PHYSICAL GEOLOGY 



ORIGINAL STRUCTURES OF SEDIMENTARY ROCKS 



Stratification. It has been noted (pp. 36-37) that sedi- 

 ments are commonly arranged in distinct layers, and that this 

 stratification is the most important structural feature of 

 sedimentary rocks. A layer may be called a bed or a stratum 

 (plural strata). A group of consecutive layers composed of 

 the same kind of rock is often called a formation. Layers 

 are separated by more or less pronounced division planes, 

 known as bedding planes (Fig. 5). An individual layer im- 

 plies essentially uniform conditions of sedimentation. A 

 notable pause in deposition, a change in the kind of sediment, 

 or a marked change in the texture of the material is indicated 

 by a new layer. The longer conditions remain constant, 

 therefore, the thicker a given layer becomes. Thickness of 

 beds is, however, only a very rough measure of time, for the 

 same material gathers at unequal rates at different times and 

 places. Very thin beds, such as those in shales, are termed 

 lamince. Lamination is absent or inconspicuous in pure 



limestones, and usually 

 pronounced in shales. 



Cross-bedding. If a 

 current (of water or air) 

 moves material along a 

 surface which terminates 

 in an abrupt slope, most 

 of the material will roll 

 down the slope and come 

 to rest. Coarse material 

 will rest at a steep angle, 

 and fine material at a 

 gentler angle. If the 

 coarseness of the material 

 moved forward to the 

 slope varies frequently, numerous inclined laminae will be 

 formed. If, in addition, the direction and strength of the 



FIG. 35. Cross-bedded sandstone, canon 

 of Virgin River, southern Utah. 

 (Fairbanks.) 



