An important class of artificial springs or wells is known as Artesian
Wells. Where bent pervious beds of rock lie between two bent impervious
beds, so as to make a basin-shaped depression, lower in the middle than
at the edges, the rain which sinks into the pervious rock where it
reaches the surface will begin to gather in the central part of the
porous rock as in a reservoir.
If a hole be now bored in the hollow of the upper impervious bed till it
reaches the water-bearing stratum, the water will flow out at the top.
The water thus obtained may have fallen a distance of many miles several
months previously, and if the gathering-ground be high the issue at the
well may be forced by the pressure of the water behind to a considerable
height.