P. V. N. Myers, A General History for Colleges and High Schools…
“ Basin of the Tigris and Euphrates.—The northern part of the Tigris and Euphrates valley, the portion that comprised ancient Assyria, consists of undulating plains, broken in places by considerable mountain ridges. But all the southern portion of the basin, the part known as Chaldæa, or Babylonia, having been formed by the gradual encroachment of the deposits of the Tigris and Euphrates upon the waters of the Persian Gulf, is as level as the sea. During a large part of the year, rains are infrequent; hence agriculture is dependent mainly upon artificial irrigation. ”
