Page 8 - dort1948

Fage 3

The position ot the northern border or the

map was determined by the position ot the southern

border ot an area mapped by Jahns in 1938, and

published 1n a report of the Carnegie Institute ot

Washington as noted in the bibliography. The remainder
ot the oartographio border was located in areas or

alluvium wherever possible, forming a natu.ral geologic

boundary, or on the phya1oal boundary ot a state highway.

The geology of the area herein discussed la

made especially interesting by the presence ot a

representative section extending trom the Upper Miocene

to the present, and by the absenoe ot two tormat1ona,

the Modelo and Pioo, trom oortains parts ot the area.

A large aeries ot river terraoea, numerou examples ot

stream piracy, and the nresence or several t7Pe9 of mass

move~ents are ot considerable geomorphlo interest. 'lhe

poss1b111ty of eommereial production of petroleum, gold,

and ilmenite adds economic importance to the investigations.

The northern two-thirds ot the territory examined

was mapped during twenty-two field days in the winter

or 1948-47 aa an exercise 1n geologio mapping tor the

California Institute ot ~eohnology-. An additional

fifteen field days were spent during the following

winter 1n re-examination ot the northern part. and in