6 Barrett — Movements of the Strand Line 



each other, yet the recurrent movements of the crust teud to 

 eliminate this mark of old age, by creating new base levels of 

 planation. The profiles for various grades of sediment will 

 also be of unlike depth. Sand derived abundantly from active 

 shore erosion may build out a subaqueous terrace at the same 

 time that finer sediment, partly of river origin, may not be 

 permitted to settle on the older and deeper platform beyond. 

 After a profile is graded, then the advance of the sea inland 

 or the retreat of the sea, provided the level is constant, requires 

 erosion or deposition on all parts. This graded protile is 



Fig. 1. 







50 



100 



ISO 



Nouhcoi miles 



zo 



1 



V: ; 



1 



1 



1 





20 



• r^-^* 











\ "60 



\ 



Fig. 1. Projected profile of the Argentine coast, South America. Section 

 50 miles wide, 25 miles on each side of a line, bearing S. 24° E. through lat. 

 S. 38° 45', long. 59° 20'. Vertical scale 890 times the horizontal scale.* 



concave upward and may be called the profile of equilibrium. 

 The depth of the profile varies greatly according to the power 

 of the waves. Off the flat shores of quiet seas it ranges, at a 

 distance of about ten miles, between ten and twenty fathoms. 

 Vaughan, from his experience in the West Indies, reaches 

 similar conclusions and considers that about ten fathoms is 

 there the depth to which submarine erosion is vigorous. Off 

 abrupt coasts exposed to on-shore heavy winds the profiles 

 make it clear that it reaches to thirty or forty fathoms at the 

 same distance. 



As observed off many continental shores these slopes at the 

 present time may continue almost flat for twenty or thirty 



* Mr. Willis has noted in the discussion of this paper that the land 

 surface of Argentina has been greatly affected by warping in late geologic 

 time and that the possibility of warping should also be entertained in 

 considering the significance of the subaqueous terraces. This is a valuable 

 suggestion and no doubt has controlled the general form of the coast 

 line. The development of these upper terraces at itniform levels over 

 broad regions, however, at approximately 30 and 50 fathoms appears 

 to indicate that their surfaces have been controlled by wave action working 

 at these two levels at two different times later than the warping. 



