 20th century. The work of Ellen Churchill Semple (Semple, Ellen Churchill) used this environmental deterministic interpretation of history. From the 1930s, historical geography gained prominence through the valuable studies in sequent occupance—i.e., the study of the human occupation of a specific region over intervals of historic time—initiated by Derwent S. Whittlesey and Carl O. Sauer (Sauer, Carl O). The establishment of the Journal of Historical Geography (1975) and historical-geography research groups by the Institute of British Geographers (1973) and the Association of American Geographers (1979) served to vindicate the historical approach in geography.