6 4 



PHYSICAL GEOLOGY 



Fig. 40. — A fissure spring. 



The oases of deserts often owe their existence to springs. The 

 oases of Kerid in the northern Sahara desert contain about 6000 



acres, which support nearly 1,000,000 

 date palms. They lie at the foot of an 

 escarpment which forms the northern 

 boundary of the desert. From the base 

 of this escarpment or cliff, numerous 

 springs gush forth and furnish a constant 

 supply of water for irrigation. The 

 water of the springs falls as rain in the highlands many miles distant. 

 After flowing as streams for a short distance, the water disappears 

 in the sand. It then follows underground courses until the escarp- 

 ment is reached. 



Constant and Intermittent Springs. — Whether springs are constant 

 or intermittent depends upon a number of factors : if the rainfall is 

 not uniformly distributed throughout the year, if the region is not 

 forested, or if the porous rock is too limited to hold a sufficient supply 

 of water, an intermittent spring may result. In such a hill as that 

 shown in Fig. 38 A the glacial deposit (till) and sand allow the 

 water to be absorbed in large 

 amounts and to sink to the 

 impervious stratum along 

 which ground water flows to 

 Sp. When the water stands 

 at A, a spring may flow which 

 will cease when the water is 

 below that level. In unusu- 

 ally dry seasons all may dis- 

 appear. It is seldom, per- 

 haps never, that a siphon 

 operates to form an inter- 

 mittent spring, but in such a 

 case as that shown in Fig. 41 

 it will be seen that the water 

 will not flow until it has 

 reached BC, after which it will discharge until the reservoir is 

 empty. This is due to the fact that the weight of the water in the 

 arm CD is greater than that of the arm B. 



Mineral Matter in Spring Water. — Since springs are derived from 

 underground water which has been in close contact with various rocks, 





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Fig. 41. — Diagram illustrating the possi- 

 bility of the occurrence of a siphon spring 

 in nature. If the vertical joints B and C do 

 not reach the surface, the water filling the 

 joints A, A, A will continue to flow as a spring 

 (Sp) until the joints are emptied, because the 

 weight of the water in the arm CD is greater 

 than in B. When once emptied the water will 

 not begin to flow until the joints are filled above 

 BC. (Modified after De Martonne.) 



