Physiographic Geography 

 That physiographers do not agree on definite 

 boundaries to the subaivisions of this great western 

 region, the varied characteristics of which have 

 been barely suggested in the foregoing paragraphs, 

 is not surprising. Sometimes one province ahnost 

 imperceptibly grades into another. As a usual thing 

 reasonably sharp limitations seem possible, but 

 exact knowledge is lacking as yet in these extensive 

 and thinly settled regions largely because geograph- 

 ical science necessarily views tne earth from so 

 many standpoints that criteria for classification are 

 many and varied compared with the relatively 

 simple conditions used as determinants in the more 

 specialized sciences. There is, however, fair ap- 

 proach to unanimity in dividing the Pacific Slope 

 primarily into four major physiographic provinces 

 each roughly parallel to the ocean: the Coast 

 Ranges, the Pacific Valleys, the Sierra Nevada-Cas- 

 cade ranges, and the Arid Plateaus. This classifi- 

 cation is based largely on a consideration of topog- 

 raphy, climate and vegetation; but these three fac- 

 tors, each involving many details, have no fixed 

 relative value in determining boundary lines of the 

 various provinces. The resulting subdivisions, how- 

 ever, have a general and easily recognized physiog- 

 raphic unity which justifies the use of varying 

 criteria in classification. The four primary regions, 

 on more detailed study, are themselves subdivided 

 by lines that sometimes roughly depend on climate, 

 sometimes on geologic structure as expressed in 

 topography or soils, or on vegetation which may be 

 taken as the surface expression of both topography 

 and climate. 



Two important and absolutely necessary condi- 

 tions for the appreciation of the diff"erent physiog- 

 raphic provinces of the west coast are, first, their 

 position on the east of a great ocean in such lati- 

 tudes that the winds prevailing from the west bring 

 inland the moderate and nearly constant tempera- 

 ture of the sea; and, secondly, the fact that the two 

 great mountain systems lie squarely athwart these 

 moisture-laden west winds. The result is an alter- 

 nation of belts of greater or less rainfall. This 

 varying amount of rain helps in determining the 

 limits of the first three provinces and is the chief 

 determinant of the Arid Plateau Province — the "rain- 

 shadow" area of the Cascade-Sierra Nevada ranges. 

 With this parallelism of the major physiographic 

 provinces and their extent practically from Canada 

 to Mexico, it follows that the visitor to the Panama- 

 Pacific International Exposition traveling through 

 the United States must cross all four of the major 



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