PLEISTOCENE GEOLOGY. 273 



farming under irrigation. No part of it, however, is sufficiently impervious 

 to resist the passage of water through it, and this passage tends to rob the 

 surface of iis soluble elements of plant food and concentrate them in the 

 lower levels. Hence the soil covering a thick deposit of loess is not 

 generally so rich as that covering an area from which the upper and 

 leached-out portions have been removed. Tims the soil of Capitol Hill is 

 inferior to that in Highlands, where the loess is comparatively thin. In 

 places where it is thin, trees and plants that have deep roots often manage 

 to dispense with irrigation, since their roots reach the moister portions near 

 its base. 



WATER. 



The loess is all more or less porous, so that water percolates through it to 

 the bed-rock below, and where a permanent supply of water is needed, wells 

 must be sunk entirely through it. This constitutes a serious impediment to 

 the settlement of the areas on the divides in the eastern part of the State 

 where great thicknesses of loess remain, as the expense of sinking a well 

 over 100 feet deep is too great to be borne by the ordinary homesteader. 

 The height of the water-saturated loess above bed-rock depends naturally 

 on the varying conditions of the bed-rock, and on the amount of water 

 received from the surface. It is hence found to be much higher below 

 irrigation ditches than above them, and a lowering of several feet is noticeable 

 in the water of wells below ditches when the water is shut off from them in 

 the autumn. 



In spite of its general porosity, there appear to be certain portions or 

 layers that are sufficiently impervious to retain some of the percolating 

 water, and thus constitute a false bed-rock. Fields resting on such layers 

 Avill require less water than others, and deep plowing will sometimes injure 

 them by breaking up the more impervious layer. The seepage of water 

 often aids in producing an artificial layer of the impervious type, so that 

 after some years of irrigation fields require less irrigation than at first. 



Below irrigating ditches the depth of wells affords good criteria for 

 determining the thickness of the loess, but is less certain above, as it is 

 often necessary to bore for a considerable distance into the bed-rock before 

 a permanent supply of water can be obtained. 



MON XXVII 18 



