6 



PHYSIOGRAPHY 





ways. Different names are given to various sorts of plains, the 

 names being intended to call attention to some one feature. Im- 

 portant classes of large plains are coastal plains, which border the 

 sea, and interior plains, which are far from the sea, or separated 

 from it by high lands. 



Coastal plains occur on the borders of many continents. They 

 are wide in some cases, and narrow in others. A narrow plain 



with a nearly flat surface 

 is shown in Fig. 3. The 

 landward edges of coastal 

 plains are not always so 

 sharply marked as in this 

 illustration. 



Plate I (p. 4) repre- 

 sents, in another way, a 

 part of the narrow coastal 

 plain of Oregon. Since 

 illustrations of the sort 

 shown in this plate will be 

 used often in the following 

 pages, the principles on which it is based must be understood. It 

 is called a contour or topographic map. 



EXPLANATION OF CONTOUR MAP 1 



The topographic map shows three sorts of features. These are (1) the 

 shape of the surface, (2) the distribution of water, and (3) the works of man. 



The shape of the surface. A land surface may be flat or uneven. If flat, 

 it may be high or low, and if uneven, it may have little relief or much. The 

 topographic map shows how flat or how rough it is. 



The height of land is reckoned from sea-level. The heights of many 

 points are measured exactly, and some of them are given on the maps in figures. 

 It is desired, however, to give the elevation of all parts of the area mapped, 

 and to show what parts are flat, and what parts have slopes. Not only this, 

 but if there are slopes, it is important to show whether they are gentle or steep. 

 All these things are done by lines called contour lines, or simply contours. A 

 contour line connects points of the same elevation above sea-level. Thus the 

 contour of 20 feet connects points which are 20 feet above sea-level, the con- 

 tour of 40 feet connects points which are 40 feet above sea-level, and so on. 

 1 From publications of the U. S. Geological Survey. 



Fig. 3. A narrow coastal plain. 



