THH GLACIAL PERIOD AND ITS RECORDS 



pendent glaciers, the ice advancing outward from their own 

 axes or centers. Thus the ice in the Lake Michigan basin 

 moved not only southward with the general movement of 

 the glacier but also radially from the central axis, southwest- 

 ward and southeastward, toward the edges of the basin. The 

 ice lobes in other basins had similar movements, which are 

 revealed in the rather complicated directions of the scratches, 

 also by curious features carved in the bedrock and by under- 

 lying deposits of earlier glaciations. 



Once past the old river valleys the ice fronts again united, 

 and the glacier — shaped somewhat like a huge cauliflower 

 with its stem to the north — moved southward almost to the 

 Ohio River. Its extent is marked by the line of hills, known 

 as terminal moraine, extending irregularly eastward from 

 Shelbyville, Illinois, to southern Ohio, thence into north- 

 western Pennsylvania, and from there southeastward to and 

 across Long Island. The advance was stopped by a slow 

 climatic change which caused the ice to begin a lingering, 

 halting retreat, marked by occasional readvances of the ice 

 front, but nevertheless a continuous withdrawal northward. 

 When the retreating ice front reached the ancient river valleys 

 the glacier once again assumed the tongue or lobate character 

 — thicker in the valleys and thinner on the uplands — each lobe 

 again with the characteristics of an essentially independent 

 glacier. 



The records of the retreat are similar to the records of re- 

 treat of living glaciers, easily studied in the Alps and in the 

 high Sierras, which furnish the clues to the work of the con- 

 tinental ice sheet. The records are mainly in long lines of 

 hummocky hills — moraines — of rock debris which the ice de- 

 posited as it melted. When the rate of forward push of the 

 ice was equalled by the rate of melting at its border, the ice 

 front remained stationary for many — probably hundreds — 

 of years, and the rock waste brought from the north settled 

 out of the melting ice at its edges, accumulating there like 



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