On the Wisconsin River Improvement 143 



let show that this change took place since the glacial period. 

 The ancient Lake Winnipeg was larger than the present Lakes 

 Superior and Michigan combined. The northern depression 

 is known to be going on along the Atlantic coast from New 

 Jersey to Greenland. Any one can test the matter of north 

 ern depression and southern elevation for himself by examin- 

 ing the published maps, and remembering that the effect of 

 bodies of water on the shore is to abrade it and spread the ma- 

 terial smoothly over the bottom, while the effect of the atmo. 

 sphere is to cut the land up in ridges and to wash the soil 

 from the rocks ; so that the land rising from the water will 

 have comparatively smooth outlines, and successive lagoons 

 parallel to the shore, while the land going under the water will 

 show jagged and sharp outlines, with deep indentations and 

 numerous islands. We know that during the cretaceous pe- 

 riod, an ocean extended from the present Gulf of Mexico to 

 the Arctic Ocean, covering a large portion of the space between 

 the Missouri River and the Rocky Mountains. At that time 

 the country through which the Upper Mississippi now flowH 

 was dry land, and its slopes must have sent its water westward 

 to that cretaceous ocean. As the continent rose this ocean 

 disappeared, and the tertiary period began with great fresh water 

 lakes along the Rocky Mountains, Into these lakes the 

 waters of the upper Mississippi region continued to drain west- 

 ward. The gradual southwestern elevation of the continent 

 throughout the testiary period is distinctly proved by the de- 

 posits of the testiary lakes. The earliest deposits were of the 

 least area, and as they become more recent they expand north- 

 eastward, and this action continued apparently to the time pre- 

 ceding the glacial epoch. Preceding the glacial period, then 

 all the water-courses westward of the upper Mississippi region 

 were westward and not southward, as now. Not only the slope 

 of the laud but the great folds of the Silurian strata compelled 

 the water to this course. Over a great deal of the region thus 

 drained, no rocks more recent than the Silurian are found, so 



