What type of aquifers are sandwiched between impermeable layers of rock?

Confined aquifers are characterized by being sandwiched between layers of impermeable rock or clay, which creates a barrier that restricts the movement of water in and out of the aquifer. This confinement allows the water within the aquifer to be under pressure, often resulting in the phenomenon where, when a well is drilled into a confined aquifer, the water can rise above the top of the aquifer or even flow out to the surface naturally, a situation known as artesian flow. The presence of impermeable layers above and below the confined aquifer is critical for maintaining the pressure and quality of the water stored within. This underlying principle differentiates confined aquifers from unconfined aquifers, which are directly connected to the surface and lack the protective impermeable layers, allowing for more direct recharge and discharge. Therefore, the definition and characteristics of confined aquifers make them the correct choice in this context.