TBRRANES. 



93 



61. 



62. 



ment of the tidal waters out of an estuary or of a stream over a sand-bed. 

 It lias been called current-bedding. The water, as it moves on, pushes up 

 some of the sand before it, and then keeps depositing the sand over the 

 front slope of the little elevation so made, pro- 

 ducing on the slope a series of thin layers 

 pitching at an angle usually of 20° to 35° in 

 the direction of the flow. During a time of 

 quiet following, as the ebb of the tide, slower 

 deposition may make a layer that is horizon- 

 tal in bedding; and thus the cross-bedded 

 layer is often made to alternate with the 

 horizontal. 



(9) In the flow-and-plunge structure the cross-bedded layer is broken up 

 into curving wave-like parts, as shown in Fig. 62. This effect is produced 

 when there is a wave-like plunging action in the rapidly flowing waters and 

 a large supply of sand or fine gravel for deposition. One of the wave-like 



parts in such a layer is usually a yard or 

 more long and six inches to a foot thick ; 

 and may be much smaller, as well as very 

 much larger. In one place in the stratified 

 drift near Kew Haven, Conn., the thickness 

 was six to eight feet. The whole thickness, 

 in all cases, was produced by one fling of the 

 waters. 



By studying the structure of layers, we 

 are enabled to determine the conditions under which rock-formations were 

 made ; and hence the facts have great interest to the geologist 



(10) The beach-structure is another of like interest, indicating a beach 

 origin. The upper part of a beach, above high-tide level, is made by the 

 toss of the waves, and especially in storms ; and it is generally irregularly 

 bedded. But the lower part, swept by the tide, has usually an even seaward 

 slope ; and the beach deposits over it have therefore a corresponding inclina- 

 tion — usually 5° to 8° when the tides are low, but 15° to 18° when high. 

 When the sands are coral or shell sands, they become cemented into a calca- 

 reous sand-rock, and show well the straticulation. 



(11) The wind-drift structure is of very different character. It is made 

 up of straticulate portions, in different positions, oblique to one another, as 

 in Fig. 63. A ridge of sand made by the drift- 

 ing winds on a coast becomes straticulate 

 parallel to its upper surface, because the dep- 

 osition by the winds is necessarily over the 

 surface. But if such a ridge has its upper 

 half shaved off obliquely in a heavy storm, 

 deposition will aftervy'ard go on parallel to the new surface, and hence at an 

 angle with the earlier layers. By repetitions of such events the wind-drift 



63. 



