THE PLEISTOCENE OR GLACIAL PERIOD. 



349 



ture from parallelism is according to a definite system, for they radiate 

 from the centers already named (Fig.- 470). Not only this, but there 

 are systematic radiations of striae within the lobes of ice which char- 

 acterized the borders of the great ice-sheets at the stages when it 

 was most influenced by the broad depressions of the Great Lake region 



Fig. 485. — Tortuous glacial grooving. The gorge is believed to be due to a sub- 

 glacial stream, into the channel of which the ice settled down, moulding itself 

 to the gorge and grooving it. Kelley's Island, Lake Erie. (U. S. Geol. Surv.) 



(Fig. 484). The direction of stria? corresponds with the direction in 

 which the drift was transported. 



Sometimes stria? and grooves follow narrow and tortuous gorges 

 (Fig. 485). Stria? are not confined to horizontal or even to gently 

 inclined surfaces. They occur on steep slopes (Fig. 486), not infre- 

 quently on the vertical faces of cliffs, and, occasionally, even on the 

 under sides of overhanging rock masses. 



