F. H. Lahee — Crescentic Fractures of Glacial Origin. 41 



Art. VI. — Crescentic Fractures of Glacial Origin ; byF. H. 



Lahee. 



The terms, ' crescentic fracture,' ' crescentic crack,' and 

 < crescentic cross-fracture, 5 have been used in the geological 

 literature* to denote certain short, curved cracks which have 

 been observed, usually in sets, on the glaciated surfaces of 

 hard, brittle, homogeneous rocks. Since these fractures bear 

 a constant and definite relation to the direction of ice motion, 

 as indicated by the striae, their origin has been reasonably 

 attributed to glacial action. 



During the past summer the writer discovered such an excel- 

 lent example of these fractures on a ledge brought to his notice 

 by Mr. R. W. Sayles, that he considered it worth while to 

 make a detailed study of them and prepare the results for pub- 

 lication. 



The ledge above referred to is one of a group of roches 

 moutonnees of quartzite, situated on the unwooded eastern end 

 of JSTorthey Hill in Lisbon, N. H. According to Hitchcock,f 

 this rock is a member of the 'Coos group' which was listed by 

 him as later than the 'Cambrian clay slate' and earlier than 

 the 'Helderberg quartzites, slates, and limestones.' J In his 

 more recent paper,§ this quartzite is represented on the map 

 (plate 43) by a heavy black line in the area of staurolite schist 

 south of Streeter Pond. 



Lithologically, this quartzite varies from a rock which is 

 friable, distinctly bedded,) and more or less argillaceous, to 

 one that is very hard, massive, and nearly pure. Microscopic 

 sections of the latter phase display a compact mass of angular 

 grains, closely cemented together, with no signs of their orig- 

 inal outlines. The compactness of the rock is demonstrated in 

 the section by the sharpness of outline of the glaciated surface 

 which cuts straight across the grains without peripheral crush- 

 ing (fig. 1, a-b). The texture of this quartzite is medium. 



Most of the outcrops, especially on the backbone and north- 

 ern slope of the hill, are of the hard variety. They are of 

 typical roche moutonnee form, with the gentle stoss side on 

 the north. The surface on which the crescentic cracks are 

 best developed, measuring roughly 600 square feet in area, is 



* See the following : Winchell, N. H., Geol. and Nat. His. Surv. of Minn., 

 6th Ann. Rept., pp. 106, 107, 1877; Andrews, Ed., this Journal (3), xxvi^ 

 1883, pp. 101, 102, 1883 ; Chamberlin, T. C.,. U. S. G. S., 3d Ann. Kept., 

 1883, pp. 363, 364, 1883; and, Gilbert, G. K., Bull. Geol. Soc. Am., xvii, pp. 

 303, 304, 1906. 



f Hitchcock, C. H., Geol. of N. H., vol. ii, pp. 275, 317, 318, 1877. 



X Ibid., p. 273. 



§ Hitchcock, C. H., Bull. Geol. Soc. Am., xv, p. 470, 1904. 

 ■ I Stratification: N.55°-65°E., 60°-80°N. 



