304 H. E. Gregory — Note on the Shape of Pebbles. 



the discovery of one " characteristic " glacial pebble. From a 

 study of a number of deposits including drumlins and reces- 

 sional remains, it appears that pebbles and bowlders which 

 may be unqualifiedly assigned to ice work do not exceed one 

 in a thousand for the Connecticut glacial deposits. 



Pebbles with " characteristic glacial forms and markings " 

 produced by processes other than glacial have been described 

 from various parts of the world, and the presence of " im- 

 pressed " pebbles in conglomerates implies pressure and differ- 

 ential motion more than sufficient for the production of striae. 

 Even from dikes, "glacial" pebbles suitable for classroom 

 demonstration have been collected. 



Of the many factors whose evaluation is essential in estab- 

 lishing distinctions between various modes of origin of con- 

 glomerate, that of shape of pebbles has perhaps the least 

 significance. No constant difference between the constituents 

 of marine, lacustrine and river gravel is likely to be established. 

 Polished, striated, and soled pebbles are not of diagnostic rank, 

 and .their absence does not imply the absence of glaciation. In 

 well-exposed and widely extended outcrops, or where proof of 

 origin rests on other grounds, pebble form may have supple- 

 mentary value ; under other circumstances this criterion should 

 be applied with extreme caution. 



Yale University, New Haven, Conn. 



