BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 VOL. 17, PP. 303-316, PLS. 37~39 JULY 26, 1905 



GRESCENTIC GOUGES ON GLACIATED SURFACES* 



BY G. K. GILBERT 



(Read before the Cordilleran Section of the Society December SO, 1905) 



CONTENTS 



Page 



Introduction and description 303 



Conoid fracture 305 



Differential pressure 307 



Deformation and rupture 309 



Rhythm 312 



Resistance of ice to flowage 313 



Nomenclature 313 



Explanation of plates 315 



Introduction and Description 



Associated with striae and other evidences of glacial abrasion are certain 

 types of rock fracture. These have been classified and described by 

 Chamberlin in his "Rock-scorings of the great ice invasions." f The 

 three principal types are chatter-marks, crescentic cracks, and crescentic 

 gouges. All of these are so associated with glacial sculpture and stria- 

 tion as to indicate that they are of glacial origin. They all occur charac- 

 teristically in sets, the members of each set succeeding one another in 

 the direction of ice motion and each individual marking having its longer 

 axis athwart the direction of, ice motion. 



Chatter-marks and crescentic gouges have a common character, in that 

 each is characteristically a shallow furrow with crescentic outline. In 

 crescentic gouges the convexity of the crescent is usually turned forward ; J 

 in chatter-marks it is usually turned backward. Chatter-marks arc 

 closely associated with grooves engraved by boulders. Crescentic gouges 

 are not thus associated; they frequently occur on surfaces exhibiting no 

 other marks of glaciation except fine striae and polish. 



*Published by permission of the Director of the U. S. Geological Survey. 



t Seventh Ann. Rept. U. S. Geological Survey, pp. 218-223. 



X In this paper the words forward and downstream are used to indicate the direction 

 toward which the glacier moves or moved, and backward and upstream to indicate the 

 opposite direction. 



XXVIII— Bull. Geol. Soc. Am., Vol. 17, 1905. (303) 



