12 trask. PRINCIPLES OF SEDIMENTATION [Ch. 1 



angle of slope as particles are swept forward off the flat part of the 

 delta. At the base of the slope the layers flatten out. The three types 

 of beds formed in this process are called top-set, fore-set, and bottom- 

 set beds. In lakes or the sea, where the bottom slopes rapidly away 

 from shore, the inclination of the fore-set layers, along the forward 

 part of the delta, may be great ; but in large bodies of water, where the 

 water deepens gradually, the inclination of the fore-set beds is likely 

 to be small. In fact, difficulty may be encountered in distinguishing 

 fore-set beds from ordinary cross-bedding caused by variable currents. 

 Geologists, when trying to determine the extent to which the angle of 

 inclination of ancient sediments is due to initial dip, should bear in 

 mind that initial dip in lakes is likely to be materially greater than in 

 the sea. Steep initial dips commonly reflect diastrophism prior to dep- 

 osition, which has led to the presence of relatively deep water close 

 to shore or to the formation of an uneven bottom on which sediments 

 are deposited. 



Wind 



Wind-blown or eolian deposits are essentially a characteristic of 

 arid or semi-arid regions rather than of humid regions, because in arid 

 regions the soil is poorly protected by vegetation and can be blown 

 about by the wind. In present arid areas most eolian deposits consist 

 of sand. However, during the Ice age, great masses of rock flour com- 

 posed of fine particles of silt size, formed by the grinding action of the 

 glaciers, were washed out by streams flowing from the ice and were de- 

 posited in great outwash plains (Thwaites, 1941). In the absence of 

 vegetation these silt particles could be blown away by the wind. The 

 extensive deposits of loess, which are found around the southern border 

 of the glaciated area, in part are believed to have formed from such 

 wind-blown silt. These loess deposits commonly weather in vertical 

 cliffs, in contrast with wind-blown sand, which forms slopes of 30° to 

 35°, representing the normal angle of repose of incoherent material. 



Wind-blown sand is well sorted because of winnowing action of the 

 air which blows the fine particles away and transports the larger par- 

 ticles essentially by rolling or jumping along the ground. For a full 

 discussion of eolian deposits see Bagnold (1941), Reiche (1945), and 

 the papers in the Loess symposium (1945). Wind-blown sand is de- 

 posited mainly in dunes or hummocks. In this process the grains 

 blow up the side of the dune on a comparatively gentle slope and 

 then, as they pass over the crest where they are not supported by the 

 slope, many particles come to rest on the lee side. Thus dunes are con- 

 tinually progressing in the direction in which the wind blows. If the 



