Complex sentence: The area was made up of farmlands to the west and the rocky areas of the Hymettus to its east; urban development replaced much of the farmland in the 1920s and the 1930s.
Split into simpler sentences: The area was made up of farmlands to the west and the rocky areas of the Hymettus to its east. Urban development replaced much of the farmlands in the 1920s and the 1930s.