Aquifers are typically made up of permeable rock such gravel, sand, sandstone, or limestone where water is able to move between rock spaces. Aquifers can be confined, meaning they covered by a layer of impermeable rock, or unconfined meaning they are easily replenished by rain or snow melt. The water table defines how deep an aquifer is. Heavy rains will likely deepen the water table and droughts will likely cause it to abate. Private wells are dug deep enough to reach aquifers and extract ground water.