CROSS-BEDDING 



705 



formed cross-bedding, but conditions for the preservation of such 

 are perhaps seldom reaHzed. Gilbert has described the mode of 

 formation of cross-bedding through the shifting of ripples on the 



Fig. 138. Cross-bedding of the Eolian type (Orange sand or La Fayette 

 Formation), Mississippi Central Railroad, Oxford, Miss. (After 

 Hilgard.) 



sea floor, due to a current accompanying oscillatory movements. He 

 considers "that sediment may be added to a rippled surface without 

 any disturbance of the pattern, but that there is usually a coinci- 

 dent gradual bodily shifting of the pattern in some direction." 



Fig. 139. Ledges of sandstone near Colorado Springs, showing Eolian cross- 

 bedding. 



"The shifting of the ripple profile during the accumulation of the 

 sediment makes the accumulation unequal on the two sides of the 

 trough (figure 3), and, if the ratio of shifting to deposition exceeds 

 a certain amount, there is deposition on only one side of the trough 

 and erosion on the other." (Gilbert-12 : /jp, Figs. ^-4.) In this 

 case two sets of oblique planes are produced, one due to deposi- 

 tion, the other to erosion, the latter representing the progress of the 



