Isopach

http://www.nvcc.edu/home/cbentley/gol_135/massanutten/massanutten.htm

This isopach map shows the thickness of the Tioga Bentonite over the mid-Atlantic region. Isopach maps are known to depict the thickness of a particular stratum with contour lines. The Tioga Bentonite was originally deposited as volcanic ash during a large volcanic eruption millions of years ago. The deposits of Tioga ash are thickest in the southwest region.