11. K Gregory — Note on the Shape of Pebbles. 301 



inal form. Likewise a retreating shore line may strand gravels 

 which have not been completely reshaped. Material furnished 

 to waves is quarried from coasts composed of all sorts of mate- 

 rial, both consolidated and unconsolidated. Glacial and river- 

 borne pebbles and bowlders, as well as joint-bounded and irreg- 

 ular fragments of rock, find their way to the beach, and the 

 shape which they assume at various stages of their life history 

 on the shore is believed to be predominantly controlled by 

 their original form. Round and oval fragments tend to retain 

 those shapes ; blocks with rectangular and square cross sections 

 develop elliptical or circular cross sections ; flat fragments tend 

 to remain flat or to become discoid. If the material supplied 

 to waves consisted of silver dollars and marbles, it is difficult 

 to imagine how any pebbles other than flat and round could 

 result from the most vigorous and long-lived wave abrasion. 

 It may be noted also that for stratigraphic purposes lacustrine 

 and delta gravels are to be included with marine sediments, 

 although their constituent pebbles are obviously of sub-aerial 

 origin. 



That pebbles may be well rounded by streams is proven by 

 direct observation and by experimentation. As pointed out 

 by Bonney,"* the results of Daubree's well known studies are 

 not of general application, since the only phase of river work 

 comparable with travel of irregular blocks in revolving cyl- 

 inders is the attrition of pebbles in potholes. Omitting the 

 time factor, the conditions surrounding Daubree's experiments 

 may be approached and a high degree of sphericity of pebbles 

 be attained in cases where irregular blocks, unmixed with much 

 finer stuff, are carried en masse. Under such circumstances 

 the pebbles experience a continuous direct bombardment among 

 themselves. As bearing on this point, it was noted that gravel 

 bars on the Navajo Reservation shift their position from year 

 to year and even during sudden floods. A deposit composed 

 of about 80 per cent of coarse gravel on Bonito Creek, Arizona, 

 moved bodily down stream between 1910 and 1913, for a dis- 

 tance of about 300 feet, without apparent change in texture or 

 composition. It was also observed at several localities that the 

 pebbles on bars and terraces exposed between floods have a 

 distinctly higher degree of rotundity where the mass is com- 

 posed almost wholly of pebbles one-half inch to three inches in 

 diameter, than where composed of saud, adobe, and pebbles. 

 In studying these deposits it was noted that highly angular 

 pebbles, and even blocks one foot in diameter, have been car- 

 ried for distances exceeding 30 miles. In the lower Chinli 

 valley a block of monzonite 5 inches in diameter, after a journey 

 of 40 miles, has a form almost identical with that of talus 

 * Geol. Mag., v, pp. 54-61. 



