AILSA ALLABY and MICHAEL ALLABY. "hardpan." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 9 Jul. 2009 <http://www.encyclopedia.com>.
AILSA ALLABY and MICHAEL ALLABY. "hardpan." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. (July 9, 2009). http://www.encyclopedia.com/doc/1O13-hardpan.html
AILSA ALLABY and MICHAEL ALLABY. "hardpan." A Dictionary of Earth Sciences. 1999. Retrieved July 09, 2009 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-hardpan.html