226 THE STRUCTURE OF STRATIFIED ROCKS 



Naturally, they are almost confined to sandstones, for conglomer- 

 ates are too coarse to retain such markings. Wave markings, 

 ripple marks, and cross-bedding are all indications of very shallow 

 water and of the immediate proximity of the shore. 



Rill Marks are made by little rills of water trickling over the 

 sand as the tide goes out. 



Fig. 82. — Rill marks on modern sandy beach. (U. S. G. S.) 



Sun-cracks are produced in mud flats, where the mud and silt, 

 exposed by the retreating tide to the heat of the sun, dry and 

 crack open in more or less regular patterns, a process which may 

 be often observed in drying rain pools. When the incoming tide 

 advances so gently as not to disturb the cracked and hardened 

 surface, but deposits a new layer upon it, filling up the cracks, the 

 latter will be preserved, and on the overlying layer the casts of the 

 cracks will stand out in relief, when the two layers are separated. 

 Such marks are common in the fine argillaceous sandstones of the 

 Connecticut valley, New Jersey, and southeastern Pennsylvania. 



