140 PHYSIOGRAPHY 



for most of its lakes to be destroyed. The oldest drift is believed 

 to be many times (probably as many as 25) as old as the youngest. 



Stratified drift. Valley trains (p. 126), outwash plains, deltas, 

 etc., were developed by streams flowing from the continental glaciers, 

 but only those of the last ice-sheet are well preserved, the older ones 

 having been destroyed by erosion. Some of the valley trains are 

 long, and in some the deposits are very deep. Thus the Rock River, 

 in southern Wisconsin, filled its valley with gravel and sand to a 

 depth of 300 to 400 feet just outside the terminal moraine of the last 

 glacial epoch. The Columbia River, swollen by the waters from 

 the melting ice, filled its valley, locally, to the depth of 700 feet 

 with sediment washed out from the ice. 



Since the ice melted, most of the valley trains have been par- 

 tially carried away, and their remnants are terraces (Fig. 105). 



Effects of Glaciation on Human Affairs 



The changes produced by glaciation have had much influence 

 on the industries of the region which the ice covered. In the United 

 States, glaciation increased the amount of mantle rock. This in- 

 crease is helpful where slopes are rather steep. This is seen by 

 comparison with driftless regions, where the soil is often very thin 

 or absent on the slopes, and where much of the land cannot be cul- 

 tivated for this reason. Abundant soil is much more likely to be 

 found on similar slopes in the glaciated area. Again, the general 

 effect of glaciation was to reduce slopes, and it therefore tended to 

 reduce the extent of areas too steep to be cultivated. 



The quality of the soil was improved in many places by glaci- 

 ation, but this was not true everywhere. It is worth noting that 

 most of the wheat and hay grown in the United States east of the 

 Rocky Mountains is within the area which was glaciated. This 

 is, however, largely because of the climate. 



On the whole, the ice left the surface less rough than it found it. 

 This made it easier to build roads, and so has helped travel and 

 transportation; but locally the surface was made rougher, with 

 disadvantageous results. 



The falls, rapids, and lakes which resulted from glaciation have 

 increased the water power, and the lakes, ponds, and marshes tend 



