THE FORMATION OF SOIL 23 



considerable disruptive force. The break- 

 ing down of rocks and stones in this way is a 

 purely mechanical process. But plant roots 

 also assist in the formation of soil by parting 

 at their apex with an acid fluid largely 

 carbonic acid which attacks stones and 

 rocks, and assists in the solution of their 

 contents. 



Over large areas in the British Isles the 

 soil of our fields owes its origin to the action 

 of glaciers, which at one time filled our valleys 

 to the depth of hundreds of feet. With the 

 exception of the higher mountain ranges, 

 the whole of Great Britain and Ireland was 

 heavily glaciated as far south as the Valley 

 of the Thames. By the action of the moving 

 ice the hills were rounded off and planed 

 down, while the valleys were deepened, the 

 whole process being accompanied by severe 

 attrition, the material so rubbed off being 

 deposited in the plains and valleys. If one 

 digs into a field in many parts of the country, 

 especially in the North, one soon comes upon 

 a stratum consisting of larger or smaller 

 stones, often smoothed, and of very diverse 

 character, such as limestone, shale, sandstone, 

 granite, whinstone, and even coal. These 

 stones and boulders, which had, in some 

 cases, been carried by the ice for hundreds 



