614 BRUCE L. CLARK 
of which were overturned. In certain localities the Cretaceous has 
been thrust over the younger formations. Though this great 
period of crustal movement had its culmination at the close of 
Saugus time, it was well under way even prior to the deposition 
of these beds. In the past geologists have generally assumed that 
the division between the Pliocene and Pleistocene occurs after this 
great folding, that is, at the end of Saugus time. But the very 
large percentage of the Recent species in the Saugus suggests that 
the group may possibly be of Pleistocene age, and therefore a 
large part of the folding which created the Coast ranges may have 
taken place during Pleistocene time. 
