H. E. Gregory — Note on the Shape of Pelbles. 303 



pencil mark had not been erased. Of the holes made in 1909 

 on the downstream side of the bowlders, none had been per- 

 ceptibly modified. On quartzite and limestone bowlders the 

 abrasion during four years amounted to less than *01 of an 

 inch. That this process is continuous is shown by the pres- 

 ence of percussion marks on upstream faces of bowlders and 

 their absence on the opposite side. In torrential streams of 

 Connecticut the upstream faces of many bowlders are percepti- 

 bly worn, while lichens cover their downstream sides. On 

 those portions of the Connecticut shore where current action 

 prevails over wave work, comparable illustrations may be 

 found. 



In streams of semi-desert regions the rate of transportation 

 appears to have high value in shaping pebbles, regardless of dis- 

 tance traveled, and to a lesser degree, of hardness and specific 

 gravity. Pebbles in " washes " are prevailingly angular, but 

 are less uniformly so in their torrential portions. At the base 

 of steep slopes, approaching the perpendicular, such as exist on 

 Navajo Mountain and the edges of Black Mesa, pebbles appear 

 to be somewhat better rounded than along the Puerco, the 

 Moencopi and the Kayenta valleys after traveling many times 

 as far. 



Moreover, many pebbles in fluviatile gravel bars are decom- 

 posed, and may be crushed in the hand, although retaining 

 their form. It appears that the conditions surrounding such 

 pebbles particularly favor chemical action, and that ancient 

 river gravels may not in all cases be distinguishable from 

 residual deposits resulting directly from weathering. 



Wind-made pebbles when maturely developed and fresh are 

 characteristic, but until faceting is far advanced and also after 

 decomposition has modified their shape, may not be unlike cer- 

 tain classes of river and glacial pebbles. 



At first sight it appears that pebbles resulting from glacia- 

 tion could readily be distinguished, and the geologic litera- 

 ture is characterized by the assumption that the presence of 

 " soled," striated, faceted, or polished pebbles demonstrates gla- 

 ciation. That this criterion should be used with great caution 

 is evident from the fact that " typical " glacial pebbles and 

 bowlders are rare. Fluvio-glacial drift together with surficial 

 and terminal moraines is many times more abundant than 

 ground moraine, and is prevailingly lacking in striated and 

 polished pebbles. The exceeding rarity of such pebbles about 

 the existing glaciers in Switzerland, Peru and California is a 

 matter worthy of comment. In the terminal moraines of 

 Long Island striated and polished pebbles must be sought for 

 with care; and in the "stony till" of Connecticut, even in 

 well-exposed sections, an hour's search may be rewarded by 



