350 



GEOLOGY. 



Besides the striae, grooves, etc., on the bed rock, then; are often 

 other details of surface wliich are equally characteristic. Minute 



Fig. 486. — Striae on two contiguous surfaces which meet each other at a large angle. 

 Southeast shore of Kelley's Island, Lake Erie. (U. S. Geol. Surv.) 



protuberances of surface often show more wear on one side than on 

 the other (Fig. 487 and 488). Minute depressions (Fig. 488), show 



Fig. 487. — Small protuberances of rock showing the effect of ice wear. Glacial knobs 

 and traps. The projections consist of chert in limestone. Near Darlington, Ind. 



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Fig. 488. — Diagram to show the effect of ice wear on slight depressions in the 



surface of rock. 



analogous features. The significant point in these features is that 

 the same sides of the protuberances, and the same sides of the depres- 



