Isopach Map

http://www.searchanddiscovery.com/documents/2007/07022lash/images/2_18.htm
An Isopach map uses contour lines to connect points of equal thickness.  These maps are often utilized in hydrographic survey, stratigraphy, sedimentology, structural geology and volcanology.  The map above is Isopach map of Ohio, Pennsylvania, and New York.