Geology . WlsdTHE 90 Fig. 484.—The radiation of strise in the Green Bay glacial lobe, and in the west partof the Lake Michigan lobe, during the last glacial epoch. The stria1 on the bed-rock beneath the drift are generally approxi-mately parallel in any given locality, and tolerably constant in direc-tion over considerable areas. When large areas are studied, the striaeare sometimes found to be far from parallel. In general, their depar- THE PLEISTOCENE OR GLACIAL PERIOD. 349 ture from parallelism is according to a definite system, for they radiatefrom the centers already named (Fig. 470). Not
Geology . WlsdTHE 90 Fig. 484.—The radiation of strise in the Green Bay glacial lobe, and in the west partof the Lake Michigan lobe, during the last glacial epoch. The stria1 on the bed-rock beneath the drift are generally approxi-mately parallel in any given locality, and tolerably constant in direc-tion over considerable areas. When large areas are studied, the striaeare sometimes found to be far from parallel. In general, their depar- THE PLEISTOCENE OR GLACIAL PERIOD. 349 ture from parallelism is according to a definite system, for they radiatefrom the centers already named (Fig. 470). Not only this, but thereare systematic radiations of striae within the lobes of ice which char-acterized the borders of the great ice-sheets at the stages when itwas 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 itselfto the gorge and grooving it. Kelleys Island, Lake Erie. (U. S. Geol. Surv.) (Fig. 484). The direction of striae corresponds with the direction inwhich the drift was transported. Sometimes stria) and grooves follow narrow and tortuous gorges(Fig. 485). Striae are not confined to horizontal or even to gentlyinclined surfaces. They occur on steep slopes (Fig. 486), not infre-quently on the vertical faces of cliffs, and, occasionally, even on theunder sides of overhanging rock masses. 350 GEOLOGY. Besides the Btria>, grooves, etc., on the bed rock, there are oftenother details of surface which are equally characteristic. Minute
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