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As water seeps into the ground, it settles in the pores and cracks of underground rocks and into the spaces between grains of sand and pieces of gravel. In time, the water trickles down into a layer of rock or other material that is water tight. This water tight zone collects the ground water, creating a saturated zone known as an aquifer. Aquifers in the United States are usually made from gravel, sandstone, limestone, or basalt (volcanic rock).