CONSERVATION OF GROUND WATERS 
189 
entire area to a depth of seventeen feet. This is equivalent to 
the rainfall for seven years. Since Iowa is in the region which 
receives at least an average amount of the total rainfall of the 
United States we may accept these figures as being substantially 
correct for our state. It appears then that there is no lack of 
ground water, that the total capacity of the reservoir is very 
large and that rainfall is reasonably sufficient to continually re- 
plenish the supply. 
If we consider the relation of the various strata of Iowa to 
her ground water supply we shall find that the glacial formations, 
the clays and sands and gravels transported and spread out by the 
great ice-sheets of Pleistocene time, form one of the most im- 
portant, if not actually the greatest of the aquifers, or water-bear- 
ing formations, of our geological column. Leaving out of consider- 
ation for the moment the water used by plants, which over most 
of the state is drawn directly from glacial drift materials, most 
of our rural population depends entirely or nearly so on the drift 
for its water supply. This supply is drawn largely from wells, 
in lesser measure from springs, to some extent from so-called 
surface waters, which, however, are replenished in part from the 
underground supplies. Again, many of our cities and towns are 
supplied from the drift — including here the sands and gravels 
found as pockets in the clays or lining the valley floors. Notable 
examples are Des Moines, Council Bluffs and Sioux City in part, 
Le Mars, Webster City, Chariton, Atlantic, Muscatine and many 
others. It is doubtless true that more than half of the state’s 
inhabitants depend directly on the drift materials for their sup- 
plies of water. 
Beneath the drift covering — • the mantle rock ■ — lie the great 
series of the bed rock. All of these strata contain water in dif- 
ferent amounts, the shales scantily, the limestones in fair abund- 
ance in crevices and fissures and pores, the sandstones in bountiful 
supply. Hence whenever a sandstone is penetrated by wells it 
is more than likely to supply a goodly store. The Cretaceous 
sandstones of western Iowa, the Des Moines sandstones in the 
central counties, the St. Peter, the New Richmond and the Cam- 
brian sandstones in northeastern and southeastern Iowa, all of 
these beds yield water in large measure to the wells which are 
sunk into their depths. The waters from some of these strata 
are rather heavily mineralized, it is true. And this mineralization 
is increasingly heavy at greater distances from the regions of 
outcrop of the strata. But it is nevertheless true that the combined 
