56o PRINCIPLES OF STRATIGRAPHY 



in the world. It is several hundred feet high, and from its advance 

 has become known as "Creeping Joe." 



(3) River Flood-Plain and River-Bottom Dunes. Rivers flow- 

 ing through broad, open valleys, especially in semiarid regions, are 

 often accompanied ,by an extensive development of border dunes, 

 while those rivers flowing through narrow carious are essentially 

 free from such accumulations. In a humid climate dunes are sel- 

 dom formed on river-bottoms, even though these may be broad and 

 supplied with an abundance of sand. In illustration of this it may 

 be noted that river border dunes are not common in western Eu- 

 rope, except in the sunny and somewhat drier regions of southern 

 France and Spain. Thus, while in the North German plains and in 

 England the few dunes which have been formed in the past are 

 low and soon covered with vegetation, the valley of the Gardon in 

 Languedoc shows dunes 10 meters high, while dunes rising to a 

 height of 25 meters are found in the sandy desert region on the 

 right bank of the Guadalquivir in Andalusia in southern Spain. In 

 eastern Europe, on the other hand, river dunes are much more ex- 

 tensively developed. Thus even the smaller streams of the northern 

 half of the Russian regions have dunes rising to a height of 5 

 meters, while southern Russia, with its dry continental climate, 

 shows them extensively developed. A nearly continuous dune belt, 

 reaching sometimes a width of 12 km., and with dunes averaging 10 

 or 12 m. in height, extends along the middle course of the Dnieper 

 below Kief, while the lower course of this stream shows a still 

 more extensive dune area, especially in the government of Taurida. 

 The sands here are found on the left bank of the Dnieper, and ex- 

 tend over an area more than 150 km. long, with a maximum width 

 of 30 km. In the midst of this area, however, are often spots free 

 from sand, while in some parts a broad area of meadow and wooded 

 land separates the sand area and the river. The height of the 

 dunes is mainly from 5 to 7 m., but exceptional cases, where the 

 height rises to 11.2 m., have been noted. The sand is almost exclu- 

 sively quartz rock, with only a very slight admixture of feldspar 

 and dark grains, probably of iron oxide. The grains are well 

 rounded, often spherical, angular grains being wholly wanting. The 

 color of the dunes as a whole is a light golden yellow. The sands 

 of the middle Dnieper are somewhat coarser, but they do not ex- 

 ceed on the average the diameter of 0.4 mm. In roundness, purity 

 and color the sands of the two areas are much alike. A still greater 

 development of river border dunes is found in the valleys of the 

 Don and the Donetz. In the Don valley the dune zone has a width 



