36 THE FIRST BOOK OF FARMING 



land to the Rocky Mountains. This vast river was 

 very deep and very heavy and into its under sur- 

 face were frozen sand, pebbles, larger stones and 

 even great rocks. Thus it acted as a great rasp or 

 file and did an immense amount of work grinding 

 rocks and making soils. It ground down moun- 

 tains and carried great beds of soil from one place 

 to another. When this great ice river melted, it 

 dropped its load of rocks and soils, and as a result 

 we find in that region of the country great boulders 

 and beds of sand and clay scattered over the land. 



Work of the Air. The air has helped in the 

 work of wearing down the rocks and making soils. 

 If a piece of iron be exposed to moist air a part of 

 the air unites with part of the iron and forms iron 

 rust. In the same way when moist air comes in 

 contact with some rocks part of the air unites with 

 part of the rock and forms rock rust which crum- 

 bles off or is washed away by water. Thus the air 

 helps to break down the rocks. Moving air or 

 wind picks up dust particles and carries them from 

 one field to another. On sandy beaches the wind 

 often blows the sand along like snow and piles it 

 into drifts. The entire surface of sandy regions is 

 sometimes changed in this way. Sands blown from 

 deserts sometimes bury forests which with their 

 foliage sift the fatal winding sheet from the dust- 

 laden winds. 



The Work of Plants. Living plants sometimes 

 send their roots into rock crevices ; there they grow, 

 expand, and split off rock fragments. Certain kinds 



