NOTES ON RIPPLE MARKS 



127 



sufficiently strong to move material of the coarseness present where 

 the rhythmic motion prevails. 



On the bottom of any billowy water, sufficiently shallow for the 

 size of the waves, there must be a to-and-fro motion for each passing 

 wave. For waves of the same size, the deeper the water the more 

 slow and the more limited will this motion be. Hence the less will 



Fig. 4. — Ripple marks in Comanchean limestone in the right bank of Bosque 

 River, near Clifton, Texas. 



be the diameter of the particles it will be able to stir. There must 

 be a certain depth where the motion will be just speedy enough 

 to stir particles of silt. Where the bottom lies at this depth, and 

 where it is covered with silt, ripple marks will form. Should not 

 their width be determined by the extent of the to-and-fro move- 

 ment in each direction ? This decreases downward according to a 

 known law. 



It is evident that the velocity of each to-and-fro movement on 

 the bottom of an agitated body of water begins with zero, rises to a 



