GEOLOGY OF PERU ADAMS. 



417 



Peru began their activity in Ter- 

 tiary times and some of them are 

 still active, although no great 

 lava flows have come from them 

 in recent times. The writer 

 has provisionally assigned the 

 Pliocene age to the Moquegua 

 formation, thus making it con- 

 temporaneous with the Pisco 

 formation to the north. There 

 appears to be no reason for con- 

 sidering it as of greater age, and 

 in outlining the history of the 

 coast the Pliocene age seems for 

 the present satisfactory. 



The thickness of the Moquegua 

 formation is variable, since it was 

 apparently deposited in a trough 

 between the coast hills and the 

 foothills of the Andes (see fig. 

 8) . From measurements made in 

 some of the canyons a thickness 

 of 1,500 feet may be assigned. 



QUATERNARY DEPOSITS. 



Pleistocene. 



The Pacasmayo Formation. 



At Pacasmayo, in the southern 

 part of the northern coastal 

 plains, the sea cliff consists of 

 stratified conglomerates mixed 

 with sand and occasional clay 

 beds (see pi. 3) . The formation is 

 also well exposed at the mouth of 

 the Jequetepeque and along that 

 stream inland. At Eten the sea 

 cliff consists of a homogeneous 

 sandy clay. To the north of Eten 

 for a considerable distance the 

 coast is low near the shore and 

 there are no good exposures, so 

 that the writer has not been able 

 to trace the Pacasmayo formation 



