Fluviatile and Marine Gravels. 505 



they are in harmony with results of exhaustive studies on the 

 English coast, where wave and current work attain maximum 

 proportions. 



Austen* concludes that " the moving power of the sea at 60 

 fathoms is limited to fine sand " and that gravels below this 

 depth date from previous cycles. This author also remarks 

 that " no abrasion has been effected over ledges (in the English 

 Channel] under depths of not more than IT fathoms." 

 Cornishf states " I reckon that they [the waves from the 

 English Channel) would move shingle at 10 fathoms, hardly at 

 all at 20 fathoms, and probably not at 30 fathoms." The 

 researches of Douglas, Stevenson, Winder and other harbor 

 engineers^ indicate that 30 fathoms is about the depth of effec- 

 tive movement of shingle even by heavy swells. That the bed 

 of the English Channel is not traversed by violent currents of 

 translation of any sort is indicated by the unworn state of 

 flints and other pebbles and by the presence of an extensive 

 fauna. If the figures for the English coast be considered as 

 representing the average, 30 fathoms may be taken as the depth 

 to which gravel under normal conditions may be deposited dur- 

 ing a single physiographic cycle. The distance from the shore 

 line at which this depth is attained in selected localities is as 

 follows : San Pedro Point, California, 8 miles ; Rosalia Point, 

 Lower California, 3 miles ; Atlantic City, New Jersey, 30 

 miles ; Shinnecock Bay, Long Island, 30 miles ; Argentine 

 coast, 40 to 70 miles ; southwest Ireland, 1 to 3 miles ; north- 

 east Ireland, 10 miles ; east Scotland, 12 miles ; western Italy, 

 1 to 10 miles; Finland, 30 miles. On those shores where the 

 30-fathom contour is more than 20 miles distant from the shore 

 line gravel is rarely indicated at that depth ; in most cases the 

 outermost gravel is shown less than 10 miles from the coast. 



On the basis of data available it appears therefore that 

 gravel deposited during a single physiographic cycle is limited 

 to a narrow zone, probably not exceeding 15 miles in width — 

 a statement in harmony with the conclusions derived from the 

 analysis of marine erosion. Moreover, the proportion of gravel 

 to finer sediments in this belt of 15 miles is not necessarily 

 large, since the coarse materials are not continuous. As pre- 

 served in the sedimentary record, a stratum of marine con- 

 glomerate exceeding 15 miles in breadth and 100 feet in thick- 

 ness would be anomalous. As compared with marine action 

 vigorous streams may carry gravel during a single cycle 3 to 

 300 times farther and distribute it much more widely. 



If the above analyses properly represent the methods of 

 river and ocean work, it follows that inland fluviatile gravels 



*Quar. Jour. Geol. Soc, vi, pp. 69-97, 1850. 



fGeog. Jour., xi, p. 541, 1898. 



:{: Discussed by Hunt, loc. cit., p. 285. 



