THE PHYSICAL PROCESSES OF SOIL FORMATION. 5 



PHYSICAL COMPOSITION' OF GLACIER MUD. 



It will be noted that over 70 per cent of this mud consists 

 of extremely fine, wholly impalpable materials ; but little of 

 which is true clay. 



The fineness of the glacier-flour renders it peculiarly suitable 

 for the rapid conversion into soil, and such soils are usually 

 excellent and remarkably durable. The great and lasting 

 fertility of the soils of southern Sweden is traced directly to 

 this mode of origin, and doubtless the great American ice 

 sheet of glacier times is similarly concerned in the high quality 

 of the soil of our '' north central " states, from the Ohio to 

 the Great Lakes and the Missouri. 



The accumulations of rocks and debris of all sizes, in the 

 *' moraines " or detrital deposits of glaciers and ice-sheets form 

 another class of glacier-made lands which cover extensive and 

 important agricultural areas (drift areas), both in the old and 

 new worlds. Such lands are undulating or slightly hilly, and 

 the soil usually contains imbedded in it stones of a great 

 variety of kinds and sixes, partly angular, partly rounded and 

 polished by friction. Of course the frequent and violent 

 changes of temperature occurring on the surface of a glacier, 

 aid materially in reducing the rocks carried by it to the con- 

 dition in which we find the material of the moraines; which 

 commonly form lateral or cross ridges in valleys formerly 

 occupied by glaciers. 



Action of flowing 7<.'<;/<v. The action of flowing water is 

 doubtless at this time the most potent mechanical agency of 

 soil formation. From the sculpturing of the original simple 

 forms in which geological agencies left the earth's surface 



