8 Barrell — Movements of the Strand Line 



pioneer in thought in many fields of Australasian geology, 

 showed in 1902 that the Great Barrier Eeef of Australia 

 stands on a platform which marks a former lower sea level. 

 As Vaughan has stated, " The problem of the depth of barrier 

 platforms is a world-wide one, for it is only an aspect of the 

 general problem of the history of continental shelves."* 



The study of the form of the subaqueous profiles shows, then, 

 that they are not generally in equilibrium with wave action 

 throughout their length. The character of the departure 

 indicates a recent submergence. Where the shore is abrupt, 

 as off the coast of Maine, the profile is overdeepened and the 

 power of the waves is concentrated upon the exposed shore. 

 Where the coast lands are very flat, as on the coastal plain, the 

 submergence has inundated a strip which was previously land. 

 The waves spend most of their energy on the flat bottom, miles 

 off shore ; they drag and throw up bars, partially recover their 

 form and advance farther inshore to where barrier beaches are 

 thrown up and cut off farther advance. The overflat bottom 

 profile thus becomes undulatory by the shifting of material which 

 is finally swept in part to the beach, in part to deeper water, 

 and a normal profile becomes established. Thus it is seen that 

 according to the form of the land submerged the profile may 

 be overdeepened or overshallowed and the wave work 

 becomes opposite in trend. But the cause, — submergence, 

 may be equally clear in each. 



Vaughan has developed additional criteria from the study 

 of the shore and offshore physiography of the West Indies. 

 He has pointed out that solution wells in the limestone show 

 former emergence. These were made by fresh waters, and in 

 the Bahamas are now submerged to a depth reaching 33 

 fathoms. There was also a still-stand of the land with the shore 

 about ten fathoms below present level, as shown by the levels 

 of the submerged terraces on the outside of the barrier reefs 

 at from 8 to about 13 fathoms, with solution wells inside the 

 barrier showing a depth from 6 to 7*5 fathoms. The filled 

 channels of Havana harbor further substantiate the conclusion. 

 Here there is a concurrence of evidence. From many regions, 

 however, the other lines of evidence will be absent. There 

 the conclusion must rest upon the testimony of the sea floor 

 alone. When understood it is as convincing, however, as are 

 any of the more familiar lines of evidence. 



As an implication from the detailed evidence of the sub- 

 aqueous profiles, it appears that the outer flats of the con- 

 tinental shelves still preserve to a greater or less degree the 

 form determined by a sea level about twenty fathoms below 



* Sketch of the Geologic History of the Florida Coral Eeef Tract, p. 33, 

 1914. 



