STRATIFICATION 267 
the land by those movements of the crust that form 
mountains, these layers are sometimes very much dis- 
turbed and moved from their position, being bent and 
tilted at all angles, even to the vertical (p. 283). 
If a series of strata are in process of deposition in 
the ocean, the lowest is first formed and is 
hence the oldest (Fig. 151). Therefore when 
these are added to the land and exposed 
to the eye, we usually feel certain that the 
lowest bed is older than the ones above it. 
This order of superposition, as it is called, 
is sometimes disturbed when the rocks have 
been folded excessively, or broken, and 
then moved one over the other (Fig. 173). 
Then, as a result of this unusual disturb- 
Fie. 151. 
ance, the older rock may actually le on a pjsgram to ik 
younger one; but these cases are rare. lustrate order 
of superposi- 
Most Sedimentary Rocks deposited in _ tion. Figures 
Shallow Water. — Absence of deep-sea de- ea 
posits. On the continents the most com- 
mon rocks are sedimentary, and the greater num- 
ber of these were formed in the sea. We know 
this because they contain fossils of ocean animals that 
were entombed while the rocks were accumulating. 
Since the greater part of the ocean is very deep, 
one might suppose that the sedimentary strata of 
the land would include many deep-sea deposits; but. 
