234 



PHYSICAL GEOLOGY 



grains of sand become cemented together to some extent, the succeed- 

 ing layer of sand will be separated from the underlying by a surface 

 which in this case will divide two beds of similar character. These 

 planes are called bedding planes. Slight and frequent changes in the 

 character of the sediments during deposition produce thin layers 

 called lamince. Laminae are often rendered distinct by the weather- 

 ing of the rock. Stratification is so characteristic of sedimentary 



Fig. 227. — Stratified limestone. Auburn, New York. (Photo. H. L. Fairchild.) 



rocks that " stratified " and " sedimentary " are used as synonymous 

 terms in describing rocks of this origin. 



Cross or False Bedding. — When sand moved either by air or water 

 currents is carried along a surface which terminates in a slope, 

 the greater part of the material will roll down the slope and come to 

 rest at a steep angle, a steeper slope being made by coarse than 

 by fine sand. If now the direction and velocity of the currents 

 vary, the inclined laminae will slope or dip in different directions and 

 meet at various angles, producing cross-bedding. Cross-bedding is 

 especially well-developed in wind-blown deposits (p. 48), where the 

 shifting winds blow the sand in one direction over an abrupt slope 

 at one time and in a different direction at a later time (p. 47). It is 

 also common in delta deposits, where the distributaries of the river 

 vary from time to time (p. 131). Currents produce cross-bedding 

 near shores and on bars, since the sand which they carry over the end 



