WAVE MARKS 



225 



Wind Drift is characteristically different from cross-bedding ; 

 here the laminae form all sorts of angles with one another. The 

 lamination is parallel to the surface as that was at the time each 

 lamina was formed ; but the sand ridges and dunes are continually 

 changing their shape, as the force and direction of the wind vary, 

 and thus a very complex arrangement results. 



Ripple Marks, exactly like those on any sandy beach of the 



FlG. 81. — Wave mark and rain prints, modern sandy beach. (U. S. G. S.) 



present, are found in rocks of almost all geological dates. Such 

 marks are formed by the wind or by the rippling movement of 

 shoal water. Ripple marks are most frequent and best shown in 

 sandstones, but other rocks, such as shales, often exhibit them 

 also. 



Wave Marks are formed by waves washing up on the beach 



after they have broken, and are preserved by the deposit of thin 



layers of sand on the edges of the waves, and indicate that the 



rock in which such marks occur was formed on the very beach. 



Q 



