FORMATION OF SWAMl'S. 



597 



denotes a high level of water in the stream and complete 

 saturation of the soil. 



(f) In hir/Ji platcanx, swampiness is due to heavy rainfall, or 

 to snowfall with subsequent tliaw, or to saturation of the air 

 combined with an impermeable subsoil accompanied by the 

 growth of swamp-forming mosses or other plants. 



{(l) Swamps on a hUlshle are generally caused by springs, the 

 water from which cannot penetrate into the ground on account 

 of a subsoil of clay or of horizontal rocky strata. If the soil 



r-x 



i. 



\. ,/^ 



Fig. 268. 

 a. Impermeable stratum, h. Permeable stratum, r. Water. 



on a hillside should be permeable above and impermeable 

 below, all the water in it descends to the base of the hill, 

 where it rises above the ground-surface and forms a swamp 

 (Fig. 268). If, however, the permeable stratum terminates in 

 the slope, drainage-water will spring out of the hillside, along 

 the line where the two strata coalesce. 



2. JJaiiKtrje done hjj Sn-anips. 



a. General Accon/il. 



The damage done by swampy ground is as follows : — 

 i. Instability of the trees, which favours windfall, especially 

 of the spruce. 



ii. Stunted growth and liability to decay in the roots and 

 stems of trees, and consequent.loss of increment and sometimes 

 death of the trees. The bad growth of trees on wet soil is due 

 to the exclusion of oxygen from the roots and to the low tem- 

 peratures of the soil and aii-, as wet soil may reduce air- tempera- 

 ture by 9' to 1-1 ¥., or to the formation of humic acid and 



