CLASSIFICATION OF SPRINGS 541 



sandstone and sandy clay overlie clay. Water that collects in the 

 sandy beds emerges in a series of springs at the contact with the 

 underlying clay, slightly above the level of the adjacent flood plain 

 of the Mississippi,' 



Mesa springs are those 

 due to an overlying ma- 

 terial which is hard and 

 forms a cliff (Fig. 6 b). =^ 

 In this class the over- ---e-^3£:-€;:^;r^=-e?c-3-:^3:?-^3-r-33->:^?z^^ 



lying material is com- Fig. 7.— Cross-section from Austin to Hickory 



monly a sandstone, Plains, Arkansas, illustrating gravity springs. 

 though it may be a po- (After Purdue.) 



rous and jointed lava 



flow. Along the east side of Chuska Mountain, New Mexico, in 

 the Navajo country, at an elevation of 8,200 feet, a rough-floored 

 terrace extends for several miles (Fig. 8). Above the shale which 

 floors the terrace a cliff of horizontal Chuska sandstone rises 200 

 to 500 feet. At the foot of the cliff are more than thirty springs."" 

 If the underlying bed is of small extent but impervious, it will 

 force water contained in overlying porous material to the surface. 

 In many places such water lies far above the ordinary water table 

 and constitutes what is called a perched water table. ^ Such 

 conditions are fairly common in unconsolidated alluvium. The 

 impervious bed is usually clay, cemented gravel, a "mortar bed," 

 or a layer of caliche or hardpan. As in popular usage all these 

 materials are known as hardpan, springs caused by them may be 

 called hardpan springs (Fig. 6 c) . The older alluvium of the 

 Sacramento Valley is generally underlain by a bed of reddish hard- 

 pan about two feet below the surface. During and after rains water 

 seeps to the surface at the contact of the hardpan with the over- 

 lying soil, forming temporary springs. A few springs of this class 



^ A. H. Purdue, "Water Resources of the Contact Region between the Paleozoic 

 and Mississippi Embayment Deposits in Northern Arkansas," U.S. Geol. Survey, 

 Water-Supply Paper 145 (1905), pp. 93, 113. 



^ H. E. Gregory, "The Navajo Country," U .S. Geol. Survey, Water-Supply Paper 

 380 (1916), pp. 140-41. 



3 A. C. Veatch, "Underground Water Resources of Long Island, N.Y.," U.S. 

 Geol. Survey, Prof. Paper 44 (1906), p. 57. 



