1918] fan Winkle: Paleontology of 'the Oligocene 73 



stratigraphic section. The rocks are composed of massive gray sandy shales dipping 

 at a low angle to the northeast. They seem to constitute the southwest limb of 

 the Chehalis synclinal trough. The basal contact with the Eocene was not observed, 

 although rocks of probable Eocene age exist not far to the south. 



On the south side of Chehalis River, between Oakville and Helsing Junction, 

 the structural relations between the Eocene and Oligocene formations can be more 

 clearly determined. The Oligocene strata rest with marked unconformity upon 

 tlif upturned and eroded edges of the Eocene shales and basalts. Exposures of 

 grayish brown sandstone containing characteristic marine Tejon fossils outcrop 

 in the cuts along the C. M. & S. P. Railway from Balch Station, in Section 36, 

 T. 1(3 X., R. 5 W., for a distance of three miles, to the southeast, where they rest 

 upon the interbedded basalts. These strata strike northwest and southeast and dip 

 from 10 to 30 to the southwest. In sections 8 and 9, T. 15 N., R. 4 W., a fine- 

 grained badly altered basalt forms the rock along the south bank of Chehalis 

 River. About one and one-half miles up Independence Creek are dark-colored 

 massive shales which strike northwest and southeast and dip to the southwest. 

 Lithologically they differ from the characteristic Oligocene sandy shales which rest 

 unconformably upon the Eocene sediments. Westerly along the south side of 

 Chehalis River from the mouth of Independence Creek are exposed light grayish 

 brown shales having good bedding planes. These strata contain typical lower Oligo- 

 cene marine fossils. They strike approximately N. 40 W. and dip to the northeast 

 at angles ranging from 55 to 65. They rest unconformably upon the Eocene 

 rocks below. The Oligocene strata exposed at the mouth of Independence Creek 

 constitute a part of the south limb of the Chehalis Valley syncline and presumably 

 extend northwesterly to Williams Creek beneath the marsh and alluvium of the 

 valley. The unconformable relations between the Eocene and Oligocene forma- 

 tions on Independence Creek suggest that during the latter part of Eocene time 

 this part of Washington was undergoing uplift, folding and erosion. 



The type exposures of the Lincoln Horizon of the Oligocene occur in cuts along 

 Chehalis River west of the mouth of Lincoln Creek. The basal beds of this section 

 are not exposed. The strata are composed of massive gray sandy shales containing 

 well preserved marine fossils. They dip to the southwest at a very low angle and 

 have a thickness of at least 500 feet. 



The area between Oakville and Gate, along the present valley of Chehalis 

 River, is deeply filled with gravel and alluvium, and the underlying bed rock is 

 nowhere exposed. The structural conditions between Porter and Oakville and 

 between Lincoln Creek and Helsing Junction suggest a direct connection of the 

 Lincoln Horizon with some portion of the lower Porter Creek section. The strata 

 at both localities seem to form a part of the northeast limb of the Chehalis 

 Valley syncline. 



Bedrock exposures are for the most part absent south of Chehalis for some 

 distance. In the rock bluffs along Olequah Creek, southwest of the town of Win- 

 lock, there are exposures of massive gray sandy shales containing an Oligocene 

 fauna similar to that at Lincoln Creek. These beds are resting almost horizontal 



