CONTINENTS; OCEANS 



191 



border of the continent. The now fertile Coastal Plain of 

 the middle and south Atlantic states was added to the 

 continent by elevation of the continental shelf. 



211. The Story of the Continents Told by the Rocks. - 

 We have seen how limestone rock was formed at the bottom 

 of the ocean. When beds of limestone are found to-day 

 in the interior of a continent, as they are in Indiana, Ken- 

 tucky, and neighboring states, it shows that there was once 

 a shallow sea in that region. If the limestone is bordered 

 on some sides by beds of sandstone and shale, it is evident 



Plate 



Mts. 



Plateau 



Plain 



Continental Shelf 



Deep Ocean 



FIG. 98. THE CONTINENTAL BLOCK 



The dotted horizontal line represents the level of the sea; the dotted 

 vertical line represents the eastern limit of a continental block. 1. To what 

 point does the continent extend? 2. What change of elevation would in- 

 crease its area? 



that the limestone was formed in a shallow sea which 

 became unfit for corals and such water animals because 

 of the wash of mud from the land. Corals live only in 

 clear, warm, and shallow waters. 



Layers of limestone are sometimes succeeded by layers of 

 sandstone and shale. This shows that corals and shell 

 animals were once abundant there; but as conditions 

 changed, cooler water or muddy water destroyed the life 

 and covered the remains with deposits of sand or mud. 



Ripple marks in the rock show where the water was very 

 shallow so that sand and mud were disturbed by wavelets. 

 Imprints made by falling raindrops prove that mud was 



