Flanked to the west by Kittatinny Mountain, the Stillwater Township is located within the Ridge and Valley Physiographic Province, and the entire township is within the Kittatinny Valley a region of rolling hills and flat valley floors that is a section of the larger Great Appalachian Valley running from eastern Canada to northern Alabama. Elevations in this valley range from . According to Snell, The surface of the country is generally uneven and hilly, and on the west exceedingly rugged and mountainous. Ponds and watercourses abound. Of the latter, the most important is the Paulinskill, a millstream of considerable consequence. All of Stillwater Township is located in the Paulins Kill watershed which flows southwest through Sussex and Warren counties before joining the Delaware River near Columbia, New Jersey. The valley floor is part of the Ordovician Martinsburg Formation (shale and slate) which make up most of the valley—and the Jacksonburg Formation (mostly limestone).