ii8 The Ottawa Naturalist. [August 



After the final emergence from the sea was accomphshed, the 

 country assumed somewhat the same character which it has at the 

 present time but with several modifications. The plateau west of 

 the present valley extended farther to the east and sloped more 

 regularly eastward while the larger channel was probably also 

 shifted eastward to run along the face of the limestone outcrop or 

 by a series of minor streams running parallel to it corresponding 

 to the basii'is of the present lakes. 



A great change in the climatic conditions next brought about 

 important changes in the surface features and also in the distribu- 

 tion of the soil. Colder winters and cooler summers were succeeded 

 by a long period ot continuous winter, in which all the natural 

 drainage was stayed and there gradually accumulated a vast thick- 

 ness of snow. The area of greatest precipitation and consequent 

 accumulation of ice and snow was at first in the country to the 

 north. As this ice increased in thickness it began to spread slowly 

 towards its outer margin. In this way there was a movement of 

 the ice southward through the valley and as the movement pro- 

 gressed this mass of ice picked up and carried along with it much 

 of the loose material on the surface, at the same time scoring and 

 polishing the harder rocks, breaking off protruding points and 

 deeply plowing along the face of the plateau of soft rocks to the 

 west. When the valley was filled there might have been a halt to 

 the forward movement for a time but it gradually over-rode the 

 edge and spread to the west as far as the Coteau du Missouri and 

 southward over Minnesota. 



Warmer conditions returned and the great mass began to 

 melt along its margin. The great amount of debris carried along 

 with the ice was thus left in great heaps where the edge of the ice 

 was stationary for some time or if the retreat caused by melting 

 was rapid the surface would be more or less evenly strewn by this 

 material which is generally called boulder-clay. As the ice melted 

 there would naturally be a vast quantity of water to be carried 

 away, and river channels were formed which appear now to have 

 little cause for origin except for this emergency. Where the slope 

 of the country was toward the ice, large lakes along its margin 

 were formed. /<\C*lCj/">v 



