THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XXXYII. — An Instance of the Action of the Ice-sheet 

 upon Slender Projecting Roclc Mcisses 'j"^ bj William 

 Herbert Hobbs. (With Plate IX.) 



The present paper will discuss what seems clearly to be a 

 <iase of degradational action by the ice mantle exerting its 

 pressure in a lateral direction against steep and slender masses 

 of projecting rock. The conditions are in some respects pecu- 

 liar to the locahty, which lies in an island-like area of ISTewark 

 traps and arkoses within protecting walls of the crystalline 

 schists — the Pomperaug Yalley area of Connecticut. The pre- 

 servation here of the ^Rewark rocks from erosion, which has 

 elsewhere so generally removed them, is a direct result of their 

 deformation by jointing and their depression in the form of a 

 composite crnstal block along the marginal joint planes as 

 fault walls.f This depression has been irregular in so far as 

 the blocks of the smaller orders of magnitude within the larger 

 composite block have been depressed by different amounts so 

 as to produce a mosaic of small prismatic blocks. The present 

 positions of basalt and arkose within the area are also dependent 

 upon the high position of the main basalt flow in the local 

 ]S"ewark series, and upon the low easterly dip (zfc20°) of the 

 beds and Hows previous to their dislocation. Their positions 

 are further affected by the fact that the displacement by fault- 

 ing was distributive largely within a marginal zone. As a 

 result of all these conditions the dense resistant basalt now 

 occupies the central portion of the depressed area and is bor- 

 dered by the softer shales and arkoses. 



Could the area have been visited subsequent to its deforma- 

 tion by faulting but previous to its degradation through the 



* Read before the Wisconsin Academy of Sciences, Arts, and Letters, at 

 Milwaukee, December, 1901. 



f Thie Newark System of the Pomperaug Valley. 21st Ann. Eept. U. S. 

 Geol. Suryey, Pt. Ill, pp. 1-160. 



A3I. Jour. Sci. — Fourth Series, Vol. XIV, No. 84. — December, 1902. 

 28 



