18 NATURE AND PROPERTIES OF SOILS 



armed. Its cutting power, therefore, depends on the amount 

 of sediment that it carries and on its velocity of flow. 

 Erosion by water deserves particular attention, as its denud- 

 ing effects are very rapid when geologically viewed. Most 

 of the changes in topography are due to such activity. The 

 material swept away is partly in suspension and partly in 

 solution. 1 The Appalachian Mountains, whose uplift was 

 complete in Carboniferous times, have lost vastly more of their 

 mass than now remains in view. 



While most of the debris from the ancient erosive cycles 

 has been changed to rock or has become a noticeable charac- 

 teristic of ocean water, remnants persist. To these remnants 

 rivers, lakes and oceans are making, year by year, substantial 

 additions. The cutting, carrying and depositing activity of 

 streams produce alluvial soils of which the Mississippi flood 

 plain is a well known example. Deltas built into oceans, lakes 

 and gulfs represent stream activity under different condi- 

 tions, while uplifted continental shelves are often bedded with 

 erosive products. The delta and marine soils of the Atlantic 

 and Gulf coastal plains afford examples of the latter types 

 of soil production. Even the pounding, grinding and sorting 

 activities of waves in ocean and lake are no mean factors in 

 the mechanics of soil formation. 



12. Glacial action. — Ice at the present time, especially 

 in temperate regions, is of little importance in soil forma- 

 tion. Nevertheless, at a comparatively recent date geolog- 

 ically, it had much to do with the preparation and deposition 

 of soil materials over great areas in central and northern 

 North America, northern Europe and the British Isles. Dur- 

 ing the Great Ice Age immense continental glaciers succes- 

 sively invaded these regions, much as the ice cap is over- 



1 The chemical denudation by streams is generally spoken of as corro- 

 sion. Abrasion is applied to the wear of the stream load upon its 

 channel and of the particules in suspension upon themselves. Erosion 

 is a broader term including corrosion and abrasion as well as trans- 

 portation. 



