PHYSICAL GEOGRAPHY 65 



State and constitute a great physical barrier between 

 Northern and Southern Cahfornia. (4) On the Pacific 

 side, to the north, a group of natural units which in- 

 cludes the Sierra Nevada, the Great Valley, and the 

 Coast Ranges proper; (5) the Peninsula Ranges of 

 Southern and Lower California, and their associated 

 features which separate the Pacific Ocean from the 

 gulf and Colorado Valley, (6) the Continental Shelf 

 with its individual ridges and projecting islands, and 

 its valley troughs..^ 



Each of these six great divisions more or less differs 

 from the others, yet each is characterized by its al- 

 ternations of ribbon-like highlands and lowland val- 

 ley plains. There are great differences in the details 

 of the several regions and most of them were made at 

 different epochs of the later periods of geologic time. 

 The physiography of each is intimately associated with 

 a certain group of master faults. 



THE CROSS TRENDS 

 We are accustomed to divide North America along 

 longitudinal lines into a series of north-south extending 

 belts like those of the Appalachian, Mid-Continent and 

 Cordilleran Regions. 



It may also be divided latitudinally or into east-west 

 extending belts, or zones, as the old-fashioned geo- 

 graphers did, to which climatic terms hke the Arctic, 

 Temperate, Sub-Tropical and Tropical Zones were for- 

 merly given. This latter subdivision into east-west 

 extending zones is accompanied by certain similarly- 

 extending lines or lineaments. One of these is better 

 known as the "glacial line," having marked the south 



'The term "CJontinental Shelf" is a misnomer, but no appropriate substi- 

 tute at present occurs to me. 



