Associated with striæx 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.”† The three principal types are chatter-marks, crescentic cracks, and crescentic gouges. All of these are so associated with glacial sculpture and striation as to indicate that they are of glacial origin. They all occur characteristically 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;‡ in chatter-marks it is usually turned backward. Chatter-marks are closely associated with grooves engraved by boulders. Cresccntie gouges . . .