Isopach

http://pubs.usgs.gov/fs/fs115-99/fs115-99.html

Contour maps come in all shapes, sizes, and types. The isopach map is one of those types. The isopach map uses isopach lines to show either rock or sediment thickness. These maps are beneficial when in search of an aquifer or gem mine. The map shown above captures the thickness of the Pittsburgh Coal bed in 1997. Judging from the graph, which uses isopach lines by thickness of feet, the thickest area of the Pittsburgh Coal bed looks to be about 9 or 10 feet and crosses over multiple counties.