[Footnote 53: H. J. L. Beadnell: A. Egyptian Oasis, London, 1909. Ellsworth Huntington: The Libyan Oasis of Kharga; Bull. Am. Geog. Soc., Vol. 42, Sept., 1910, pp. 641-661.] [Footnote 54: S. S. Visher: The Bajada of the Tucson Bolson of Southern Arizona; Science, N. S., Mar. 23, 1913. Ellsworth Huntington: The Basins of Eastern Persia and Seistan, in Explorations in Turkestan.] [Footnote 55: Griffith Taylor: Australian Meteorology, 1920, p. 189.] CHAPTER IX THE ORIGIN OF LOESS One of the most remarkable formations associated with glacial deposits consists of vast sheets of the fine-grained, yellowish, wind-blown material called loess. Somewhat peculiar climatic conditions evidently prevailed when it was formed. At present similar deposits are being laid down only near the leeward margin of great deserts. The famous loess deposits of China in the lee of the Desert of Gobi are examples. During the Pleistocene period, however, loess accumulated in a broad zone along the margin of the ice sheet at its maximum extent. In the Old World it extended from France across Germany and through the Black Earth region of Russia into Siberia. In the New World a still larger area is loess-covered. In the Mississippi Valley, tens of thousands of square miles are mantled by a layer exceeding twenty feet in thickness and in many places approaching a hundred feet. Neither the North American nor the European deposits are associated with a desert. Indeed, loess is lacking in the western and drier parts of the great plains and is best developed in the well-watered states of Iowa, Illinois, and Missouri. Part of the loess overlies the non-glacial materials of the great central plain, but the northern portions overlie the drift deposits of the first three glaciations. A few traces of loess are associated with the Kansan and Illinoian, the second and third glaciations, but most of the America loess appears to have been formed at approximately the time of the Iowan or fourth glaciation, while only a little overlies the drift sheets of the Wisconsin age. The loess is thickest near the margin of the Iowan till sheet and thins progressively both north and south. The thinning southward is abrupt along the stream divides, but very gradual along the larger valleys. Indeed, loess is abundant along the bluffs of the Mississippi, especially the east bluff, almost to the Gulf of Mexico.[56]