1. n. [Geology]

A contour connecting points of equal true vertical thickness of strata, formations, reservoirs or other rock units. A map that displays isochores is an isochore map. The terms isopach and isopach map are incorrectly used interchangeably to describe isochores and isochore maps. Isopachs and isochores are equivalent only if the rock layer is horizontal.

See: contour,  contour map,  formation,  isopach,  reservoir,  stratigraphy,  true stratigraphic thickness,  true vertical thickness