Fluviatile and Marine Gravels. 497 



Gravels of Marine Origin. 



Definitions. — For the sake of uniformity in the discussion 

 of coastal physiography it seems advisable to retain the terms 

 suggested by Gulliver,* viz : Shore line is the line of intersec- 

 tion of sea and land, the region landward of the shore line is 

 the coast, and the region seaward of this line is the shore.\ 

 Coastal belt is the zone formed of coast, shore line and shore. 

 Wave base is the controlling plane of marine levelling — a 

 mathematical plane toward which the mature profile of marine 

 denudation tends but does not reach. The submarine platform 

 (or subaqueous shore terrace, or plain of marine denudation) 

 bears the same relation to wave base that a peneplain does to 

 river base level. Initial is a technical term defining the form 

 at the beginning of a physiographic cycle. Forms developed 

 at later stages are sequential. The term shifting is used in 

 the present paper for change in position of a shore line result- 

 ing from crustal movements, recession for the change in 

 position due to wave cutting, and migration for the total move- 

 ment of the shore line regardless of causes. 



Physiographic Development of a Coastal Belt. 



The formation, distribution, and preservation of marine 

 gravels are controlled by marine currents conditioned by rock 

 structure and physiographic age. A brief analysis of marine 

 action within a coastal belt may therefore serve as an aid in 

 the interpretation of conglomerates. 



The net result of the combined work of waves and currents 

 is to establish maturity of outline within the two zones which 

 comprise the coastal belt, namely the coast and the shore. 

 Maturity of outline of the coast is attained when erosion, trans- 

 portation and deposition are so adjusted with reference to each 

 other that all portions of the shore retreat at equal rates ; that 

 the landward migration of the shore line in areas of resistant 

 materials is the same as for the less resistant portions of the 

 coast. Under these conditions the shore line tends to straight- 

 ness and retains that quality so long as sea level remains 

 constant. Development of the coastal contour of continents 

 cannot proceed further so long as any land remains above sea 

 level. On many small islands, however, landward migration 

 of the shore line has continued until all land has been consumed. 

 In such cases a mature coast is succeeded by no coast ; land 



* Shore line Topography, Proc. Am. Acad. Arts and Sciences, vol. xxxiv, 

 p. 152, 1899. 



f This use of the term " shore " is obviously technical. The common prac- 

 tice is to consider the " shore " as part of the land. Satisfactory discriminat- 

 ing terms are needed for the loosely used words "coast," "shore," and 

 "strand." 



