Fluviatile and Marine Gravels. 501 



Application to the Formation and Distribution of Gravels. 



On an excursion to Marblehead and Nahant in. 1898 raj 

 attention was called to the tendency of sediments, particularly 

 those of coarse texture, to remain near shore. Observation of 

 shores of New England, California, North Carolina, and Peru 

 have produced a sympathetic attitude toward the erroneous 

 view of Rear Admiral Davis that the sea " rejects or repels" 

 the debris of continents.* While this statement is obviously 

 an exaggerated figure of speech, the coast ward urging of coarse 

 materials during the process of coast erosion, has, to my mind, 

 been too little emphasized in texts for students' use. Without 

 taking an extreme view, it follows from the above summary 

 analysis of the development of coast contour and shore profile, 

 that clastic marine deposits laid down during a single physio- 

 graphic cycle have no great extent or thickness, and that their 

 limits may be roughly calculated. If the coarser materials, 

 gravel and cobbles, are alone considered, the belt of marine and 

 lacustrine sedimentation is narrowed to a degree not generally 

 recognized. If we could determine the thickness and extent 

 of marine gravel deposits laid down under average conditions 

 upon various types of shore and at different stages in their 

 physiographic history, a useful criterion for the recognition of 

 ancient conglomerates would be established. This I believe 

 maj r be done with a fair degree of approximation on the basis 

 of the physiographic studies outlined above. 



Distribution and Preservation of Gravel during a Single 

 Physiographic Cycle. 



Whatever the initial topographic expression of a coastal 

 belt, the coast contour and the shore profile experience a series 

 of changes leading from youth through adolescence to maturity 

 following a defined order. The nature and amount of the 

 work to be performed within a cycle and the time required to 

 reach maturity depends upon the initial form, the strength 

 of the ocean forces and the composition and structure of the 

 coast. Gravel is supplied from stream sediments, from older 

 coast gravels or from wave-beaten cliffs, and in the absence of 

 supplies from these sources the sea may expend its power for 

 long periods upon fine stuff alone. In general, however, youth 

 is the time of maximum erosion and maximum deposition for 

 all classes of material. Two types of shore may be distin- 

 guished : over-flat, and over-steep initial profiles. 



Initial over-flat profile. — If the initial shore profile is too 

 flat for the most effective wave work, a nip is cut in the sub- 



* Quoted by Mitchell, U. S. Coast and Geodetic Survey, 1869, Appendix 5, 

 p. 85. 



