6 



THE SOIL AND ITS ORIGIN 



Rock Weathering the SoU-forming Process. — The formation 

 of soil is the result of the slow process of rock weathering, meaning 

 the breaking up and decay of rocks brought about by the destruc- 

 tive action of the forces of nature. These forces include: 



(a) Heat and cold — causing cracking and splitting of rocks 

 through sudden temperature changes. 



(6) Frost — causing splitting and cracking by freezing water. 



Fig. 2. — Sugar Bowl Rock, Dells of the Wisconsin River. Water wears away and helps to 



disintegrate rocks. 



(c) Wind — causing abrasion through the action of dust and sand 

 particles carried by it. 



(d) Ice — causing grinding as in glacial action. 



(e) Water — a wearing and dissolving agent, also producing 

 chemical changes^ (Figs. 2 and 3.) 



Oxygen,^ producing chemical changes, 

 (/) Gases — ^ Carbon dioxide, or carbonic acid gas, producing 

 ( chemical changes. 



2 Chemical changes are the decay processes in rock weathering. 

 ^ Oxygen is a gas composing 20.7 per cent of the air. Carbon dioxide is a 

 heavy gas consisting of the elements carbon and oxygen (CO2). 



