418 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1908. 



I w 



farther in that direction. To the south of Pacas- 

 mayo the coastal plains narrow until the mountains 

 descend to the shore south in the valley of Viru. 

 Throughout this extent the Pacasmaj'^o formation 

 is represented in its various phases. The age and 

 relations of this formation will be more clearly 

 understood when it is considered in connection with 

 the Barranco formation next to be described, with 

 which it has been correlated (see fig. 9). It is to 

 be regretted that in the region of the Sechura desert 

 the relations of the Tertiary formations of the north- 

 ern part of the northern coastal plains and the 

 Pacasmayo of the southern portion are obscured by 

 the drifting sands, which obliterates any exposures 

 which might otherwise be seen in this area of slight 

 relief. 



Barranco Formation. 



At the valleys of the Pativilca, Huaura, Chancay, 

 and Eimac rivers there are sea cliffs cut in what 

 appear to be raised delta formations. In other val- 

 leys to the south and north smaller areas of a 

 similar formation may be seen (see pi. 4). At 

 Tambo de Mora the sea cliff has the same character 

 as at the mouths of the rivers, but there the forma- 

 tion extends inland and northward continuously to 

 the valley of the Caiiete. The writer regards this 

 area, which constitutes a part of the south-central 

 coastal plains, as furnishing the key to the proper 

 understanding of the Barranco formation. It un- 

 doubtedly lies upon the Pisco formation, although 

 its relations to the latter south of the Chincha River 

 are not very clear because of the intervention of 

 the wide stream valley. Its relation to the Pisco 

 formation may also be seen in the Caiiete Valley. 

 The character of the materials and the degree of 

 cementation in the Pacasmayo and Barranco forma- 

 tions is similar. 



No fossils have been found with the excep- 

 tion of comminuted shells and occasional branches 

 of trees. The writer has assigned the Pleistocene 

 age to these deposits and would correlate the coarse 

 sediments and bowlders which have been deposited 

 in the form and structure of deltas with the in- 



