THE QUATERNARY PERIOD 



455 



Lakes are especially abundant in the terminal moraines. In 

 Minnesota and Wisconsin thousands of them mark the posi- 

 tions of these belts. 



Stretching southward from the moraines, gently sloping 

 plains mark the outwash deposits which were built by 

 the overloaded glacial streams. Owing to the porous, well- 



FIG. 473. Lobate edge of the American ice sheet with marginal lakes left 

 during its retreat. (Modified after Taylor and Leverett, U.S. Geol. Surv.) 



drained soil, some of these plains make excellent farming 

 land, although others are too sandy. Down every valley 

 leading away from the moraines, gravel and silt were strewn, 

 forming a flood plain. When the glaciers disappeared and 

 the streams became relatively free from detritus they were 

 able to cut down into these valley trains and have left portions 

 of them as terraces. 



