CHAPTER VIII 

 THE LAVA PLAIN OF THE NORTH 



An Immense Lava Plain 



A part of what is probably the largest lava field in the world 

 is in northern California. This vast outpouring of molten rock 

 covers an area of approximately 200,000 square miles in north- 

 ern California, Oregon, Washington, Idaho, and Montana. It 

 was during Tertiary time that this great outpouring of lava was 

 emitted from the depths of the earth. It spread far, covering 

 plains and low hills and filling valleys. Mounts Shasta, Hood 

 and Rainier are three outstanding volcanic peaks which con- 

 tributed to the molten mass. A great number of volcanoes of 

 lesser size, now extinct craters, occur in the great lava field and 

 contributed to the flow. 



A broad high plateau of lava in northeastern California 

 extends to the east, including Modoc County, and beyond into 

 Nevada. What is known as Lassen Peak Ridge extends across 

 Shasta, Lassen, Plumas, and Tehama counties, from the north 

 fork of American River to Klamath River. Lassen Peak Ridge 

 is a vast hill of lava 50 miles long and 25 miles wide, built up 

 in a valley which lies between the north end of the Sierra 

 Nevada Range and the southern end of the Cascade Range. 

 The ridge has been built up by outpourings of lava. The orig- 

 inal valley was an ancient depression in which a sound or arm 

 of the ocean once lay, now filled with lava. It was a valley in 

 very ancient (Palaeozoic) time. The ancient valley is a con- 

 tinuation of the Great Central Valley of California. 



The Northern End of the Sierra Range 



The Sierra Nevada Range may be said to end and the Cas- 

 cade Range to begin in northern California. Lassen Peak, in 



