Bivariate choropleth maps

http://ga.water.usgs.gov/edu/earthgwsinkholes.html

A bivariate choropleth map uses colors or shading to show it's variables of data across a geographic location like any other choropleth map, however it shows only two variables specifically. This is accomplished by using a color system for one variable and using a shading system for another. The map displayed here is one of possible sinkhole locations across the United States as of 1972. The karst landscapes shown here by two different colors make up the first variable while the lines for shading make up the second variable representing evaporite rocks that are known to be in an area. Florida, parts of Texas, and the Midwest are susceptible to sinkholes as this map shows.