Thomas J. Major Ronald G. Borman 1977 <p>The northern High Plains of Colorado is an area of about 9,500 mi<span id="_mce_caret" data-mce-bogus="true">^2<span id="_mce_caret" data-mce-bogus="true"> (24,600 km<span id="_mce_caret" data-mce-bogus="true">^2<span id="_mce_caret" data-mce-bogus="true">) in the eastern part of the State (index map). The boundaries of the High Plains of Colorado are the State line and the limit of the Ogallala Formation of late Tertiary age. The Ogallala Formation is an unconsolidated or partly consolidated deposit of sand, gravel, clay, and silt and is the major aquifer in the northern High Plains.</span></span></span></span><br></p> application/pdf 10.3133/wri7742 en U.S. Geological Survey Water-level changes in the northern high plains of Colorado, 1964 to 1976 and 1972 to 1976 reports