FORMATIONS AND PHYSICAL HISTORY 559 



By the beginning of the Eocene, the Puget Sound depression, 

 l>rrh;ip.- to be correlated with the great valley of California and the 

 dull" of California, had begun to show itself. 1 The lands east and 

 west of the sound were high, but not mountainous; and the region 

 of the sound was a great estuary, in and about which deposition was 

 in progress. Some of the sediments accumulated in brackish water 

 and on land, and resulted in the thick coal-bearing Puget series of 

 Washington, the upper part of which is Oligocene or even Miocene. 

 The series is said to contain 125 beds of coal thick enough to attract 

 prospectors. Most of the workable coal is in its lower part. The 

 area of deposition extended south into Oregon, and east toward the 

 Blue Mountains of that state. The system has an estimated thick- 

 ness of 10,000 to 12,000 feet in southern Oregon, and but little less 

 in southern California. 



British Columbia appears to have been land during the period, 

 but Eocene beds, much disturbed (Kenai series), have been recog- 

 nized in Alaska, where they are coal-bearing in places. 



After the Eocene there was a time of temporary 'elevation, 

 erosion, and volcanic activity along the Pacific coast, with consider- 

 able basaltic flows in Washington and Oregon. 



The western interior. The warpings, faultings, and the intru- 

 sions and extrusions of lava which marked the close of the Meso- 

 zoic era in the west appear to have developed lands which were 

 relatively high, adjacent to tracts which were relatively low. The 

 steep slopes of the mountain folds, fault scarps, and volcanic piles 

 seem to have afforded the conditions for rapid erosion, while the 

 adjacent lowlands furnished places of lodgment for much of the 

 sediment. Some of it took the form of fans and alluvial plains, and 

 some of it probably lodged in lake basins formed by warping and 

 faulting, or by the obstruction of valleys by lava flows. The \\ind 

 also made its contribution to the deposits of the time, and the 

 Eocene system contains much pyroclastic material. The result 

 was a combination of lacustrine, fluvial, pluvial, eolian, and vol- 

 canic deposits. 



The sites of principal sedimentation shifted somewhat from 

 time to time, and among the widely distributed deposits referred 

 to this period there are great differences of age. Several more or 

 less distinct stages of deposition have been made out, the distinc- 

 tions being based partly on the superposition of the beds, and partly 



1 Willis, Tacoma folio, U. S. Geol. Surv. 



