PROCESSES IN OPERATION 



171 



It follows that the changes effected by diastrophism are more obvious 

 in -hallow water than in deep. Emergence or submergence shifts 

 the zone of contact of ocean and land, and so the areas of aggrada- 

 tion and degradation, and changes the region concerned from one 

 appropriate for sea life to one appropriate for land life, or vice versa. 

 Over the continental shelves the water is shallow and the bottom 

 relatively smooth. If the sea-level were drawn down, or if the con- 

 tinental shelf were elevated evenly, the new shore-line on the smooth 

 surface of the former submerged shelf would be regular relatively, 

 fvrn though the coast was notably irregular before the change. 

 This is illustrated by Fig. 174. Subsidence of a coast-line (or rise 

 of the sea-level) tends to the opposite result, for in this case the 

 sea advances on a surface which has. relief, and the water covers 

 every low place sunk to its level. Thus the numerous bays at 

 the lower ends of the streams along the Atlantic coast from Long 

 Island Sound to Carolina are the results of recent sinking. From 

 the present configuration of coast-lines it has been inferred that the 

 present is an era of continental depression. Some river valleys, the 

 lower ends of which are embayed, are found to be continuous with 



Fig. 175. The submerged valley which has been interpreted as the continua- 

 tion of the Hudson Valley. The position of the valley is indicated by contours. 

 (Data from C. and G. Survey.) 



