780 .T. BARRELL ]S[KASIJRKMJhJNTS OF (lEOLOlUC TIME 



ligure for the effective wave-base of tlie ocean.-- This is based on tiie actual 

 evidence of profiles rather than on the theory of waves, or the ability f)f 

 exceptional waves to agitate mud in suspension at greater dei3ths. 



To gain conceptions as to the depth of ancient interior seas several 

 classes of water bodies must be studied. Stormy epeiric seas are repre- 

 sented by Lake Erie and the North Sea. The full de])th of water, so far 

 as wave action is concerned, is attained Avithin 100 miles from the shore. 

 In both of these water bodies there are areas below wave-base which permit 

 an excess of sediment to be swept into such reservoirs, and thus prevent 

 the profile from being shallowed. Quiet epeiric seas are found in tropic 

 waters and are illustrated by the shelf sea east of the Mississippi delta, 

 but more typically by the Persian Gulf.--"^ The latter is an epeiric sea 

 which has a central portion unfilled and below wave-base. Large lagoons 

 are represented by Lake Pontchartrain on the Mississippi delta. Tn addi- 

 tion what may be called j)laya seas formed recurrently important features 

 in the past. These were annually, or at longer intervals, alternately 

 flooded with shallow marine waters and exposed to tbe air. Such playa 

 seas are represented at present by the Eann of Cutch, an area of 10,000 

 square miles east of the Indus delta, flooded from July to November, dur- 

 ing the southwest monsoon, to an average depth of five feet, owing to a 

 rise of sealevel due to wind ^Dressure, but constituting a l)arren and saline 

 mud-flat during the remainder of the year. Over very flat areas facing 

 shallow water bodies the seasonal direction and intensity of winds is thus 

 seen to be an important factor. 



A tabulation of the relations of depths to distance from shore for these 

 various types of water bodies is given as follows : 



Bottom Profiles established hy Wave Action 

 Depths Of water in feet 



Distances from shore in miles 



Type of water body 5 10 20 80 100 



Stormy shelf seas 95 110 140 300 300 



Stormy epeiric seas 55 70 90 110 110 



Quiet epeiric seas 35 50 70 90 90 



Wide lagoons 15 15 



Playa seas to 5 to 10 



-- Josepli Barren : Factors in the movements of tbe strand-line and their results in the 

 Pleistocene and post-Pleistocone. Am. Jour. Sci., vol. xl, 1015, pp. 6, 7. 



A good illustration of this limitation in depth is shown in the bathy metric contours 

 of the Boston quadrangle, scale 1 : 1,000,000. TT. S. Geological Survey. Sheet North K 10. 

 International Map of the World. 



-The writer is indebted to Doctor Bowman, director of the American Geographical 

 Society, for data from the British Admiralty charts showing the details of the bottom 

 of the Persian Gulf. 



