Vol. VI] WEAVER— POST-EOCENE OF WESTERN WASHINGTON 23 



tains in Chehalis County, Oligocene and Miocene formations ' 

 extend southerly to the middle of Pacific County. Outlying 

 residuals occur in western Thurston and Lewis counties. Sim- 

 ilar deposits of post-Tejon age are present in Wahkiakum 

 County and in the southwestern part of Pacific County. They 

 have been cut by the Columbia River and their southern exten- 

 sion forms a part of the well-known series of outcrops occurring 

 at Astoria, Oregon. No marine deposits of Oligocene or Mio- 

 cene age are known to occur within the Cascade Mountains 

 or within the great basin area of eastern Washington. In 

 these regions deposits of corresponding age are of igneous 

 or freshwater orig^in. 



■ts' 



SEDIMENTATION 



From the standpoint of stratigraphy the post-Tejon sedi- 

 ments occurring in the w^estern part of the state are divided 

 into two broad groups, separated by a well-marked uncon- 

 formity. The older division includes deposits of Oligocene 

 and lower Miocene age, while the younger involves sediments 

 of upper Miocene and possibly lower Pliocene ages. During 

 the Oligocene, embayments of the ocean were widely ex- 

 tended over western W^ashington with the exception of the 

 central and western portions of the Olympic Peninsula. Dur- 

 inof the lower Miocene their extent became much smaller and 

 by the opening of the upper Miocene they w^ere almost en- 

 tirely withdrawn. During the upper Miocene two small, shal- 

 low basins of deposition were formed. One of these existed in 

 the basin of Grays Harbor and the other near the junction of 

 Bogachiel and Solduc rivers in southwestern Clallam County. 



During the Eocene epoch, southwestern Washington was 

 occupied by an embaynient of the ocean which seems to have 

 extended as far north as the middle portion of the present 

 Puget Sound basin. This fact is indicated by the occurrence 

 of narrow bands of marine strata interbedded with those of 

 purely brackish water origin. On the eastern shores of this 

 embayment were situated extensive estuaries in which over 

 10,000 feet of brackish water sediments were deposited and 

 which now form extensive outcrops in King, Pierce and Lewis 

 counties. Igneous activity was characteristic of the larger 



