Geology from a Motor Car 399 



Nevada on the east. It embraces the north half of the Great 

 Central Valley of California, of which the San Joaquin Valley 

 is the south half. Into the Sacramento Valley from both east 

 and west enter streams which have carried detritus to the valley, 

 forming the alluvial floor of the nearly flat plain. In the flatter 

 parts of the valley overflow from the incoming rivers is prac- 

 tically a yearly occurrence. The overflow lands on a large area 

 have been protected by dikes. Overflowed lands which support 

 a luxurious swamp vegetation are among the most fertile in 

 the valley when reclaimed by levees. 



Rainfall is seasonable. The months October to March con- 

 stitute the rainy season. The rest of the year is practically 

 rainless. Cattle-raising is an important industry. Lands that 

 are overflowed yearly (tule-lands) are grazed during the dry 

 summers, and with the coming of the fall rains cattle and sheep 

 are moved to grazing lands in the mountains. Cereal grains 

 are sown in the fall and harvested in the early summer. All 

 deciduous fruits bear abundantly, and are rarely damaged by 

 frost. Many fruits and nuts are grown, and there are large 

 areas of vineyards. 



The gently undulating, nearly flat, plain of Sacramento 

 Valley, which is crossed en route to Roseville, merges into the 

 broadly eroded depression of the valley of American River. 

 This broad valley, east of Roseville and about Folsom, has been 

 formed by the erosion of the comparatively soft and erodable 

 granodiorite which is the body rock of the Sierra Nevada. Off 

 to the north and east Lincoln looks like a great table-land. 

 This is known as the Auburn Ridge. It is a hard volcanic 

 rock (technically andesite) of Tertiary age. The rock is hard, 

 and the softer formations surrounding are eroded away leaving 

 the flat-topped table-land. The foothills which lie along the 

 edge of the valley at the base of the Sierra Range are the dis- 

 turbed, folded and wrinkled metamorphic sedimentary rocks 

 of the Mariposa (Jurassic) formation, intermixed with lavas, 

 together with later deposits of nearly horizontal beds of sand- 



