Geological strata giving rise to an Artesian well

An artesian aquifer is an aquifer whose water is overpressurized. Water will thus flow out of an artesian well without pumping.

Why is a well artesian?

Because its recharge zone is higher than the head of the well.

Pascal's Law predicts the overpressure:

 Poverpressure = ρg(zrecharge - zwellhead) 

where ρ is the volumic mass of the fluid, g the acceleration due to gravity, and z is height.

Origin

Artesian wells are named after the town of Artois in France, where the first one was drilled by Carthusian monks in 1126. [1] (http://scholar.chem.nyu.edu/tekpages/artesian.html) (see also Note 1: )

Notes

Note 1: Frances and Joseph Gies, Cathedral, Forge, and Waterwheel subtitled "Technology and Invention in the Middle Ages". Harper Perennial, 1995 ISBN 0060165901, page 112.