BOUNDARY OF THE GLACIATED AREA. 



191 



perature, to secure all the additional force required to extend 

 the present glaciers of southeastern Alaska, British Columbia, 



Fia. 62. — Map of area covered by the North American ice sheet of tha Pleistocene epoch 

 at its maximum extension, showing the approximate southern limit of glaciation, 

 the three main centers of ice accumulation, and the drif tless area within the glacia- 

 ted region. It is now proven that the maximum advance from the Keewatin center 

 preceded that from Labrador ; since bowldery deposits from the northeast extend 

 over those from the norlkwest throughout a large part of Indiana and Illinois and 

 for quite a di3tanceinto Iowa. Lake Superior copper isfound in the oldest deposits 

 in Pennsylvania, while red jasper conglomerate bowlders from Lake Huron are 

 found in the newer deposfte west of the Mississippi a f aw miles from Keokuk. 

 (From the United States Geological Survey.) 



and of the Cascade Range of State of Washington and Ore- 

 gon, until they should gorge all the channels of the Alex- 



