Scientists who study groundwater (often called “hydrologists” or “hydrogeologists”) generally distinguish between two types of aquifers:
Porous Media Aquifers
Individual particles the size of sand or gravel and up to large boulders. Groundwater moves through the openings, between the individual grains or rocks, called pore spaces.
Porous rock layers where grains are cemented to each other are called “consolidated.” Sandstones are examples of consolidated porous material.
Solid rock layer where groundwater is found in fractures, joints or cracks in the rock. Fractured aquifers can be found in many different types of rocks including granite and basalt.