438 STUDIES FOR STUDENTS 



It is seen, therefore, that in places where the httoral attains a 

 maximum width of 30 miles there are at the same time far greater 

 areas receiving land and ocean sediments, so that the proportion of 

 the littoral zone is hardly greater than before. 



These few places where the littoral attains a maximum width 

 are oijset by the thousands of miles of coast-line where the sea is 

 cutting into the land, and the tide rises and falls against a narrow 

 beach at the foot of a sea-cliff; so that the previous conclusion may 

 be further extended to include all continental coast-lines; and it 

 may be stated that, taking the world as a whole, the width of the 

 littoral zone does not average a mile, and therefore comprises but a 

 small fraction of the earth's surface compared with the great extent 

 of marginal marine and even of continental deposits.' 



With respect to the shallow- water marine formations this remains 

 true under all geological conditions, and therefore remains true for all 

 geological time. But occasionally, in periods when the continental 

 surfaces were physiographically old, of greatly diminished area and 

 supplying but little sediment, continental deposits may well have 

 sunk to less importance than the littoral. During such periods, 

 however, there is a corresponding expansion of epicontinental marine 

 sediments, though at such times of a calcareous character, so that 

 the insignificant proportion of the littoral to the sum-total of other 

 sediments deposited upon the continental platforms would not be 

 greatly changed, and has been a constant feature of the earth's sur- 

 face. 



CAUSES RESTRICTING THE WIDTH OF THE LITTORAL ZONE 



The previous examination of open sea and estuarine coasts shows 

 that the width of the littoral is not dependent merely upon the flatness 

 of the shores, nor upon the magnitude of the tidal range. These, 

 while contributory factors, can operate only within certain limits. 

 In order to see why this is so, it may be well to state briefly some of 

 the causss which influence the result. 



I. The influence of varying slope of shore may be seen in Fig. 5. 



I Chamberlin and Salisbury, Geology, Vol. I, p. 352, give the width of the littoral 

 zone as half a mile, the length of the coast-lines of the world as 125,000 miles, and 

 consequently the area of the littoral zone as 62,500 square miles, as against 10,000,000 

 square miles for the area of the shallow-water zone within the loo-fathom line. 



