72 University of Washington Publications in Geology [Vol. 1 



and Centralia. The third area is situated to the southeast between Winlock and 

 the Cowlitz River. For purposes of reference this arm of the sea may be spoken 

 of as the Oligocene Chehalis Valley embayment. 



The Oligocene formations exposed on Porter Creek are entirely composed of 

 marine sediments, which attain a thickness of at least 1,200 feet. They rest im- 

 conformably upon sandstones and basalts of Eocene Tejon age. The contact 

 between the Tejon basalts and the basal Oligocene sediments occurs on Porter 

 Creek three and one-half miles above its junction with Chehalis River. The basal 

 Oligocene beds are composed of a medium-grained conglomerate in which the pebbles 

 range in diameter from two inches down to a fine grit. These lower beds exhibit a 

 rough stratification and are nearly always stained a reddish brown color, due largely 

 to the circulating waters, which are charged with iroji derived from the nearby 

 basaltic masses. The pebbles in the basal conglomerates are clearly derived from 

 the underlying Tejon basalts. Exposures of Oligocene sediments occur at intervals 

 in the banks and bed of Porter Creek southward from the contact. These strata 

 have a prevailing northwest and southeast strike with a dip ranging from 1 to 20 

 to the southwest. 



About seventy-five feet above the base of the Oligocene on Porter Creek, the 

 gritty phase of the sediments grades over into a grayish brown, medium grained 

 micaceous shaly sandstone, which in turn grades into a sandy shale. The middle 

 and upper strata in this section are prevailingly massive and well developed bedding 

 planes are usually absent. The rock is prevailingly a shaly sandstone possessing 

 a light grayish brown color. The uppermost beds exposed in the railway and 

 wagon road cuts at the town of Porter contain numerous rounded concretions 

 averaging from two to four inches in diameter. In the interior of these are com- 

 monly the fossil remains of mollusks or crustaceans. 



The contact as observed between the Eocene and Oligocene on Porter Creek, 

 in the east half of Section 11, T. 17 N., R. 5 W., extends northerly and again 

 appears on Mox Chehalis Creek about nine miles above its junction with Chehalis 

 River, in Section 13, T. 18 N., R. 5 W. The Oligocene sediments at this locality 

 rest unconformably upon the older Eocene basalts. 



Basal Oligocene conglomerates are exposed at the northwest end of a quarry 

 composed of Tejon basalt situated one mile west of Oakville on the Northern 

 Pacific Railway tracks. These conglomerates rest unconformably upon the basalt 

 and have a thickness of about twenty feet. Above the conglomerates are massive 

 gritty sandstones which are dipping at a low angle to the southwest. 



The Eocene-Oligocene contact is exposed on Cedar and Gibson creeks about 

 one and one-half miles east of their junction with Chehalis River. The bed rock 

 exposures east of this contact consist entirely of Eocene basalt. On the western 

 side of the contact the basal beds are mainly gritty or conglomeratic, while the upper 

 beds are for the most part composed of gray sandy shales. 



On the western side of Chehalis River, midway between Porter and Oakville, 

 are exposures of massive sandy shales which may be seen in places along Williams 

 Creek. The lack of good exposures renders it almost impossible to construct a 



