552 KIRK BRYAN 



are those of an artesian basin. There are four classes of springs 

 which fulfil these conditions. 



Dip artesian springs (Fig. i?, a) occur in more or less regularly 

 bedded rocks that have been tilted and eroded in such a manner 

 that the porous bed receives water from the rain or streams in its 

 upper end and that the lower end is exposed at the surface. Sedi- 

 mentary rocks, alternating lava flows, tuffs and gravels, and 

 unconsolidated alluvial material supply these conditions. 



In a series of beds that has been folded, if the proper conditions 

 of inflow and outflow exist, a porous stratum constitutes an inverted 

 siphon for the conveyance of water. Springs due to the outflow 

 from the low side of such a basin may be called siphon artesian 

 springs (Fig. i8 6). The artesian basin of the Great Plains is the 



111! 



Fig. 19. — Cross-section of the Great Plains showing siphon artesian springs near 

 Sioux City, Iowa. (After Darton.) 



largest and most remarkable in the world, because of the great 

 distance traveled by the water and the heavy pressure under which 

 it exists. Along the eastern border of the basin the beds have a 

 distinct westward tilt, but the water-bearing Dakota sandstone is 

 not everywhere exposed (Fig. 19). However, near Sioux City, 

 Iowa, numerous springs and seeps are found along the western 

 bluffs of Missouri River, where the Dakota first emerges from 

 beneath its impervious cover.^ 



In certain unconsolidated deposits not regularly bedded, a 

 mass of porous material may be so exposed as to receive water at a 

 high level and discharge this water at a lower level. The springs 

 resulting from this structure may be caUed unbedded artesian 



^G. E. Condra, "Geology and Water Resources of a Portion of the Missouri 

 River Valley in Northeastern Nebraska," U.S. Geol. Survey, Water-Supply Paper 21$ 

 (1908), pp. 27, 28. 



