■^'s^l Existing glacial fields. 



"1 Exposed after disappearance of PLEISTOCENE ice. 



[ 'Cl. j Driftless and nunatak areas wholly or partly exposed during PLEISTOCENE. 

 |iil_^_j Areas of mountain and valley glaciation during the PLEISTOCENE. 

 Y'//^ Atlantic coastal plain and Pacific areas exposed during the QUATERNARY. 

 1^^^^^^ Probable land area available since close of the T E R T I A R Y . 

 II I II I I II Probable land area available since close of the CRETACEOUS. 

 Probable land area available since close of the JURASSIC. 



I Probable land area available since close of the PALAEOZIC.or unknown. 



Fig. 44. — IVIap showing periods since when various areas of present-day North 



American land have supposedly been free from major ice-sheets, etc. (Harvard 



University handout, modified after Flint and others.) 



