CONTINENTAL SHELF 257 



the earth where the life is so varied and the struggle for 

 existence so great as on these shallow continental borders. 

 Here the mud and sand brought down by the rivers is 

 spread out and the sedimentary rocks formed. It is the 

 elevation of this shelf which has formed the low-lying 

 coastal plains which border many of the continents. There 

 is good reason to believe that the deep floors of the sea 

 have never been raised into dry land, and that the vast 

 extent of sedimentary rocks which make up the larger por- 

 tion of the land has almost all been laid down in regions 

 which were at the time continental shelves. 



Coast Effects Resulting from Upward Movement of the 

 Earth's Crust. Experiment 82. Tack enough sheet lead to a 

 very rough board so that it will remain submerged when placed in 

 water. Place the board in a shallow dish of water, lead side down. 

 Taking the board by one edge, gradually lift this edge above the 

 water surface. What kind of line does the water form where it 

 meets the board? In what way would this line be changed if the 

 board were smoother? If it were rougher? If the edge of the 

 board is lifted higher, does the position of the water line change? 

 Does its form materially alter? 



Soundings show that a continental shelf has a compara- 

 tively smooth surface and a gentle slope. If the shelf is 

 elevated, a strip of level sea bottom is added to the dry 

 land, and the water will meet this new shore in almost a 

 straight line. The material forming the shore, both above 

 and below the water line, will be easily eroded since it has 

 been recently deposited and has not had time to be consoli- 

 dated into solid rock. 



Waves rolling in from shore will strike the bottom of this 

 gently sloping shelf at a considerable distance off shore. 

 The water thus loses velocity, and deposits much of the solid 



