or THE NORTHWESTERN STATES. 21 



of the rocky strata. This movement, enduring for a long period of time, served to 

 remove a large portion of the broken fragments of the northern rocks to regions 

 farther south ; leaving, at its close, the strata towards the pole bare, or with less 

 earthy covering, and those south of the Arctic circle with more. 



As the glacial era drew towards its close, the transporting force changed from 

 one in which ice predominated, to a modified movement of ice -and water, which, 

 of course, affected the condition of the materials. They show everywhere the 

 effects of two kinds of force. Sometimes, especially upon and north of Lake Supe- 

 rior, the unstratified, confused drift, with coarse gravel and boulders, and the 

 stratified water-washed sand, clay, and gravel are in close connection. 



Towards the south the materials are finer, and a larger proportion stratified. 

 Beyond the coarse boulder portion and the striated rocks, the proportion of finer 

 materials predominates. These beds are composed also of clay, loam, sand, and 

 gravel, due to this modified form of deposition. In Southern Illinois and in Mis- 

 souri the prairie land commences towards the south on these deposits. 



The prairies are not, however, confined to them, but extend northerly into Wis- 

 consin and Iowa, over the true drift regions, and westerly and northwesterly over 

 the cretaceous and tertiary formations, showing that the treeless country is not 

 wholly due to geological causes. 



Earth thrown out of the wells of Ohio and the mineral pits of Wisconsin, im- 

 mediately sends up weeds, grasses, and shrubs that appear to come from seeds 

 preserved in the drift beds. Where timber is so well preserved below the efi"ects 

 of the atmosphere, the seeds of plants might well retain their vitality. 



As to forest trees, their germs would be of varieties belonging to an Arctic or 

 subarctic climate, brought southward out o-f their proper place, and would not 

 flourish well in their new position. The southerly portions of the drift, and the 

 superficial beds between it and the tertiary strata of the Mississippi valley, have 

 more loam and clay, with less sand and coarse gravel, than the northern drift, 

 which form a rich soil, stimulating to annuals and grasses. When vegetation 

 became again prolific, the surface of the country along the southern edge of the 

 glacier regions must have been a long time flooded with water and floating ice, the 

 motion of which would also be towards the south. In the central parts of the 

 continent this water would be fresh. The leading valleys would bear away the 

 largest portion, and the currents thus produced would be most powerful along the 

 valleys. For a time there would be a mixture of ice, water, boulders, gravel, and 

 mud, in a fluid or semifluid state. 



In this way the finer materials were transported farther to the south, and scat- 

 tered there after the glacier motion had ceased. The northwestern States are so 

 little elevated, that large portions of the upland would be for a time submerged, 

 over which the finer sediments were dispersed. Immense floes of moving ice, 

 grounded upon the higher lands with currents between, would produce many of ■ 

 the effects we witness. 



The unstratified hardpan, and the half-worn and striated fragments of adjacent 

 rocks, convey to an observer evidence of pressure and mechanical forces Avholly 

 inexplicable by hydrostatic action. When the thawing agencies commenced, the 



