QUATERNARY PERIOD ON THE WESTERN SIDE OF THE CONTINENT. 563 



Subsequent subsidence and partial re-elevation are still more clear- 

 ly shown by. raised sea-margins. During this period of subsidence 

 (Ohamplain), the Bay of San Francisco covered all the flat lands about 

 the bay, and all the valley continuations of the bay, north and south, 

 such as Sonoma and Napa Valleys on the north and Santa Clara Val- 

 ley on the south. Also the sea then passed through the Strait of Car- 

 quinez and covered the whole San Joaquin and Sacramento plains, 

 forming a great interior sea 300 miles long and 50 miles wide. The 

 margins of this sea are still visible in the upper Sacramento Valley. 

 At the same time the sea entered the Columbia River and spread over 

 the Willamette Valley, forming a great sound, and passed up to, and 

 possibly beyond, the Cascades. About Puget Sound similar evidences 

 of former extension are plain, especially at the southern end ; while in 

 British Columbia Dawson finds old sea-margins up to 2,000 or even 

 3,000 feet above the present sea-level. During the Terrace period, the 

 coast-line was re-elevated to its present level, leaving successive lower 

 terraces which are conspicuous in some places. Lake Tulare is a rem- 

 nant of the interior San Joaquin Sea, although it was probably first 

 freshened by an outlet into the San Joaquin River, and again salted by 

 loss of its outlet. 



2. Ice. — We have already (p. 265) spoken of a great elevation of the 

 Sierra Range, which occurred at the beginning of the Quaternary. 

 This mountain-lifting doubtless contributed to the development of gla- 

 cial phenomena at this time, but must not be confounded with the 

 general continental elevation which took place at the same time, as 

 shown by the sea-margin phenomena. 



During the fullness of glacial times — as shown by Dawson * — a 

 continental ice-sheet covered nearly the whole of British Columbia, 

 Northwest Territory, and Alaska, connecting in high latitudes with the 

 Eastern sheet. The center of radial movement was a high area ex- 

 tending from 55° to 59° north latitude. From this area the ice moved 

 southward, southwestward, westward, and even northwestward. South- 

 ward it certainly reached beyond 48°. Westward it flowed over the 

 Coast Ranges, filled the valleys (now submerged), separating the great 

 coast islands from the mainland, flowed over these islands, and ran into 

 the sea beyond. 



At the same time it is certain that the Sierra f was completely 

 mantled with snow ; and great glaciers, some of them 40 to 50 miles 

 long filled all the profound canons which trench its flanks. At the 

 same time also there is some evidence that even the Coast Ranges, 



* Geological Magazine, ^ol. v, p. 347, 1888. 



f For a fuller account of the glaciers of the Sierra, and the condition of things during 

 the Glacial epoch, see American Journal of Science, vol. iii, p. 825, and vol. x, p. 26. 



