Isopach Map

http://www.geo.utexas.edu/faculty/barker/kempter/rbtephra.html

An isopach map shows the variation in thickness of a stratum or unit, many times a rock unit. The lines are called isopachs, and are contour lines that display equal thickness over a certain area. The map above is an isopach map of the Rio Blanco tephra deposit. When the Active Crater erupted, the Rio Blanco tephra was spread by prevailing winds and deposited in the contoured areas of the map.