62 Physics of the Soil 



to permit the soil to become dry enough to allow the air 

 to penetrate deeply and freely, the organic matter accu- 

 mulates and soils are formed containing a large proportion 

 of humus ; even beds of peat may develop. 



Under other conditions, where rivers ap- 

 proach their outlet across a very flat country 

 I and are no longer able to scour their chan- 

 nels and keep them clean, the moving sedi- 

 | ment finally raises the banks and the bed un- 

 til the water is flowing above the surround- 

 a ing country. Under these conditions with a 

 continual seepage and frequent overflows 

 swamps are developed in which marsh vege- 

 tation grows luxuriantly and, falling under 

 g conditions where free oxidation cannot oc- 

 | cur, the remains only partially decay, giving 

 i rise to beds of peat and rich humus soils. 



In other cases, where a river often shifts 

 | its course and especially where the cut-offa 

 'o or ox-bows illustrated in Fig. 15 are formed, 

 | these places, with the poor drainage which 

 they must have and with the occasional over- 

 ,8 flows to keep the cut-offs filled with water, 

 o are maintained wet long and continuously 

 j enough to allow humus soils to form. 

 | With the final withdrawal of the great ice 



* sheet from the glaciated parts of America 

 S and Europe there were left large numbers of 

 J3 shallow lakes whose flat margins were wet 

 ^ enough to support marsh vegetation and 

 S very often this vegetation came to form a 

 | floating fringe steadily encroaching upon 



* the lake in the manner represented in Fig. 

 20. As the vegetation continued to grow 



and die the fringe became heavier and sank more deeply in 

 the water until finally the whole lake was overgrown and 

 until the organic matter, together with the sediments 

 brought down by the rains and the winds and washed in 



