Unconsolidated deposits are further classified according to the processes responsible for their formation. One of the most extensive and important alluvial deposits in the US is the Ogallala formation, which resulted from deposition of sediment eroded from the eastern face of the Rocky Mountains. Consolidated Rocks Consolidated Rocks are usually classified according to their origin. Important types of sedimentary rock aquifers include sandstones and carbonate rocks. Sandstones can vary widely in permeability, depending upon the degree of cementation and the presence of secondary fractures.
Sandstone formations serve as significant aquifers in many areas.
These deposits exhibit mostly secondary porosity due to fracturing and dissolution openings, because CaCO3 is soluble in rainwater. If the carbonate formations are extensive and extend close to the land surface, a unique terrain, known as karst terrain can exist. Karst areas are characterized by caverns, sinkholes, lost rivers, and large springs. The Ozark region of Missouri and Arkansas is an example. Carbonate formations form significant aquifers in northern Illinois and along the gulf plains of Texas and Florida.