STRATIFICATION 971 
is often marked by a series of undulations, which are 
known as ripple marks. These are caused by the rising 
and falling of the waves. They can be formed only 
where the depth is so slight that the effect of the 
waves is felt on the bottom. Ordinarily this depth 
is less than a score or two of feet. On the sandy or 
clayey bottom of lake or sea-shore, ripple marks may 
Fie. 154. 
Rainprints on a surface of mud. 
be seen forming to-day, giving rise to outlines which 
in no respect differ from those preserved in the strata. 
Change in Level of Land and Sea Bottom. — While 
the majority of sedimentary rocks are thus of shallow- 
water origin, we nevertheless find accumulations of 
sedimentary strata with a depth of thousands of feet, 
one layer over another. So we have the apparent 
anomaly of a great thickness of layers, each of which 
